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Anatomy of digestive organs: Quick Anatomy Lesson: Human Digestive System

Quick Anatomy Lesson: Human Digestive System


Media Backgrounder

The human digestive system, (also known as the digestive tract, the GI tract, the alimentary canal) is a series of connected organs leading from the mouth to the anus. The digestive system allows us to break down the food we eat to obtain energy and nourishment.

The digestive system — which can be up to 30 feet in length in adults — is usually divided into eight parts: the mouth, the esophagus, the stomach, the small intestine (or “small bowel”) and the large intestine (also called “large bowel” or “colon”) with the liver, pancreas, and gallbladder adding secretions to help digestion. These organs combine to perform six tasks: ingestion, secretion, propulsion, digestion, absorption, and defecation.

The mouth starts the process by ingesting and mechanically breaking down the food we eat into a swallowable form, adding some early secretions to start the process of digestion. The esophagus is the muscular tube connecting the mouth to the stomach. A ring-like muscle at the end of the esophagus controls the passage of food into the stomach. This ring-like muscle also controls the food from going back up into the esophagus. The stomach then further liquefies the food and adds digestive acids.

From there the stomach contents are slowly released into the small intestine. The small intestine is where most of the digestion and absorption of nutrients takes place. There are millions of tiny finger-like projections lining the small intestine called villi (pronounced: “VILL-ee”). These villi aid in the absorption of nutrients. Secretions from the liver, pancreas, and gallbladder are emptied into the small intestine and also aid in the digestive process.

After traveling through the nearly 20 feet of small intestine, unabsorbed material moves into the colon (large bowel or large intestine). Here the remaining liquids and salts (electrolytes) are absorbed from the digested material. Bacteria further break down the undigested material, which continues to solidify and eventually passes from the body as feces through the rectum and anus during a bowel movement.

Reviewed August 2014  



Digestive System (Anatomy): How It Works

Your digestive system is uniquely designed to turn the food you eat into nutrients, which the body uses for energy, growth and cell repair. Here’s how it works.


The mouth is the beginning of the digestive tract. In fact, digestion starts here as soon as you take the first bite of a meal. Chewing breaks the food into pieces that are more easily digested, while saliva mixes with food to begin the process of breaking it down into a form your body can absorb and use.


Also called the pharynx, the throat is the next destination for food you’ve eaten. From here, food travels to the esophagus or swallowing tube.


The esophagus is a muscular tube extending from the pharynx to the stomach. By means of a series of contractions, called peristalsis, the esophagus delivers food to the stomach. Just before the connection to the stomach there is a “zone of high pressure,” called the lower esophageal sphincter; this is a “valve” meant to keep food from passing backwards into the esophagus.


The stomach is a sac-like organ with strong muscular walls. In addition to holding the food, it’s also a mixer and grinder. The stomach secretes acid and powerful enzymes that continue the process of breaking down the food. When it leaves the stomach, food is the consistency of a liquid or paste. From there the food moves to the small intestine.

Small Intestine

Made up of three segments, the duodenum, jejunum, and ileum, the small intestine is a long tube loosely coiled in the abdomen (spread out, it would be more than 20 feet long). The small intestine continues the process of breaking down food by using enzymes released by the pancreas and bile from the liver. Bile is a compound that aids in the digestion of fat and eliminates waste products from the blood. Peristalsis (contractions) is also at work in this organ, moving food through and mixing it up with digestive secretions. The duodenum is largely responsible for continuing the process of breaking down food, with the jejunum and ileum being mainly responsible for the absorption of nutrients into the bloodstream.

Three organs play a pivotal role in helping the stomach and small intestine digest food:


Among other functions, the oblong pancreas secretes enzymes into the small intestine. These enzymes break down protein, fat, and carbohydrates from the food we eat.


The liver has many functions, but two of its main functions within the digestive system are to make and secrete bile, and to cleanse and purify the blood coming from the small intestine containing the nutrients just absorbed.


The gallbladder is a pear-shaped reservoir that sits just under the liver and stores bile. Bile is made in the liver then if it needs to be stored travels to the gallbladder through a channel called the cystic duct. During a meal, the gallbladder contracts, sending bile to the small intestine.

Once the nutrients have been absorbed and the leftover liquid has passed through the small intestine, what is left of the food you ate is handed over to the large intestine, or colon.

Colon (Large Intestine)

The colon is a 5- to 6-foot-long muscular tube that connects the cecum (the first part of the large intestine to the rectum (the last part of the large intestine). It is made up of the cecum, the ascending (right) colon, the transverse (across) colon, the descending (left) colon, and the sigmoid colon (so-called for its “S” shape; the Greek letter for S is called the sigma), which connects to the rectum.

Stool, or waste left over from the digestive process, is passed through the colon by means of peristalsis (contractions), first in a liquid state and ultimately in solid form as the water is removed from the stool. A stool is stored in the sigmoid colon until a “mass movement” empties it into the rectum once or twice a day. It normally takes about 36 hours for stool to get through the colon. The stool itself is mostly food debris and bacteria. These bacteria perform several useful functions, such as synthesizing various vitamins, processing waste products and food particles, and protecting against harmful bacteria. When the descending colon becomes full of stool, or feces, it empties its contents into the rectum to begin the process of elimination.


The rectum (Latin for “straight”) is an 8-inch chamber that connects the colon to the anus. It is the rectum’s job to receive stool from the colon, to let you know there is stool to be evacuated, and to hold the stool until evacuation happens. When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters (muscles) relax and the rectum contracts, expelling its contents. If the contents cannot be expelled, the sphincters contract and the rectum accommodates, so that the sensation temporarily goes away.


The anus is the last part of the digestive tract. It consists of the pelvic floor muscles and the two anal sphincters (internal and external muscles). The lining of the upper anus is specialized to detect rectal contents. It lets us know whether the contents are liquid, gas, or solid. The pelvic floor muscle creates an angle between the rectum and the anus that stops stool from coming out when it is not supposed to. The anal sphincters provide fine control of stool. The internal sphincter keeps us from going to the bathroom when we are asleep, or otherwise unaware of the presence of stool. When we get an urge to go to the bathroom, we rely on our external sphincter to keep the stool in until we can get to the toilet.

Functional Anatomy of the Digestive System

Functional Anatomy of the Digestive System

The digestive system is composed of the digestive or alimentary tube and accessory digestive organs. The basic terminology used to describe parts of the digestive system is shown below and more detailed description of each is presented in later sections.

The digestive system depicted above – a carnivore – is the simplist among mammals. Other species, even humans, have a more or very much more extensive large intestine, and ruminants like cattle and sheep have a large set of forestomachs through which food passes before it reaches the stomach.

Each of the organs shown above contributes to the digestive process in several unique ways. If you were to describe their most important or predominant function, and summarize shamelessly, the list would look something like this:

  • Mouth: Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant. In some species, saliva contains amylase, an enzyme that digests starch.
  • Esophagus: A simple conduit between the mouth and stomach – clearly important but only marginally interesting compared to other regions of the tube.
  • Stomach: Where the real action begins – enzymatic digestion of proteins initiated and foodstuffs reduced to liquid form.
  • Liver: The center of metabolic activity in the body – its major role in the digestive process is to provide bile salts to the small intestine, which are critical for digestion and absorption of fats.
  • Pancreas: Important roles as both an endocrine and exocrine organ – provides a potent mixture of digestive enzymes to the small intestine which are critical for digestion of fats, carbohydrates and protein.
  • Small Intestine: The most exciting place to be in the entire digestive system – this is where the final stages of chemical enzymatic digestion occur and where almost almost all nutrients are absorbed.
  • Large Intestine: Major differences among species in extent and importance – in all animals water is absorbed, bacterial fermentation takes place and feces are formed. In carnivores, that’s about the extent of it, but in herbivores like the horse, the large intestine is huge and of critical importance for utilization of cellulose.

Last updated May 20, 2020. Send comments to [email protected]

A Russian language translation of this page by Patrick Wright and can be found at Russian translation

23.1 Overview of the Digestive System – Anatomy and Physiology

Learning Objectives

By the end of this section, you will be able to:

  • Identify the organs of the alimentary canal from proximal to distal, and briefly state their function
  • Identify the accessory digestive organs and briefly state their function
  • Describe the four fundamental tissue layers of the alimentary canal
  • Contrast the contributions of the enteric and autonomic nervous systems to digestive system functioning
  • Explain how the peritoneum anchors the digestive organs

The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process (Figure 23.2).

Figure 23.2 Components of the Digestive System All digestive organs play integral roles in the life-sustaining process of digestion.

As is the case with all body systems, the digestive system does not work in isolation; it functions cooperatively with the other systems of the body. Consider for example, the interrelationship between the digestive and cardiovascular systems. Arteries supply the digestive organs with oxygen and processed nutrients, and veins drain the digestive tract. These intestinal veins, constituting the hepatic portal system, are unique; they do not return blood directly to the heart. Rather, this blood is diverted to the liver where its nutrients are off-loaded for processing before blood completes its circuit back to the heart. At the same time, the digestive system provides nutrients to the heart muscle and vascular tissue to support their functioning. The interrelationship of the digestive and endocrine systems is also critical. Hormones secreted by several endocrine glands, as well as endocrine cells of the pancreas, the stomach, and the small intestine, contribute to the control of digestion and nutrient metabolism. In turn, the digestive system provides the nutrients to fuel endocrine function. Table 23.1 gives a quick glimpse at how these other systems contribute to the functioning of the digestive system.

Contribution of Other Body Systems to the Digestive System

Body system Benefits received by the digestive system
Cardiovascular Blood supplies digestive organs with oxygen and processed nutrients
Endocrine Endocrine hormones help regulate secretion in digestive glands and accessory organs
Integumentary Skin helps protect digestive organs and synthesizes vitamin D for calcium absorption
Lymphatic Mucosa-associated lymphoid tissue and other lymphatic tissue defend against entry of pathogens; lacteals absorb lipids; and lymphatic vessels transport lipids to bloodstream
Muscular Skeletal muscles support and protect abdominal organs
Nervous Sensory and motor neurons help regulate secretions and muscle contractions in the digestive tract
Respiratory Respiratory organs provide oxygen and remove carbon dioxide
Skeletal Bones help protect and support digestive organs
Urinary Kidneys convert vitamin D into its active form, allowing calcium absorption in the small intestine

Table 23.1

Digestive System Organs

The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body. Accessory digestive organs, despite their name, are critical to the function of the digestive system.

Alimentary Canal Organs

Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = “to nourish”) is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body’s “inner space.”

Accessory Structures

Each accessory digestive organ aids in the breakdown of food (Figure 23.3). Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.

Histology of the Alimentary Canal

Throughout its length, the alimentary tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit their specific functions. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa, which is continuous with the mesentery (see Figure 23.3).

Figure 23.3 Layers of the Alimentary Canal The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa.

The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of gut epithelium. The membrane consists of epithelium, which is in direct contact with ingested food, and the lamina propria, a layer of connective tissue analogous to the dermis. In addition, the mucosa has a thin, smooth muscle layer, called the muscularis mucosae (not to be confused with the muscularis layer, described below).

Epithelium—In the mouth, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the alimentary canal. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the mouth) to about a week (in the gut). This process of rapid renewal helps preserve the health of the alimentary canal, despite the wear and tear resulting from continued contact with foodstuffs.

Lamina propria—In addition to loose connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the alimentary canal to other parts of the body. The lamina propria also serves an immune function by housing clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the distal ileum where they are known as Peyer’s patches. When you consider that the alimentary canal is exposed to foodborne bacteria and other foreign matter, it is not hard to appreciate why the immune system has evolved a means of defending against the pathogens encountered within it.

Muscularis mucosae—This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption.

As its name implies, the submucosa lies immediately beneath the mucosa. A broad layer of dense connective tissue, it connects the overlying mucosa to the underlying muscularis. It includes blood and lymphatic vessels (which transport absorbed nutrients), and a scattering of submucosal glands that release digestive secretions. Additionally, it serves as a conduit for a dense branching network of nerves, the submucosal plexus, which functions as described below.

The third layer of the alimentary canal is the muscularis (also called the muscularis externa). The muscularis in the small intestine is made up of a double layer of smooth muscle: an inner circular layer and an outer longitudinal layer. The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal. In the most proximal and distal regions of the alimentary canal, including the mouth, pharynx, anterior part of the esophagus, and external anal sphincter, the muscularis is made up of skeletal muscle, which gives you voluntary control over swallowing and defecation. The basic two-layer structure found in the small intestine is modified in the organs proximal and distal to it. The stomach is equipped for its churning function by the addition of a third layer, the oblique muscle. While the colon has two layers like the small intestine, its longitudinal layer is segregated into three narrow parallel bands, the tenia coli, which make it look like a series of pouches rather than a simple tube.

The serosa is the portion of the alimentary canal superficial to the muscularis. Present only in the region of the alimentary canal within the abdominal cavity, it consists of a layer of visceral peritoneum overlying a layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the alimentary canal in place near the ventral surface of the vertebral column.

Nerve Supply

As soon as food enters the mouth, it is detected by receptors that send impulses along the sensory neurons of cranial nerves. Without these nerves, not only would your food be without taste, but you would also be unable to feel either the food or the structures of your mouth, and you would be unable to avoid biting yourself as you chew, an action enabled by the motor branches of cranial nerves.

Intrinsic innervation of much of the alimentary canal is provided by the enteric nervous system, which runs from the esophagus to the anus, and contains approximately 100 million motor, sensory, and interneurons (unique to this system compared to all other parts of the peripheral nervous system). These enteric neurons are grouped into two plexuses. The myenteric plexus (plexus of Auerbach) lies in the muscularis layer of the alimentary canal and is responsible for motility, especially the rhythm and force of the contractions of the muscularis. The submucosal plexus (plexus of Meissner) lies in the submucosal layer and is responsible for regulating digestive secretions and reacting to the presence of food (see Figure 23.3).

Extrinsic innervations of the alimentary canal are provided by the autonomic nervous system, which includes both sympathetic and parasympathetic nerves. In general, sympathetic activation (the fight-or-flight response) restricts the activity of enteric neurons, thereby decreasing GI secretion and motility. In contrast, parasympathetic activation (the rest-and-digest response) increases GI secretion and motility by stimulating neurons of the enteric nervous system.

Blood Supply

The blood vessels serving the digestive system have two functions. They transport the protein and carbohydrate nutrients absorbed by mucosal cells after food is digested in the lumen. Lipids are absorbed via lacteals, tiny structures of the lymphatic system. The blood vessels’ second function is to supply the organs of the alimentary canal with the nutrients and oxygen needed to drive their cellular processes.

Specifically, the more anterior parts of the alimentary canal are supplied with blood by arteries branching off the aortic arch and thoracic aorta. Below this point, the alimentary canal is supplied with blood by arteries branching from the abdominal aorta. The celiac trunk services the liver, stomach, and duodenum, whereas the superior and inferior mesenteric arteries supply blood to the remaining small and large intestines.

The veins that collect nutrient-rich blood from the small intestine (where most absorption occurs) empty into the hepatic portal system. This venous network takes the blood into the liver where the nutrients are either processed or stored for later use. Only then does the blood drained from the alimentary canal viscera circulate back to the heart. To appreciate just how demanding the digestive process is on the cardiovascular system, consider that while you are “resting and digesting,” about one-fourth of the blood pumped with each heartbeat enters arteries serving the intestines.

The Peritoneum

The digestive organs within the abdominal cavity are held in place by the peritoneum, a broad serous membranous sac made up of squamous epithelial tissue surrounded by connective tissue. It is composed of two different regions: the parietal peritoneum, which lines the abdominal wall, and the visceral peritoneum, which envelopes the abdominal organs (Figure 23.4). The peritoneal cavity is the space bounded by the visceral and parietal peritoneal surfaces. A few milliliters of watery fluid act as a lubricant to minimize friction between the serosal surfaces of the peritoneum.

Figure 23.4 The Peritoneum A cross-section of the abdomen shows the relationship between abdominal organs and the peritoneum (darker lines).

Disorders of the…

Digestive System: Peritonitis

Inflammation of the peritoneum is called peritonitis. Chemical peritonitis can develop any time the wall of the alimentary canal is breached, allowing the contents of the lumen entry into the peritoneal cavity. For example, when an ulcer perforates the stomach wall, gastric juices spill into the peritoneal cavity. Hemorrhagic peritonitis occurs after a ruptured tubal pregnancy or traumatic injury to the liver or spleen fills the peritoneal cavity with blood. Even more severe peritonitis is associated with bacterial infections seen with appendicitis, colonic diverticulitis, and pelvic inflammatory disease (infection of uterine tubes, usually by sexually transmitted bacteria). Peritonitis is life threatening and often results in emergency surgery to correct the underlying problem and intensive antibiotic therapy. When your great grandparents and even your parents were young, the mortality from peritonitis was high. Aggressive surgery, improvements in anesthesia safety, the advance of critical care expertise, and antibiotics have greatly improved the mortality rate from this condition. Even so, the mortality rate still ranges from 30 to 40 percent.

The visceral peritoneum includes multiple large folds that envelope various abdominal organs, holding them to the dorsal surface of the body wall. Within these folds are blood vessels, lymphatic vessels, and nerves that innervate the organs with which they are in contact, supplying their adjacent organs. The five major peritoneal folds are described in Table 23.2. Note that during fetal development, certain digestive structures, including the first portion of the small intestine (called the duodenum), the pancreas, and portions of the large intestine (the ascending and descending colon, and the rectum) remain completely or partially posterior to the peritoneum. Thus, the location of these organs is described as retroperitoneal.

The Five Major Peritoneal Folds

Fold Description
Greater omentum Apron-like structure that lies superficial to the small intestine and transverse colon; a site of fat deposition in people who are overweight
Falciform ligament Anchors the liver to the anterior abdominal wall and inferior border of the diaphragm
Lesser omentum Suspends the stomach from the inferior border of the liver; provides a pathway for structures connecting to the liver
Mesentery Vertical band of tissue anterior to the lumbar vertebrae and anchoring all of the small intestine except the initial portion (the duodenum)
Mesocolon Attaches two portions of the large intestine (the transverse and sigmoid colon) to the posterior abdominal wall

Table 23.2

Interactive Link

By clicking on this link you can watch a short video of what happens to the food you eat, as it passes from your mouth to your intestine. Along the way, note how the food changes consistency and form. How does this change in consistency facilitate your gaining nutrients from food?

How Your Gastrointestinal Tract Works

If you are considering gastric bypass surgery, it’s important you understand how your gastrointestinal (GI) tract works. At University of Missouri Health Care, our bariatric surgery experts change your gastrointestinal tract anatomy and function during weight loss surgeries.

Surgeries may change the size of your stomach or even remove part of your small intestine, depending on which weight loss option you choose. By changing the amount of food you can eat and how you digest, our surgeons help you lose weight.

What is the gastrointestinal tract?

The GI tract is the pathway food takes from your mouth, through the esophagus, stomach, small and large intestine. In the GI tract, nutrients and water from foods are absorbed to help keep your body healthy. Whatever isn’t absorbed keeps moving through your GI tract until you get rid of it by using the bathroom.

Your stomach and esophagus

The first part of the GI tract is the esophagus, the tube that connects your mouth and stomach. Food passes through the esophagus before it is stored in the stomach. Your stomach can hold as much as a quart and a half of food as it creates acids to digest the food. Because the stomach doesn’t absorb food at all, food only stays there a short time before small muscle contractions push the food into your small intestine.

Your small and large intestines

The small intestine consists of three parts: the duodenum, the jejunum and the ileum. In these three parts, food is mixed with digestive fluids that break it down and digest it further. The small intestine also absorbs nutrients into the bloodstream.

In the duodenum, bile, fluid from the walls of the intestine and other fluids from the pancreas mix with the foods. This mixture of food and fluids moves into the jejunum, where food gets broken down into its essential elements like carbohydrates, fats, proteins and other nutrients.

Finally, food moves into the ileum, where a lot of the nutrients and water are absorbed into the bloodstream. Food and digestive fluids that aren’t absorbed keep moving into the large intestine (your colon), where it is stored until you use the restroom.

Learn more about Bariatric Surgery Services at MU Health Care.

Digestive System Anatomy and Physiology

Children are fascinated by the workings of the digestive system: they relish crunching a potato chip, delight in making “mustaches” with milk, and giggle when their stomach growls. As adults, we know that a healthy digestive system is essential for good health because it converts food into raw materials that build and fuel our body cells.

Functions of the Digestive System

The functions of the digestive system are:

  1. Ingestion. Food must be placed into the mouth before it can be acted on; this is an active, voluntary process called ingestion.
  2. Propulsion. If foods are to be processed by more than one digestive organ, they must be propelled from one organ to the next; swallowing is one example of food movement that depends largely on the propulsive process called peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall).
  3. Food breakdown: mechanical digestion. Mechanical digestion prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces, and examples of mechanical digestion are: mixing of food in the mouth by the tongue, churning of food in the stomach, and segmentation in the small intestine.
  4. Food breakdown: chemical digestion. The sequence of steps in which the large food molecules are broken down into their building blocks by enzymes is called chemical digestion.
  5. Absorption. Transport of digested end products from the lumen of the GI tract to the blood or lymph is absorption, and for absorption to happen, the digested foods must first enter the mucosal cells by active or passive transport processes.
  6. Defecation. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces.

Anatomy of the Digestive System

The organs of the digestive system can be separated into two main groups: those forming the alimentary canal and the accessory digestive organs.

Organs of the Alimentary Canal

The alimentary canal, also called the gastrointestinal tract, is a continuous, hollow muscular tube that winds through the ventral body cavity and is open at both ends. Its organs include the following:


Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane-lined cavity.

  • Lips. The lips (labia) protect its anterior opening.
  • Cheeks. The cheeks form its lateral walls.
  • Palate. The hard palate forms its anterior roof, and the soft palate forms its posterior roof.
  • Uvula. The uvula is a fleshy finger-like projection of the soft palate, which extends inferiorly from the posterior edge of the soft palate.
  • Vestibule. The space between the lips and the cheeks externally and the teeth and gums internally is the vestibule.
  • Oral cavity proper. The area contained by the teeth is the oral cavity proper.
  • Tongue. The muscular tongue occupies the floor of the mouth and has several bony attachments- two of these are to the hyoid bone and the styloid processes of the skull.
  • Lingual frenulum. The lingual frenulum, a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements.
  • Palatine tonsils. At the posterior end of the oral cavity are paired masses of lymphatic tissue, the palatine tonsils.
  • Lingual tonsil. The lingual tonsils cover the base of the tongue just beyond.


From the mouth, food passes posteriorly into the oropharynx and laryngopharynx.

  • Oropharynx. The oropharynx is posterior to the oral cavity.
  • Laryngopharynx. The laryngopharynx is continuous with the esophagus below; both of which are common passageways for food, fluids, and air.


The esophagus or gullet, runs from the pharynx through the diaphragm to the stomach.

  • Size and function. About 25 cm (10 inches) long, it is essentially a passageway that conducts food by peristalsis to the stomach.
  • Structure. The walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same four basic tissue layers or tunics.
  • Mucosa. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen, of the organ; it consists primarily of a surface epithelium, plus a small amount of connective tissue (lamina propria) and a scanty smooth muscle layer.
  • Submucosa. The submucosa is found just beneath the mucosa; it is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic vessels.
  • Muscularis externa. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells.
  • Serosa. The serosa is the outermost layer of the wall that consists of a single layer of flat serous fluid-producing cells, the visceral peritoneum.
  • Intrinsic nerve plexuses. The alimentary canal wall contains two important intrinsic nerve plexuses- the submucosal nerve plexus and the myenteric nerve plexus, both of which are networks of nerve fibers that are actually part of the autonomic nervous system and help regulate the mobility and secretory activity of the GI tract organs.


Different regions of the stomach have been named, and they include the following:

  • Location. The C-shaped stomach is on the left side of the abdominal cavity, nearly hidden by the liver and the diaphragm.
  • Function. The stomach acts as a temporary “storage tank” for food as well as a site  for food breakdown.
  • Cardiac region. The cardiac region surrounds the cardioesophageal sphincter, through which food enters the stomach from the esophagus.
  • Fundus. The fundus is the expanded part of the stomach lateral to the cardiac region.
  • Body. The body is the midportion, and as it narrows inferiorly, it becomes the pyloric antrum, and then the funnel-shaped pylorus.
  • Pylorus. The pylorus is the terminal part of the stomach and it is continuous with the small intestine through the pyloric sphincter or valve.
  • Size. The stomach varies from 15 to 25 cm in length, but its diameter and volume depend on how much food it contains; when it is full, it can hold about 4 liters (1 gallon) of food, but when it is empty it collapses inward on itself.
  • Rugae. The mucosa of the stomach is thrown into large folds called rugae when it is empty.
  • Greater curvature. The convex lateral surface of the stomach is the greater curvature.
  • Lesser curvature. The concave medial surface is the lesser curvature.
  • Lesser omentum. The lesser omentum, a double layer of peritoneum, extends from the liver to the greater curvature.
  • Greater omentum.  The greater omentum, another extension of the peritoneum, drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall, and is riddled with fat, which helps to insulate, cushion, and protect the abdominal organs.
  • Stomach mucosa. The mucosa of the stomach is a simple columnar epithelium composed entirely of mucous cells that produce a protective layer of bicarbonate-rich alkaline mucus that clings to the stomach mucosa and protects the stomach wall from being damaged by acid and digested by enzymes.
  • Gastric glands. This otherwise smooth lining is dotted with millions of deep gastric pits, which lead into gastric glands that secrete the solution called gastric juice.
  • Intrinsic factor. Some stomach cells produce intrinsic factor, a substance needed for the absorption of vitamin b12 from the small intestine.
  • Chief cells. The chief cells produce protein-digesting enzymes, mostly pepsinogens.
  • Parietal cells. The parietal cells produce corrosive hydrochloric acid, which makes the stomach contents acidic and activates the enzymes.
  • Enteroendocrine cells. The enteroendocrine cells produce local hormones such as gastrin, that are important to the digestive activities of the stomach.
  • Chyme. After food has been processed, it resembles heavy cream and is called chyme.

Small Intestine

The small intestine is the body’s major digestive organ.

  • Location. The small intestine is a muscular tube extending from the pyloric sphincter to the large intestine.
  • Size. It is the longest section of the alimentary tube, with an average length of 2.5 to 7 m (8 to 20 feet) in a living person.
  • Subdivisions. The small intestine has three subdivisions: the duodenum, the jejunum, and the ileum, which contribute 5 percent, nearly 40 percent, and almost 60 percent of the small intestine, respectively.
  • Ileocecal valve. The ileum meets the large intestine at the ileocecal valve, which joins the large and small intestine.
  • Hepatopancreatic ampulla. The main pancreatic and bile ducts join at the duodenum to form the flasklike hepatopancreatic ampulla, literally, the ” liver-pacreatic-enlargement”.
  • Duodenal papilla. From there, the bile and pancreatic juice travel through the duodenal papilla and enter the duodenum together.
  • Microvilli. Microvilli are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance, sometimes referred to as the brush border; the plasma membranes bear enzymes (brush border enzymes) that complete the digestion of proteins and carbohydrates in the small intestine.
  • Villi. Villi are fingerlike projections of the mucosa that give it a velvety appearance and feel, much like the soft nap of a towel.
  • Lacteal. Within each villus is a rich capillary bed and a modified lymphatic capillary called a lacteal.
  • Circular folds. Circular folds, also called plicae circulares, are deep folds of both mucosa and submucosa layers, and they do not disappear when food fills the small intestine.
  • Peyer’s patches. In contrast, local collections of lymphatic tissue found in the submucosa increase in number toward the end of the small intestine.

Large Intestine

The large intestine is much larger in diameter than the small intestine but shorter in length.

  • Size. About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus.
  • Functions. Its major functions are to dry out indigestible food residue by absorbing water and to eliminate these residues from the body as feces.
  • Subdivisions. It frames the small intestines on three sides and has the following subdivisions: cecum, appendix, colon, rectum, and anal canal.
  • Cecum. The saclike cecum is the first part of the large intestine.
  • Appendix. Hanging from the cecum is the wormlike appendix, a potential trouble spot because it is an ideal location for bacteria to accumulate and multiply.
  • Ascending colon. The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal cavity.
  • Transverse colon. The ascending colon makes a turn and continuous to be the transverse colon as it travels across the abdominal cavity.
  • Descending colon. It then turns again at the left colic (or splenic) flexure, and continues down the left side as the descending colon.
  • Sigmoid colon. The intestine then enters the pelvis, where it becomes the S-shaped sigmoid colon.
  • Anal canal. The anal canal ends at the anus which opens to the exterior.
  • External anal sphincter. The anal canal has an external voluntary sphincter, the external anal sphincter, composed of skeletal muscle.
  • Internal involuntary sphincter. The internal involuntary sphincter is formed by smooth muscles.

Accessory Digestive Organs

Other than the intestines and the stomach, the following are also part of the digestive system:


The role the teeth play in food processing needs little introduction; we masticate, or chew, by opening and closing our jaws and moving them from side to side while continuously using our tongue to move the food between our teeth.

  • Function. The teeth tear and grind the food, breaking it down into smaller fragments.
  • Deciduous teeth. The first set of teeth is the deciduous teeth, also called baby teeth or milk teeth, and they begin to erupt around 6 months, and a baby has a full set (20 teeth) by the age of 2 years.
  • Permanent teeth. As the second set of teeth, the deeper permanent teeth, enlarge and develop, the roots of the milk teeth are reabsorbed, and between the ages of 6 to 12 years they loosen and fall out.
  • Incisors. The chisel-shaped incisors are adapted for cutting.
  • Canines. The fanglike canines are for tearing and piercing.
  • Premolars and molars. Premolars (bicuspids) and molars have broad crowns with round cusps ( tips) and are best suited for grinding.
  • Crown. The enamel-covered crown is the exposed part of the tooth above the gingiva or gum.
  • Enamel. Enamel is the hardest substance in the body and is fairly brittle because it is heavily mineralized with calcium salts.
  • Root. The outer surface of the root is covered by a substance called cementum, which attaches the tooth to the periodontal membrane (ligament).
  • Dentin. Dentin, a bonelike material, underlies the enamel and forms the bulk of the tooth.
  • Pulp cavity. It surrounds a central pulp cavity, which contains a number of structures (connective tissue, blood vessels, and nerve fibers) collectively called the pulp.
  • Root canal. Where the pulp cavity extends into the root, it becomes the root canal, which provides a route for blood vessels, nerves, and other pulp structures to enter the pulp cavity of the tooth.

Salivary Glands

Three pairs of salivary glands empty their secretions into the mouth.

  • Parotid glands. The large parotid glands lie anterior to the ears and empty their secretions into the mouth.
  • Submandibular and sublingual glands. The submandibular and sublingual glands empty their secretions into the floor of the mouth through tiny ducts.
  • Saliva. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids.
  • Salivary amylase. The clear serous portion contains an enzyme, salivary amylase, in a bicarbonate-rich juice that begins the process of starch digestion in the mouth.


Only the pancreas produces enzymes that break down all categories of digestible foods.

  • Location. The pancreas is a soft, pink triangular gland that extends across the abdomen from the spleen to the duodenum; but most of the pancreas lies posterior to the parietal peritoneum, hence its location is referred to as retroperitoneal.
  • Pancreatic enzymes. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid that neutralizes the acidic chyme coming in from the stomach.
  • Endocrine function. The pancreas also has an endocrine function; it produces hormones insulin and glucagon.


The liver is the largest gland in the body.

  • Location. Located under the diaphragm, more to the right side of the body, it overlies and almost completely covers the stomach.
  • Falciform ligament. The liver has four lobes and is suspended from the diaphragm and abdominal wall by a delicate mesentery cord, the falciform ligament.
  • Function. The liver’s digestive function is to produce bile.
  • Bile. Bile is a yellow-to-green, watery solution containing bile salts, bile pigments, cholesterol, phospholipids, and a variety of electrolytes.
  • Bile salts. Bile does not contain enzymes but its bile salts emulsify fats by physically breaking large fat globules into smaller ones, thus providing more surface area for the fat-digesting enzymes to work on.


While in the gallbladder, bile is concentrated by the removal of water.

  • Location. The gallbladder is a small, thin-walled green sac that snuggles in a shallow fossa in the inferior surface of the liver.
  • Cystic duct. When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored.

Physiology of the Digestive System

Specifically, the digestive system takes in food (ingests it), breaks it down physically and chemically into nutrient molecules (digests it), and absorbs the nutrients into the bloodstream, then, it rids the body of indigestible remains (defecates).

Activities Occurring in the Mouth, Pharynx, and Esophagus

The activities that occur in the mouth, pharynx, and esophagus are food ingestion, food breakdown, and food propulsion.

Food Ingestion and Breakdown

Once food is placed in the mouth, both mechanical and chemical digestion begin.

  • Physical breakdown. First, the food is physically broken down into smaller particles by chewing.
  • Chemical breakdown. Then, as the food is mixed with saliva, salivary amylase begins the chemical digestion of starch, breaking it down into maltose.
  • Stimulation of saliva. When food enters the mouth, much larger amounts of saliva pour out; however, the simple pressure of anything put into the mouth and chewed will also stimulate the release of saliva.
  • Passageways. The pharynx and the esophagus have no digestive function; they simply provide passageways to carry food to the next processing site, the stomach.

Food Propulsion – Swallowing and Peristalsis

For food to be sent on its way to the mouth, it must first be swallowed.

  • Deglutition. Deglutition, or swallowing, is a complex process that involves the coordinated activity of several structures (tongue, soft palate, pharynx, and esophagus).
  • Buccal phase of deglutition. The first phase, the voluntary buccal phase, occurs in the mouth; once the food has been chewed and well mixed with saliva, the bolus (food mass) is forced into the pharynx by the tongue.
  • Pharyngeal-esophageal phase. The second phase, the involuntary pharyngeal-esophageal phase, transports food through the pharynx and esophagus; the parasympathetic division of the autonomic nervous system controls this phase and promotes the mobility of the digestive organs from this point on.
  • Food routes. All routes that the food may take, except the desired route distal into the digestive tract, are blocked off; the tongue blocks off the mouth; the soft palate closes off the nasal passages; the larynx rises so that its opening is covered by the flaplike epiglottis.
  • Stomach entrance. Once food reaches the distal end of the esophagus, it presses against the cardioesophageal sphincter, causing it to open, and food enters the stomach.

Activities of the Stomach

The activities of the stomach involve food breakdown and food propulsion.

Food Breakdown

The sight, smell, and taste of food stimulate parasympathetic nervous system reflexes, which increase the secretion of gastric juice by the stomach glands

  • Gastric juice. Secretion of gastric juice is regulated by both neural and hormonal factors.
  • Gastrin. The presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin, which prods the stomach glands to produce still more of the protein-digesting enzymes (pepsinogen), mucus, and hydrochloric acid.
  • Pepsinogen. The extremely acidic environment that hydrochloric acid provides is necessary, because it activates pepsinogen to pepsin, the active protein-digesting enzyme.
  • Rennin. Rennin, the second protein-digesting enzyme produced by the stomach, works primarily on milk protein and converts it to a substance that looks like sour milk.
  • Food entry. As food enters and fills the stomach, its wall begins to stretch (at the same time as the gastric juices are being secreted).
  • Stomach wall activation. Then the three muscle layers of the stomach wall become active; they compress and pummel the food, breaking it apart physically, all the while continuously mixing the food with the enzyme-containing gastric juice so that the semifluid chyme is formed.

Food Propulsion

Peristalsis is responsible for the movement of food towards the digestive site until the intestines.

  • Peristalsis. Once the food has been well mixed, a rippling peristalsis begins in the upper half of the stomach, and the contractions increase in force as the food approaches the pyloric valve.
  • Pyloric passage. The pylorus of the stomach, which holds about 30 ml of chyme, acts like a meter that allows only liquids and very small particles to pass through the pyloric sphincter; and because the pyloric sphincter barely opens, each contraction of the stomach muscle squirts 3 ml or less of chyme into the small intestine.
  • Enterogastric reflex. When the duodenum is filled with chyme and its wall is stretched, a nervous reflex, the enterogastric reflex, occurs; this reflex “puts the brakes on” gastric activity and slows the emptying of the stomach by inhibiting the vagus nerves and tightening the pyloric sphincter, thus allowing time for intestinal processing to catch up.

Activities of the Small Intestine

The activities of the small intestine are food breakdown and absorption and food propulsion.

Food Breakdown and Absorption

Food reaching the small intestine is only partially digested.

  • Digestion. Food reaching the small intestine is only partially digested; carbohydrate and protein digestion has begun, but virtually no fats have been digested up to this point.
  • Brush border enzymes. The microvilli of small intestine cells bears a few important enzymes, the so-called brush border enzymes, that break down double sugars into simple sugars and complete protein digestion.
  • Pancreatic juice. Foods entering the small intestine are literally deluged with enzyme-rich pancreatic juice ducted in from the pancreas, as well as bile from the liver; pancreatic juice contains enzymes that, along with brush border enzymes, complete the digestion of starch, carry out about half of the protein digestion, and are totally responsible for fat digestion and digestion of nucleic acids.
  • Chyme stimulation. When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones; two of these are secretin and cholecystokinin which influence the release of pancreatic juice and bile.
  • Absorption. Absorption of water and of the end products of digestion occurs all along the length of the small intestine; most substances are absorbed through the intestinal cell plasma membranes by the process of active transport.
  • Diffusion.  Lipids or fats are absorbed passively by the process of diffusion.
  • Debris. At the end of the ileum, all that remains are some water, indigestible food materials, and large amounts of bacteria; this debris enters the large intestine through the ileocecal valve.

Food Propulsion

Peristalsis is the major means of propelling food through the digestive tract.

  • Peristalsis. The net effect is that the food is moved through the small intestine in much the same way that toothpaste is squeezed from the tube.
  • Constrictions. Rhythmic segmental movements produce local constrictions of the intestine that mix the chyme with the digestive juices, and help to propel food through the intestine.

Activities of the Large Intestine

The activities of the large intestine are food breakdown and absorption and defecation.

Food Breakdown and Absorption

What is finally delivered to the large intestine contains few nutrients, but that residue still has 12 to 24 hours more to spend there.

  • Metabolism. The “resident” bacteria that live in its lumen metabolize some of the remaining nutrients, releasing gases (methane and hydrogen sulfide) that contribute to the odor of feces.
  • Flatus. About 50 ml of gas (flatus) is produced each day, much more when certain carbohydrate-rich foods are eaten.
  • Absorption. Absorption by the large intestine is limited to the absorption of vitamin K, some B vitamins, some ions, and most of the remaining water.
  • Feces. Feces, the more or less solid product delivered to the rectum, contains undigested food residues, mucus, millions of bacteria, and just enough water to allow their smooth passage.

Propulsion of the Residue and Defecation

When presented with residue, the colon becomes mobile, but its contractions are sluggish or short-lived.

  • Haustral contractions. The movements most seen in the colon are haustral contractions, slow segmenting movements lasting about one minute that occur every 30 minutes or so.
  • Propulsion. As the haustrum fills with food residue, the distension stimulates its muscle to contract, which propels the luminal contents into the next haustrum.
  • Mass movements. Mass movements are long, slow-moving, but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum.
  • Rectum. The rectum is generally empty, but when feces are forced into it by mass movements and its wall is stretched, the defecation reflex is initiated.
  • Defecation reflex. The defecation reflex is a spinal (sacral region) reflex that causes the walls of the sigmoid colon and the rectum to contract and anal sphincters to relax.
  • Impulses. As the feces is forced into the anal canal, messages reach the brain giving us time to make a decision as to whether the external voluntary sphincter should remain open or be constricted to stop passage of feces.
  • Relaxation. Within a few seconds, the reflex contractions end and rectal walls relax; with the next mass movement, the defecation reflex is initiated again.

Practice Quiz: Digestive System Anatomy and Physiology

Here’s a 10-item quiz about the study guide. Please visit our nursing test bank page for more NCLEX practice questions.

1. All of these are among the four tunics found throughout the digestive tract EXCEPT:

A. Mucosa
B. Glandulosa
C. Submucosa
D. Muscularis

1. Answer: B. Glandulosa

  • B: The fourth, or outermost, layer of the digestive tract is either a serosa or an adventitia.
  • A: The innermost tunic, the mucosa, consists of mucous epithelium, a loose connective tissue called the lamina propria, and a thin smooth muscle layer, the muscularis mucosa.
  • C: The submucosa lies just outside the mucosa. It is a thick layer of loose connective tissue containing nerves, blood vessels, and small glands. An extensive network of nerve cell processes from a plexus.
  • D: The next tunic is the muscularis, which in most parts of the digestive tube consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle.

2. Which of the following are functions of the tongue? Select all that apply.

A. Crucial organ for speech
B. Primary organ for taste
C. Vital for swallowing food
D. Manipulates food for mastication
E. All of these are functions of the tongue

2. Answer: E. All of these are functions of the tongue

  • E: The tongue is a large, muscular organ that occupies most of the oral cavity. It moves food in the mouth and, in cooperation with the lips and cheeks, holds the food in place during mastication. It also plays a major role in the process of swallowing. The tongue is a major sensory organ for taste, as well as being one of the major organs of speech.

3. Each quadrant of the adult mouth holds ___ incisors, ___ canines, ___ premolars, and ___ molars.

A. 1, 2, 3, 2
B. 1, 2, 2, 3
C. 2, 1, 3, 2
D. 2, 1, 2, 3

3. Answer: D. 2, 1, 2, 3

  • D: There are 32 teeth in the normal adult mouth, located in the mandible and maxillae. The teeth can be divided into quadrants: right upper, left upper, right lower, and left lower. In adults, each quadrant contains one central and one lateral incisor; one canine; first and second premolars; and first, second, and third molars.

4. Which of these is NOT a pair of salivary glands?

A. Parotid
B. Submandibular
C. Submucosal
D. Sublingual

4. Answer: C. Submucosal

  • A: The largest of the salivary glands, the parotid glands, are serous glands located just anterior to each ear. Parotid ducts enter the oral cavity adjacent to the second upper molars.
  • B: The submandibular glands produce more serous than mucous secretions. Each gland can be felt as a soft lump along the inferior border of the mandible. The submandibular ducts open into the oral cavity on each side of the frenulum of the tongue.
  • D: The sublingual glands, the smallest of the three paired salivary glands, produce primarily mucous secretions. They lie immediately below the mucous membrane in the floor of the oral cavity.

5. Which statement about the esophagus is TRUE? Select all that apply.

A. It is a cartilaginous tube.
B. It extends from the nasal cavity to the stomach.
C. It lies anterior to the trachea.
D. It has upper and lower sphincters.
E. All statements describe the esophagus.

5. Answer: D. It has upper and lower sphincters.

  • D: Upper and lower esophageal sphincters, located at the upper and lower ends of the esophagus, respectively, regulate the movement of food into and out of the esophagus.
  • A: The esophagus is a muscular tube, lined with moist stratified squamous epithelium.
  • B: It extends from the pharynx to the stomach. It is about 25 centimeters (cm) long.
  • C: It lies anterior to the vertebrae and posterior to the trachea within the mediastinum.

6. All of these structures are matched with the correct description EXCEPT:

A. Cardiac region: the area closest to the small intestine
B. Fundus: the most superior portion of the stomach
C. Pyloric opening: opening from the stomach into the small intestine
D. Body: the largest portion of the stomach

6. Answer: A. Cardiac region: the area closest to the small intestine

  • A: The opening from the esophagus into the stomach is called the cardiac opening because it is near the heart. The region of the stomach around the cardiac opening is called the cardiac region.
  • B: The most superior part of the stomach is the fundus.
  • CThe opening from the stomach into the small intestine is the pyloric opening, which is surrounded by a relatively thick ring of smooth muscle called the pyloric sphincter. The region of the stomach near the pyloric opening is the pyloric region.
  • D: The largest part of the stomach is the body, which turns to the right, forming a greater curvature on the left, and a lesser curvature on the right.

7. The saliva does NOT contain:

A. Amylase
B. Lysozyme
C. Mucin
D. Hydrochloric acid

7. Answer: D. Hydrochloric acid

  • D: Saliva does not contain hydrochloric acid.
  • A: The serous part of saliva, produced mainly by the parotid and submandibular glands, contains a digestive enzyme called salivary amylase.
  • B: Saliva prevents bacterial infection in the mouth by washing the oral cavity, and it contains lysozyme, which has a weak antibacterial action.
  • C: The serous part of saliva dissolves molecules, which must be in solution to stimulate taste receptors. The mucous secretions of the submandibular and sublingual glands contain a large amount of mucin, a proteoglycan that gives a lubricating quality tot he secretions of the salivary glands.

8. Which statements best describe mechanical digestion? Select all that apply.

A. It breaks large food particles into smaller ones.
B. It involves the breaking of covalent chemical bonds in organic molecules by digestive enzymes.
C. It begins in the stomach, where some small, lipid-soluble molecules, such as alcohol and aspirin, can diffuse through the stomach epithelium into the circulation.
D. It requires carrier molecules and includes facilitated diffusion, cotransport, and active transport.
E. All the statements describe mechanical digestion.

8. Answer: A. It breaks large food particles into smaller ones.

  • A: Mechanical digestion breaks large food particles down into smaller ones.
  • B: Chemical digestion involves the breaking of covalent chemical bonds in organic molecules by digestive enzymes.
  • C: Absorption begins in the stomach, where some small, lipid-soluble molecules, such as alcohol and aspirin, can diffuse through the stomach epithelium into the circulation.
  • D: Transport requires carrier molecules and includes facilitated diffusion, cotransport, and active transport.

9. The greatest volume of gastric secretion occurs during:

A. Cephalic phase
B. Gastric phase
C. Intestinal phase
D. Deglutition

9. Answer: B. Gastric phase

  • B: The gastric phase is the period of greatest gastric secretion. This phase is responsible for the greatest volume of gastric secretions, and it is activated by the presence of food in the stomach.
  • A: The cephalic phase of stomach secretion is anticipatory and prepares the stomach to receive food. In the cephalic phase, sensations of taste, the smell of food, stimulation of tactile receptors during the process of chewing and swallowing, and pleasant thoughts of food stimulate centers within the medulla oblongata that influence gastric secretions.
  • C: The intestinal phase of gastric secretion primarily inhibits gastric secretions. It is controlled by the entrance of acidic chyme into the duodenum. The presence of chyme in the duodenum initiates both neural and hormonal mechanisms.
  • D: Deglutition the action or process of swallowing.

10. Which of the following produce gastrin?

A. Mucous neck cells
B. Parietal cells
C. Chief cells
D. Endocrine cells
E. Surface mucous cells

10. Answer: D. Endocrine cells

  • D: Endocrine cells produce gastrin.
  • A: Mucous neck cells are found in the necks of the gastric pits.
  • B: Parietal cells produce HCl.
  • C: Chief cells produce pepsinogen
  • E: Foveolar cells or surface mucous cells are mucus-producing cells which cover the inside of the stomach, protecting it from the corrosive nature of gastric acid.

See Also

Other anatomy and physiology study guides:

Further Reading

  1. Nursing Diagnosis Handbook: An Evidence-Based Guide to Planning Care
  2. Medical-Surgical Nursing: Assessment and Management of Clinical Problems
  3. Medical-Surgical Nursing: Patient-Centered Collaborative Care
  4. Saunders Comprehensive Review for the NCLEX-RN Examination
  5. Brunner & Suddarth’s Textbook of Medical-Surgical Nursing

Digestive Anatomy and Physiology of the Horse

Horses are non-ruminant herbivores, meaning they eat mainly plant material. The horse’s gastrointestinal tract consists of the mouth, esophagus, stomach, small intestine and the highly developed large intestine composed of the caecum, large colon, small colon and rectum (figure 1).

The Mouth

Anatomical features of the mouth include the teeth, tongue and salivary glands. Digestion of feeds begins when food enters the mouth. The horse chews reducing feed particle size and mixing it with saliva to begin the digestive process. Saliva acts as a lubricant to provide easier passage through the esophagus and buffers acid in the stomach. Once swallowed the bolus of feed moves from the esophagus to the stomach.

The Stomach

The stomach of the horse is the smallest unit of the digestive tract with a capacity of approximately 2-4 gallons, comprising around 10% of the total volume of the horse’s digestive tract. The horse has the smallest stomach in relation to body size of all domestic animals. Due to the small capacity, smaller, frequent meals are recommended. The stomach’s main functions include mixing, storage and controlled release of feed into the small intestine; and secretion of pepsin to begin protein digestion. Very little absorption of nutrients occurs in the stomach. Once feed is released from the stomach it enters the small intestine.

Small Intestine

The horse’s small intestine is approximately 70 feet long, comprising 30% of the total digestive system. The passage of feed through the horse’s small intestine is rapid, moving at approximately 1 foot/min and delivering the digesta to the cecum in as little as 45 minutes after a meal. volume of feed consumed and rate of passage affect digestion and absorption of nutrients – larger volume and increased rate of passage will decrease digestion and absorption

In the small intestine a majority of non-structural carbohydrate (starch), protein and fat is digested by enzymes and absorbed. Starch is digested by amylase enzymes, oil is digested by lipase enzymes and protein is digested by protease enzymes. These enzymes, which are produced either in the pancreas or the small intestine, reduce starch into glucose, fats (oil) into glycerol and fatty acids, and protein into amino acids.

The digestion of oils and protein is extensive in the small intestine. The digestion of starch can often be incomplete due to the starch present in cereal grains being protected by the grains seed coat. If starch is not digested in the small intestine it will be delivered to the hindgut where it will be rapidly fermented by bacteria, causing lactic acid production and accumulation, hindgut acidosis and diseases such as colic, metabolic acidosis and laminitis/founder.


The hindgut of the horse comprises the cecum, large colon, small colon and rectum. The cecum consists of 12-15% of tract capacity and the colon 40-50% of tract capacity. The major functions of the hindgut are the microbial digestion (fermentation) of dietary fiber (structural carbohydrates primarily from forages in the horse’s diet). Important end-products of the fermentation are volatile fatty acids (acetic, propionic and butyric) which can serve as an energy source for horses fed mostly forages such as pasture or hay. Fermentation also produces methane, carbon dioxide and water, as well as most of the B-vitamins and some amino acids. Another function of the hindgut is water reabsorption.

The diet composition affects the makeup of the microbial population. When starch is delivered to the hindgut the starch fermenters (amylolytic bacteria) begin to rapidly ferment the starch, producing large quantities of lactic acid and volatile fatty acids (VFA). Because of the acidic nature of these products of fermentation, the pH in the hindgut begins to fall. A low pH favors pathogenic bacterial which can then contribute to serious diseases such as, laminitis or founder, colic, endotoxemia and metabolic acidosis.

Figure 1. Equine Digestive System

Management Suggestions

  • maintain regular feeding schedule
  • base a feeding program on high quality forage – only feed concentrate to meet nutrient requirements not met by the forage
  • feed small meals, especially concentrates, in small amounts (< 4-5 pounds of concentrate/meal)
  • minimize the NSC levels in the concentrate while assuring adequate supply of energy and other non-calorie nutrients
    • Utilize highly digestible fiber and fat for increased calorie needs in performance horses, lactating mares and growing horses wherever possible
  • make feed changes slowly
    • when changing to a richer forage such as pasture or legume hay allow 7 to 10 days for adjustment of microbes in hindgut
  • follow a regular schedule for dental care and deworm

90,000 General structure of the digestive system – lesson. Biology, grade 9.

Nutrition is a set of processes of intake, digestion, absorption and assimilation by the body of nutrients necessary for the normal functioning of the body.

Water, mineral salts and vitamins in the gastrointestinal tract are absorbed unchanged. Large molecules of proteins, fats and carbohydrates undergo mechanical and chemical processing – digestion (because they themselves cannot pass through the wall of the alimentary canal).

Digestion is the process by which absorbed food is converted into a form suitable for use by the body.

Food is digested as it moves through the digestive system.

The digestive system is a system of organs in which mechanical and chemical processing of food is carried out, the absorption of processed substances and the excretion of undigested and undigested food constituents.

The digestive system is subdivided into digestive tract and digestive glands .

The digestive tract consists of the following sections: oral cavity, pharynx, esophagus, stomach, small intestine, large intestine (the total length of the human digestive tract is 8-10 m).

The wall of the alimentary canal consists of three layers:

  • outer (connective tissue),
  • medium (muscle tissue),
  • internal (mucous). It consists of epithelial tissue containing numerous glands that produce digestive enzymes and mucus necessary for the digestion and movement of food.

The muscle layer of the oral cavity, pharynx and the upper third of the esophagus consists of striated muscles, and the muscle layer of the underlying sections is represented by smooth muscles. Due to the wave-like contractions of the muscles, food moves along the digestive tract.

Digestive glands : three pairs of salivary glands (the largest), the liver and pancreas are located outside the digestive tract.Through special ducts, they secrete juices containing enzymes (secretions) into its cavity. Digestive enzymes secreted by them provide chemical breakdown of food, and some enzymes break down carbohydrates, others – proteins, and others – fats.

Basic functions of the digestive system:

  • mechanical and chemical processing of food;
  • absorption of nutrients into the internal environment of the body;
  • Elimination of undigested food residues from the body.


Pasechnik V.V., Kamensky A.A., Shvetsov G.G. / Ed. V.V. Pasechnik Biology. Grade 8 – M .: Education

Lyubimova Z.V., Marinova K.V. Biology. Man and his health. Grade 8 – M .: Vlados

Lerner G.I. Biology: A complete guide for preparing for the exam: AST, Astrel


Human digestive system. The structure | organs

One of the most significant components of human life is digestion, because it is during this process that the body receives the necessary proteins, fats, carbohydrates, vitamins, minerals and other useful ingredients – a kind of “building blocks” on which all physiological reactions are based.That is why the correct functioning of the human digestive system serves as the basis for full-fledged life support: during the main processes taking place in the gastrointestinal tract, each cell is saturated with nutrients, which are subsequently converted into energy or spent for metabolic needs. In addition, the digestive system is also responsible for the water-electrolyte balance, regulating the rate of fluid intake from food.

How does this complex mechanism work and how does food pass through the gastrointestinal tract, turning from familiar and familiar dishes into millions of molecules, useful and not so useful? The basics of the physiology and anatomy of the digestive system of the body will help you understand the key points of this process, evaluate the importance of each stage of digestion and rethink the principles of proper nutrition, which is the key to health and proper functioning of the gastrointestinal tract.

Digestion is a combination of mechanical, chemical and enzymatic processing of food from the daily diet. The initial stages of this long-term process are represented by mechanical grinding, which greatly facilitates the subsequent digestion of nutrients. It is achieved mainly through the physical impact of the teeth, gums and oral cavity on each absorbed piece. Chemical splitting, in turn, acts more subtly and scrupulously: under the action of enzymes secreted by the glands of the digestive system, finely chewed food is split into its constituent ingredients, gradually breaking down into the initial nutrients – lipids, proteins and carbohydrates.

Each of the digestive departments has its own internal environment, which serves as the basis for the functions assigned to it. The organs of the gastrointestinal tract, together with the auxiliary glands, gradually break down each component of the food, excreting what the body needs, and sending the rest of the absorbed food to the junk. If at any of these stages a failure occurs, organs and systems receive less energy resources and, therefore, cannot fully perform their functions, causing an imbalance of the whole organism.

The digestive system itself is conditionally subdivided into 3 key sections: anterior, middle and posterior. The processes of food digestion begin in the anterior section, represented by the oral cavity, pharynx and esophagus – here large pieces are crushed, softened by the incoming salivary fluid and pushed to the stomach. Chemical processing of food products occurs in the middle section, which includes the stomach, intestines (thick and thin), as well as enzymatic organs – the liver and pancreas.It is in this area of ​​the gastrointestinal tract that the optimal balance of microflora and pH is provided, due to which the main nutrient components are absorbed and residual masses are formed, the so-called ballast, which is subsequently released through the caudal rectum. It is here, in the back of the digestive tract, that the digestive chain ends.

Conventionally, all the functions assigned to the human digestive system can be divided into 4 key categories:

  1. Mechanical.This stage involves grinding the incoming food for further splitting and processing.
  2. Secretory. This function is rather complex and consists in the production of enzymes necessary for digestive processes – gastric and intestinal juices, bile, saliva.
  3. Suction. After the products are broken down into nutrient molecules, the food chain does not end, it is still necessary that they are assimilated in the gastrointestinal tract and be able to perform the functions assigned to them – energy supply, metabolism, various physiological processes, etc.d.
  4. Excretory. Not everything that comes with food is equally beneficial for the body. In the digestive tract, the necessary nutrients are filtered out, and the rest is formed into feces and excreted from the body.

All these functions are performed in stages: first, the food is crushed and softened due to the liquid part of saliva, then it is split into various substances, the useful part of which is absorbed by the body, and the ballast part is removed outside.At the slightest failure at any of the indicated stages, this chain is interrupted, and in this case several outcomes are possible, each of which is associated with certain complications. Either the body receives less nutritional components, suffering from a lack of energy resources, or the unfulfilled functions are compensated for at the expense of other parts of the digestive system, which sooner or later causes even more serious problems. Therefore, it is very important to know how well each organ that is part of the digestive system performs the function assigned to it; not only full-fledged digestion, but also the health of the body as a whole depends on this.

All organs related to the digestive system are most often classified based on their location, highlighting the front, middle and rear sections, which are described above. However, from the point of view of functionality, it is much easier to consider the digestive system as a complex of organs of the gastrointestinal tract, along which food passes the main path from the usual dish to complete breakdown, and the enzymatic system, which is responsible for the release of certain substances that greatly facilitate the movement and breakdown of food masses.Let’s take a closer look at each organ in this chain in order to visually assess its importance in the most complex mechanism of food digestion.

1. Oral cavity

The oral cavity is an opening through which food enters the body directly in the form of ready-made meals of the everyday menu, which is familiar to us. This includes the lips, dentition, tongue and salivary glands, which greatly facilitate the mechanical grinding process.The lips are the closing link and hold food in the oral cavity, the teeth cope with crushing larger and harder pieces, the tongue and gums grind small soft pieces, forming a food lump that is moistened with saliva and therefore easily passes to the distant parts of the digestive tract.

The main function of mechanical grinding is performed by the dentition. In 99.8% of newborn babies, teeth are missing, so they can only eat special homogenized food.However, by six months, as a rule, babies have one or even several milk teeth, which is a signal for the introduction of complementary foods – the child can already perceive other products, in addition to breast milk or adapted infant formula. As the number of teeth increases, the menu becomes more varied, and by the age of 10–12, when all milk teeth are replaced by permanent ones, a child can grind and digest food on a par with an adult.

However, not only the mechanical process of grinding food takes place in the oral cavity: other, much more significant functions are performed here.The papillae located on the tongue allow you to assess the temperature, taste and quality of food, preventing possible poisoning from spoiled foods, thermal burns and damage to the mucous membrane. And the salivary glands secrete not only the liquid part of saliva, which softens the food lump, but also enzymes, under the influence of which the primary breakdown of foods and their preparation for further digestion occurs.


The pharynx is a funnel-shaped digestive tube that connects the mouth and the esophagus itself. Its only function is the swallowing process, which occurs reflexively. Its length is about 10 cm, which are divided approximately equally between the mouth, nasopharynx and larynx. It is here that the respiratory and digestive systems intersect, separated by the epiglottis, which normally prevents food from entering the lungs.However, with insufficient work or spontaneous swallowing, this protective process is disrupted, as a result of which asphyxia may appear.

3. Esophagus

The anterior part of the gastrointestinal tract ends with a hollow tube about 25 cm long, the upper part of which is formed mainly by striated muscle fibers, and the lower one is smooth. Due to this alternation, a wave-like contraction and relaxation occurs in the esophagus, which gradually moves the crushed and prepared food for digestion into the stomach cavity.This process is the only significant function of the esophagus; no other physical, chemical or metabolic processes take place here.

4. Stomach

The stomach looks like a hollow muscular organ located in the left hypochondrium. It is an enlargement of the esophagus with highly developed muscular walls, which contract perfectly, facilitating the digestion of food. Due to the coordinated work of muscle fibers, the shape and size of the stomach can change depending on dietary habits and a particular phase of the digestive chain.For example, the empty stomach of an average adult has a volume of no more than one and a half liters, but after eating it can easily increase to 3 or even 4 liters, that is, more than 2 times.

The same applies to people who are prone to frequent overeating: regular consumption of large portions leads to overstretching of muscle fibers, due to which the walls of the stomach become flabby, and the total volume increases.This, in turn, causes a disturbance in eating habits and contributes to the accumulation of excess weight. Therefore, all nutritionists, without exception, recommend eating often, but in fractional portions: such a diet is more physiological.

During swallowing, the muscles that form the walls of the stomach relax, allowing the lump of food, or, as it is called in dietetics, chyme, to pass inside. This happens until the meal is over (or the stomach is full), after which the walls contract again – this is how the metabolic process begins.Under the pressure of peristalsis, the chyme is mixed, frayed and loosened, being exposed to gastric juice. The acidic component of the internal environment of the stomach is produced in the folds of the mucous membrane, where special secretory glands are located. Food is gradually saturated with this secret, crushed, becomes softer and friable, which contributes to its speedy decomposition into molecules.

Then special enzymes of gastric juice – proteases begin the process of breaking down protein structures.However, the process does not end with this, in the stomach proteins are only prepared for complete decomposition, breaking down into complex multicomponent substances. In addition, the cleavage of emulsified lipids into glycerols and fatty acids occurs here, and the metabolism of starches is completed.

The composition and concentration of gastric juice directly depends on a person’s dietary habits. So, the largest amount is synthesized in response to protein foods, and the smallest – to fatty foods. That is why lipids are much more difficult to break down and often lead to excess weight than other substances that make up the diet.

5. Small intestine

The small intestine is the longest part of the human digestive system. Its total length can reach 5–6 meters, which fit into the abdominal cavity only due to its well-thought-out loop-like arrangement. The following areas are distinguished in the small intestine:

  • Duodenum 12 (about 30 cm),
  • jejunum (about 2.5 meters),
  • iliac (2.5-3.5 m).

From the pylorus to the large intestine, the lumen of the small intestine is constantly narrowing. Peristaltic contraction gradually advances the chyme, continuing to break it down into nutrient molecules. Here the food lump is mixed several more times, softened and gradually absorbed by the cells of the mucous membrane.

The inner side of the small intestine has many circular folds, inside which are hidden numerous villi.Due to this, the total area of ​​the mucous membrane increases several times, which means that the absorption capacity of the intestine also increases. Each villi has its own network of lymphatic and blood capillaries, through the thin walls of which molecules of proteins, fats and lipids seep into the blood, spreading throughout the body and forming an energy depot. This allows you to get the maximum of nutrients from the absorbed food.

6. Large intestine

The large intestine ends the digestive chain.The total length of this intestine is about one and a half meters, from which at the very beginning a small blind process – the appendix – departs. A very small organ is a kind of sac, which in some cases can become inflamed and cause an acute condition requiring immediate surgical intervention.

Under the influence of the mucus of the large intestine, certain vitamins, glucose, amino acids synthesized by flora microorganisms are absorbed. In addition, most of the fluids and electrolytes needed to maintain water balance in the cells of the body are absorbed here.

The final part of the intestine is the rectum, which ends in the anus, through which the body leaves the unnecessary substances formed into feces. If the entire digestive process is not disturbed, in total it takes about 3 days, of which 3–3.5 hours are spent for the delivery of the chyme to the large intestine, another 24 hours for its filling and a maximum of 48 for emptying.

1. Salivary glands

The salivary glands are located in the mouth and are responsible for the synthesis of an enzymatic fluid that moistens food and prepares it for breakdown.This organ is represented by several pairs of larger glands (parotid, sublingual, submandibular), as well as numerous small glands. Normally, human saliva contains a watery and slimy secretion, as well as enzymes that provide the initial chemical breakdown of the products that make up the food consumed.

Normally, the following enzymes are present in salivary fluid:

  • amylase breaks down starches to disaccharides,
  • maltase completes this process by converting disaccharides into glucose molecules.

The concentration of these enzymes is usually very high, since food is in the mouth for an average of 18–23 seconds before being swallowed. However, this time is not always enough, therefore gastroenterologists recommend thoroughly and for a long time to chew each piece, then the starches will have time to completely break down, and the food itself will become softer and more homogeneous.

2. Pancreas

The pancreas is another auxiliary enzymatic organ that synthesizes the substances necessary for the complete digestion of nutrients.Pancreatic juice is produced in its cells, which contains all the necessary chemical compounds for the preparation and subsequent breakdown of lipids, proteins and carbohydrates. In addition, the pancreatic juice contains a pancreatic substance produced by duct cells. Due to the bicarbonate ions, this liquid neutralizes the acidic component of the residual products of digestion, thereby preventing irritation and damage to the mucous membranes.


Due to its multifunctionality, the liver belongs to several body systems at once, one of which is the digestive system. In the liver cells, the transformation of amino acids, free fatty acids, lactic acid and glycerol into glucose takes place, which serves as an energy reserve for the human body. In addition, the liver plays a key role in neutralizing toxic compounds that have entered the digestive system. Such a protective reaction prevents the severe consequences of food poisoning and cleanses the gastrointestinal tract of harmful components that have entered the body.

4. Gall bladder

Anatomically, the gallbladder is an appendage of the liver, in which a supply of bile accumulates in case of an urgent need for the body. When a large amount of food is ingested, especially harmful (fatty, fried, smoked, etc.), the accumulated bile is thrown into the lumen of the small intestine to support and accelerate metabolic processes. However, such a mechanism is far from always necessary, therefore, the intake of bile is clearly dosed using valves and bile ducts and increases only if food that is heavy for splitting gets into the digestive tract.


Human digestion is a complex and filigree mechanism, the quality of which directly depends on the correct functioning of each organ, each cell that forms this system. Such a balance is possible only in the case of a careful and delicate attitude to one’s own digestive tract. Do not overload it with exorbitant portions, fatty, heavy and fried foods, meat products that pollute the body and do nothing but harm, and then you will not be bothered by metabolic problems, and the body will always be provided with enough energy without the risk of deficiency, or vice versa , excess fat accumulation and excess weight.Take care of the correct diet today, and tomorrow you will not have to go to a gastroenterologist and waste time on expensive and sometimes ineffective treatment of the digestive system!

Lesson 1. Organs and processes of digestion

Eating is a process for the sake of which each person leaves all his affairs and worries several times a day, because nutrition supplies his body with energy, strength and all the substances necessary for normal life. It is also important that food provides it with material for plastic processes, so that body tissues can grow and repair, and destroyed cells are replaced with new ones.After all that was needed from food, the body has received, it turns into waste, which is excreted from the body in a natural way.

The well-coordinated work of such a complex mechanism is possible thanks to the digestive system, which carries out the digestion of food (physical and chemical processing), the absorption of cleavage products (they are absorbed into the lymph and blood through the mucous membrane) and the elimination of undigested residues.

Thus, the digestive system performs several important functions:

  • Motor-mechanical (food is crushed, moved and excreted)
  • Secretory (enzymes, digestive juices, saliva and bile are produced)
  • Suction (absorbed proteins, fats, carbohydrates, vitamins, minerals and water)
  • Excretory (undigested food residues, excess of a number of ions, salts of heavy metals are removed)

Next, we will talk in detail about how the digestion process takes place, and also talk in detail about each of the organs of the digestive system.But as an introduction, let us briefly touch on the issue of their development.

A little about the development of the digestive system

The digestive system begins to form in the early stages of development of the human embryo. After 7-8 days of development of the fertilized egg, the primary intestine is formed from the endoderm (internal germ layer). On the 12th day, it is divided into two parts: the yolk sac (extra-embryonic part) and the future digestive tract – the gastrointestinal tract (intra-embryonic part).

Initially, the primary intestine is not connected to the oropharyngeal and cloacal membranes. The first melts after 3 weeks of intrauterine development, and the second after 3 months. If, for some reason, the process of membrane melting is disrupted, anomalies appear in the development.

After 4 weeks of embryo development, sections of the digestive tract begin to form:

  • Pharynx, esophagus, stomach, duodenal segment (liver and pancreas begin to form) – derivatives of the anterior intestine
  • Distal, jejunum and ileum – midgut derivatives
  • Colon divisions – hindgut derivatives

The basis of the pancreas is the outgrowths of the anterior intestine.Simultaneously with the glandular parenchyma, pancreatic islets are formed, consisting of epithelial strands. 8 weeks later, the hormone glucagon is determined by the immunochemical method in the alpha cells, and the hormone insulin is determined in the beta cells at the 12th week. Between the 18th and 20th weeks of gestation (pregnancy, the duration of which is determined by the number of full weeks of gestation that have passed from the 1st day of the last menstruation to the moment the newborn’s umbilical cord is cut), the activity of alpha and beta cells increases.

After the baby is born, the gastrointestinal tract continues to grow and develop. The formation of the gastrointestinal tract ends by about three years of age.

The digestive organs and their functions

Simultaneously with the study of the digestive organs and their functions, we will analyze the path taken by food from the moment it enters the oral cavity.

The main function of converting food into substances necessary for the human body, as it has already become clear, is performed by the gastrointestinal tract.It is not just called the tract for nothing, because is a food path, thought out by nature, and its length is about 8 meters! The gastrointestinal tract is filled with all kinds of “adjusting devices” with the help of which food, making stops, gradually goes its way.

Oral cavity

The beginning of the digestive tract is the oral cavity, in which solid food is moistened with saliva and ground with teeth. Saliva is secreted into it by three pairs of large and many small glands.In the process of eating, the secretion of saliva increases many times over. In general, in 24 hours, the glands secrete about 1 liter of saliva.

Saliva is required to wet food lumps so that they can move further more easily, and also supplies an important enzyme – amylase or ptyalin, with the help of which carbohydrates begin to break down already in the oral cavity. In addition, saliva removes from the cavity any substances that irritate the mucous membrane (they enter the cavity by accident, and are not food).

Lumps of food, chewed by teeth and moistened with saliva, when a person makes swallowing movements, pass through the mouth into the pharynx, bypass it and then go into the esophagus.


The esophagus can be characterized as a narrow (about 2-2.5 cm in diameter and about 25 cm long) vertical tube that connects the pharynx and stomach. Despite the fact that the esophagus is not actively involved in food processing, its structure is similar to the structure of the lower parts of the digestive system – the stomach and intestines: each of these organs has walls consisting of three layers.

What are these layers:

  • The inner layer is formed by the mucous membrane. It contains different glands, which differ in their characteristics in all parts of the gastrointestinal tract. Digestive juices are secreted from the glands, thanks to which food products can be broken down. Also, mucus is secreted from them, which is necessary to protect the inner surface of the alimentary canal from the effects of spicy, rough and other irritating food.
  • The middle layer lies under the mucous membrane.It is a muscular layer composed of longitudinal and circular muscles. The contractions of these muscles allow you to tightly grasp the food lumps, and then, using wave-like movements (these movements are called peristalsis), push them further. Note that the muscles of the alimentary canal are muscles of a group of smooth muscles, and their contraction occurs involuntarily, in contrast to the muscles of the limbs, trunk and face. For this reason, a person cannot relax or contract them at will. Only the rectum with striated rather than smooth muscle can be deliberately contracted.
  • The outer layer is called the serous membrane. It has a shiny and smooth surface, and is mainly composed of dense connective tissue. A wide connective tissue plate called the mesentery originates from the outer layer of the stomach and intestines along its entire length. With the help of it, the digestive organs are connected to the back wall of the abdominal cavity. The mesentery contains lymphatic and blood vessels – they supply lymph and blood to the digestive organs and nerves, which are responsible for their movement and secretion.

These are the main characteristics of the three layers of the walls of the digestive tract. Of course, each department has its own differences, but the general principle is the same for everyone, from the esophagus to the rectum.

After passing through the esophagus, which takes about 6 seconds, the food enters the stomach.


The stomach is a so-called sac, which has an elongated shape and is obliquely located in the upper abdominal region. The main part of the stomach is to the left of the central section of the trunk.It begins at the left dome of the diaphragm (the muscular septum that separates the abdominal and chest cavities). The entrance to the stomach is where it connects to the esophagus. Just like the exit (gatekeeper), it is distinguished by circular obturator muscles – pulp. Thanks to the contractions, the pulp separates the gastric cavity from the duodenum, which is located behind it, as well as from the esophagus.

Figuratively speaking, the stomach “knows” that food will soon enter it. And he begins to prepare for her new reception even before the moment when the food gets into his mouth.Remember for yourself the moment when you see some delicious food, and your mouth water starts to flow. Together with these “saliva” that occurs in the oral cavity, digestive juice begins to be secreted in the stomach (this is what happens before a person begins to eat directly). By the way, this juice was named by Academician I.P. Pavlov as “hot” or appetizing juice, and the scientist assigned it a big role in the process of subsequent digestion. Appetizing juice serves as a catalyst for more complex chemical processes that are mainly involved in the digestion of food that has entered the stomach.

Note that if the appearance of the food does not evoke appetizing juice, if the eater is absolutely indifferent to the food in front of him, this can create certain obstacles to successful digestion, which means that the food will enter the stomach, which is not sufficiently prepared for its digestion. That is why it is customary to attach such great importance to a beautiful table setting and appetizing appearance of dishes. Be aware that in the central nervous system (CNS) of a person, conditioned reflex connections are formed between the smell and the type of food and the work of the gastric glands.These connections contribute to the determination of a person’s attitude to food at a distance, i.e. in some cases, he experiences pleasure, and in others – no feelings or even disgust.

It will not be superfluous to note one more aspect of this conditioned reflex process: in the case when the ignition juice has already been caused for some reason, i.e. if the “drooling” has already “flowed”, it is not recommended to postpone the meal. Otherwise, the connection between the activity of the gastrointestinal tract areas is disrupted, and the stomach starts to work “idle”.If these disorders are frequent, the likelihood of certain ailments, such as stomach ulcers or catarrh, increases.

When food is in the mouth, the intensity of secretion of the glands of the gastric mucosa increases; inborn reflexes come into force in the work of the above-named glands. The reflex is transmitted along the sensitive endings of the taste nerves of the pharynx and tongue to the medulla oblongata, and then goes to the nerve plexuses embedded in the layers of the walls of the stomach.Interestingly, digestive juices are released only when only edible products enter the oral cavity.

It turns out that by the time the food chopped up and moistened with saliva is in the stomach, it is already absolutely ready for work, representing like a machine for digesting food. Lumps of food, getting into the stomach and automatically irritating its walls with the chemical elements present in them, contribute to an even more active release of digestive juices that affect individual food elements.

The digestive juice of the stomach contains hydrochloric acid and pepsin, a special enzyme. Together they break down proteins into albumoses and peptones. The juice also contains chymosin, a rennet that curdles dairy products, and lipase, an enzyme necessary for the initial breakdown of fats. Among other things, mucus is secreted from some glands, which protects the inner walls of the stomach from overly irritating food. A similar protective function is performed by hydrochloric acid, which helps to digest proteins – it neutralizes toxic substances that enter the stomach with food.

Almost no food breakdown products get into the blood vessels from the stomach. For the most part, alcohol and substances containing alcohol, for example, dissolved in alcohol, are absorbed in the stomach.

“Metamorphoses” of food in the stomach are so great that in cases where digestion is disturbed for some reason, all parts of the gastrointestinal tract are affected. Based on this, you must always adhere to the correct diet. This can be called the main condition for protecting the stomach from any kind of disturbance.


The food is in the stomach for about 4-5 hours, after which it is redirected to another part of the gastrointestinal tract – the duodenum. It passes into it in small parts and gradually.

As soon as a new portion of food has entered the intestine, the muscle pulp of the pylorus contraction occurs, and the next portion does not leave the stomach, until the hydrochloric acid, which is in the duodenum together with the already received lump of food, is neutralized by alkalis contained in the intestinal juices.

The duodenum was named by ancient scientists, the reason for which was its length – somewhere 26-30 cm, which can be compared with the width of 12 fingers located next to each other. In shape, this intestine resembles a horseshoe, and the pancreas is located in its bend.


Digestive juice is secreted from the pancreas and flows into the duodenal cavity through a separate canal. Also, bile gets here, which is produced by the liver.Together with the enzyme lipase (it is contained in the juice of the pancreas), bile breaks down fats.

There is also the enzyme trypsin in the juice of the pancreas – it helps the body to digest proteins, as well as the enzyme amylase – it promotes the breakdown of carbohydrates to the intermediate stage of disaccharides. As a result, the duodenum serves as a place where all the organic components of food (proteins, fats and carbohydrates) are actively affected by a variety of enzymes.

Being transformed in the duodenum into food gruel (called chyme), food continues on its way and enters the small intestine.The presented segment of the gastrointestinal tract is the longest – about 6 meters in length and 2-3 cm in diameter. Enzymes finally break down complex substances on this path into simpler organic elements. And already these elements become the beginning of a new process – they are absorbed into the blood and lymphatic vessels of the mesentery.

Small intestine

In the small intestine, the food taken by a person is finally transformed into substances that are absorbed into the lymph and blood, and then used by the cells of the body for their own purposes.The small intestine has loops in continuous motion. Such peristalsis provides complete mixing and movement of food masses to the large intestine. This process is quite long: for example, the usual mixed food included in the human diet passes through the small intestine in 6-7 hours.

Even if you look closely at the mucous membrane of the small intestine even without a microscope, you can observe small hairs over its entire surface – villi with a height of approximately 1 mm.One square millimeter of mucosa is present with 20-40 villi.

When food passes through the small intestines, the villi are constantly (and each of the villi has its own rhythm) contracted by about ½ of its size, and then pulled up again. Thanks to the combination of these movements, a suction effect appears – it is this that allows the split food products to pass from the intestines into the blood.

A large number of villi contribute to an increase in the absorption surface of the small intestine.Its area is 4-4.5 sq. m (and this is almost 2.5 times the outer surface of the body!).

But not all substances are absorbed in the small intestine. The remains are sent to the large intestine about 1 m long and about 5-6 cm in diameter. The large intestine is separated from the small intestine by a valve – the Bauginium valve, which from time to time passes parts of the chyme to the initial segment of the large intestine. The large intestine is called the cecum. On its lower surface there is a process that resembles a worm – this is the well-known appendix.

Large intestine

The large intestine is characterized by a U-shape and raised upper corners. It consists of several segments, including the blind, ascending, transverse colon, descending and sigmoid colon (the latter is curved like the Greek letter sigma).

The large intestine is home to many bacteria that produce fermentation processes. These processes help break down the fiber, abundant in plant foods.And along with its absorption, water is also absorbed, which enters the large intestine with chyme. Feces immediately begin to form.

The large intestines are not as active as the small ones. For this reason, chyme stays in them much longer – up to 12 hours. During this time, food goes through the final stages of digestion and dehydration.

The entire volume of food (as well as water) ingested into the body undergoes a lot of all kinds of changes. As a result, it significantly decreases in the large intestine, and from a few kilograms of food remains from 150 to 350 grams.These remnants are subject to defecation, which occurs due to the contraction of the striated muscles of the rectum, muscles of the abdominal and perineum. The process of defecation completes the path of food passing through the digestive tract.

A healthy body spends 21 to 23 hours to completely digest food. If any deviations are noticed, in no case should they be ignored, because they indicate that there are problems in some parts of the alimentary canal or even in certain organs.In case of any violation, it is necessary to consult a specialist – this will not allow the onset of the disease to become chronic and lead to complications.

Speaking about the digestive organs, it should be said not only about the main, but also about the auxiliary organs. We have already talked about one of them (this is the pancreas), so it remains to mention the liver and gallbladder.


The liver belongs to the vital unpaired organs. It is located in the abdominal cavity under the right dome of the diaphragm and performs a wide variety of physiological functions.

Hepatic tracts are formed from liver cells, receiving blood from the artery and portal vein. From the beams, blood flows to the inferior vena cava, where the pathways along which bile is diverted into the gallbladder and duodenum begin. And bile, as we already know, takes an active part in digestion, like pancreatic enzymes.


The gallbladder is a sac-shaped reservoir located on the lower surface of the liver where bile produced by the body is collected.The reservoir is characterized by an elongated shape with two ends – wide and narrow. In length, the bubble reaches 8-14 cm, and in width – 3-5 cm. Its volume is approximately 40-70 cubic meters. see

The bladder has a bile duct that connects to the hepatic duct at the hilum of the liver. The fusion of the two ducts forms a common bile duct, which joins with the pancreatic duct and opens into the duodenum through the sphincter of Oddi.

The importance of the gallbladder and bile function should not be underestimated, i.e.because they perform a number of important operations. They are involved in the digestion of fats, create an alkaline environment, activate digestive enzymes, stimulate intestinal motility and remove toxins from the body.

In general, the gastrointestinal tract is a real conveyor for the continuous movement of food. His work is subject to strict sequence. Each stage affects food in a specific way, so that it supplies the body with the energy it needs to function properly.And another important characteristic of the gastrointestinal tract is that it adapts quite easily to different types of food.

However, the gastrointestinal tract is “needed” not only for food processing and removal of unusable food residues. In fact, its functions are much broader, since as a result of metabolism (metabolism), unnecessary products appear in all cells of the body, which must be removed, otherwise their poisons can poison a person.

A large proportion of toxic metabolic products enter the intestines through the blood vessels.There, these substances break down and are excreted along with the feces during bowel movements. It follows from this that the gastrointestinal tract helps the body get rid of many toxic substances that appear in it in the process of life.

The clear and harmonious work of all systems of the alimentary canal is the result of regulation, for which the nervous system is mainly responsible. Some processes, for example, the act of swallowing food, the act of chewing it or the act of defecation, are controlled by a person’s consciousness. But others, such as the secretion of enzymes, the breakdown and absorption of substances, contractions of the intestines and stomach, etc.are carried out on their own, without conscious effort. The autonomic nervous system is responsible for this. In addition, these processes are associated with the central nervous system, and in particular with the cerebral cortex. So any changes in a person’s mental state (joy, fear, stress, excitement, etc.) immediately affect the activity of the digestive system. But this is already a conversation on a slightly different topic. We summarize the first lesson.

In the second lesson, we will talk in detail about what food consists of, tell you why the human body needs certain substances, and also give a table of the content of useful elements in foods.

Test your knowledge

If you want to test your knowledge of the topic of this lesson, you can take a short test consisting of several questions. In each question, only 1 option can be correct. After you have selected one of the options, the system automatically proceeds to the next question. The points you receive are influenced by the correctness of your answers and the time spent on passing. Please note that the questions are different each time, and the options are mixed.

Kirill Nogales