What are the main types of cell signaling in the human body. How does the endocrine system regulate physiological processes. What are the key components and functions of the endocrine system. How do hormones work to maintain homeostasis.

The Endocrine System: An Overview

The endocrine system is a complex network of glands and hormones that play a crucial role in regulating various bodily functions. This intricate system works in harmony with the nervous system to maintain homeostasis and control long-term physiological processes. Unlike the nervous system, which produces rapid, short-term responses, the endocrine system creates slower-acting but longer-lasting changes throughout the body.

At its core, the endocrine system relies on a sophisticated method of communication known as endocrine signaling. This process involves the secretion of hormones directly into the bloodstream, allowing these chemical messengers to reach target cells and organs throughout the body.

Key Components of the Endocrine System

• Endocrine glands
• Hormones
• Target cells with specific receptors
• Feedback mechanisms

Understanding the intricacies of the endocrine system is essential for grasping how our bodies maintain balance and respond to various internal and external stimuli.

Cell Signaling: The Language of Intercellular Communication

Cell signaling is the foundation of communication within the body. There are four main types of cell signaling, each with its unique characteristics and range of influence. These signaling mechanisms allow cells to coordinate their activities and respond to changes in their environment.

Autocrine Signaling: Self-Regulation at the Cellular Level

Autocrine signaling is a process where cells communicate with themselves. In this type of signaling, a cell releases a signaling molecule that binds to receptors on the same cell, triggering internal changes. This self-regulatory mechanism is particularly important in the immune system, where cells use autocrine signaling to modulate their responses to threats.

How does autocrine signaling work in immune cells? Immune cells release cytokines, which are small proteins that act as messengers. These cytokines then bind to receptors on the same cell, fine-tuning its response to pathogens or other stimuli. This self-regulation helps ensure that immune responses are appropriately calibrated.

Juxtacrine Signaling: Communication Between Neighboring Cells

Juxtacrine signaling involves direct communication between adjacent cells. Unlike other forms of signaling, juxtacrine signaling does not involve the release of signaling molecules into the extracellular space. Instead, it relies on direct contact between cells or interactions with the extracellular matrix.

There are three subtypes of juxtacrine signaling:

1. Surface protein interactions between signaling and receiving cells
2. Binding of extracellular matrix proteins to cell receptors
3. Signal transmission through channels connecting the cytoplasm of adjacent cells

An intriguing example of juxtacrine signaling is the interaction between the “bride of sevenless” protein and its receptor, sevenless. This whimsically named protein pair plays a crucial role in cell fate determination during eye development in fruit flies.

Paracrine Signaling: Local Communication in Tissues

Paracrine signaling allows cells to influence their immediate neighbors within a tissue. In this process, cells release paracrine factors into the surrounding area, creating a concentration gradient. The strength of the cellular response depends on the concentration of these factors, which decreases with distance from the source.

How does paracrine signaling function in the nervous system? In synapses, neurotransmitters are released into the synaptic cleft, acting as paracrine factors. These molecules diffuse across the small gap between neurons, binding to receptors on the postsynaptic cell and triggering a response. The concentration gradient ensures that only nearby neurons are affected, allowing for precise control of neural communication.

Endocrine Signaling: Long-Distance Communication via Hormones

Endocrine signaling is the primary mode of communication used by the endocrine system. This type of signaling involves the release of hormones into the bloodstream, allowing for long-distance communication between cells and organs throughout the body.

Hormones are classified into five main categories based on their chemical composition:

• Protein hormones
• Amino acid hormones
• Steroid hormones
• Eicosanoid hormones
• Gas hormones

Each type of hormone has unique properties that influence its mode of action, target specificity, and duration of effect.

The Endocrine System’s Role in Homeostasis

The endocrine system plays a pivotal role in maintaining homeostasis, the body’s ability to maintain a stable internal environment despite external changes. Through the action of hormones, the endocrine system regulates a wide array of physiological processes, ensuring that the body functions optimally under various conditions.

How does the endocrine system maintain homeostasis? The system operates through a series of feedback loops. When a physiological parameter deviates from its set point, hormones are released to bring it back into balance. This can occur through negative feedback, where the hormone’s effects counteract the initial change, or positive feedback, where the hormone amplifies the change to achieve a specific outcome.

Key Physiological Processes Regulated by the Endocrine System

• Metabolism and energy balance
• Growth and development
• Reproduction and sexual function
• Stress response
• Fluid and electrolyte balance
• Blood pressure regulation
• Blood glucose levels

By finely tuning these processes, the endocrine system ensures that the body can adapt to both internal and external changes while maintaining overall health and well-being.

Hormones: The Chemical Messengers of the Endocrine System

Hormones are the primary effectors of the endocrine system, carrying messages throughout the body to coordinate physiological responses. These chemical messengers are produced by endocrine glands and released into the bloodstream, where they travel to target cells equipped with specific receptors.

What makes hormones so effective at long-distance signaling? Hormones are highly specific molecules that can be produced in small quantities yet elicit significant responses in target cells. Their ability to circulate through the bloodstream allows them to reach distant organs and tissues, making them ideal for coordinating complex physiological processes.

The Mechanism of Hormone Action

1. Hormone production and release by endocrine glands
2. Transport through the bloodstream
3. Binding to specific receptors on target cells
4. Activation of intracellular signaling cascades
5. Changes in cellular function or gene expression

The specificity of hormone-receptor interactions ensures that hormones only affect cells that express the appropriate receptors, allowing for precise control of physiological responses.

Endocrine Glands: The Hormone Factories

Endocrine glands are specialized organs that produce and secrete hormones directly into the bloodstream. These glands are distributed throughout the body and work together to regulate various physiological processes.

Major Endocrine Glands and Their Functions

• Hypothalamus: Produces releasing and inhibiting hormones that control the pituitary gland
• Pituitary gland: Often called the “master gland,” it produces hormones that regulate other endocrine glands
• Thyroid gland: Produces thyroid hormones that regulate metabolism
• Parathyroid glands: Secrete parathyroid hormone to control calcium levels
• Adrenal glands: Produce stress hormones and regulate metabolism
• Pancreas: Secretes insulin and glucagon to control blood sugar levels
• Gonads (ovaries and testes): Produce sex hormones for reproduction and sexual characteristics
• Pineal gland: Secretes melatonin to regulate sleep-wake cycles

Each of these glands plays a crucial role in maintaining the body’s overall balance and function. Their coordinated actions ensure that the body can respond appropriately to both internal and external stimuli.

Endocrine Disorders: When the System Malfunctions

While the endocrine system is remarkably robust, various factors can disrupt its delicate balance, leading to endocrine disorders. These conditions can result from either overproduction or underproduction of hormones, or from problems with hormone receptors or signaling pathways.

What are some common endocrine disorders? Some frequently encountered endocrine disorders include diabetes mellitus, thyroid disorders (such as hypothyroidism and hyperthyroidism), adrenal insufficiency, and growth hormone deficiency. These conditions can have wide-ranging effects on the body due to the far-reaching influence of hormones on various physiological processes.

Causes of Endocrine Disorders

• Genetic factors
• Autoimmune diseases
• Tumors (benign or malignant) in endocrine glands
• Infections
• Injury or trauma to endocrine glands
• Environmental factors and toxins
• Aging

Understanding the underlying causes of endocrine disorders is crucial for developing effective diagnostic and treatment strategies. Many endocrine disorders can be managed with hormone replacement therapy or other targeted treatments that aim to restore hormonal balance.

The Endocrine System and Human Development

The endocrine system plays a vital role in human development, from prenatal stages through adolescence and into adulthood. Hormones orchestrate the complex processes of growth, sexual maturation, and metabolic regulation that shape our bodies and influence our behavior throughout life.

How does the endocrine system influence human development? During fetal development, hormones guide the formation of organs and tissues. In childhood, growth hormone and thyroid hormones are crucial for physical growth and cognitive development. During puberty, sex hormones trigger the development of secondary sexual characteristics and reproductive maturity.

Key Developmental Processes Regulated by Hormones

• Fetal growth and organ development
• Childhood growth and metabolism
• Puberty and sexual maturation
• Bone growth and density
• Muscle development
• Brain development and cognitive function
• Reproductive system maturation

The intricate interplay of hormones throughout development ensures that the body grows and matures in a coordinated manner, preparing individuals for the various stages of life.

The Future of Endocrine Research: Emerging Trends and Challenges

As our understanding of the endocrine system continues to evolve, researchers are exploring new frontiers in endocrinology that promise to revolutionize our approach to health and disease. Emerging trends in endocrine research are opening up exciting possibilities for more targeted and personalized treatments for endocrine disorders.

What are some cutting-edge areas of endocrine research? Some of the most promising areas include the study of endocrine disruptors in the environment, the role of the endocrine system in aging, and the development of artificial endocrine organs. Additionally, researchers are investigating the complex interactions between the endocrine system and other bodily systems, such as the immune system and the microbiome.

Emerging Trends in Endocrine Research

• Epigenetics and hormone regulation
• Endocrine-disrupting chemicals and their long-term effects
• Chronobiology and the endocrine system
• Artificial intelligence in endocrine diagnostics
• Gene therapy for endocrine disorders
• Bioengineered hormone-producing cells
• Neuroendocrine interactions and mental health

These emerging areas of research hold the potential to transform our understanding of the endocrine system and its role in health and disease. As we unravel the complexities of hormonal signaling and regulation, we move closer to developing more effective treatments for endocrine disorders and improving overall human health.

Endocrine System: Structure & Functions

The endocrine system is one of the key regulatory systems within the body. It consists of a series of glands that interact with each other and external factors to control organs located far away from them by secreting hormones into the bloodstream. It works in concert with the nervous system, however, instead of short term rapid changes, it creates slower acting, longer-lasting changes in the body.

Types Of Cell Signalling

There are four main types of cell signalling used to convey messages within the body. These vary in the range and type of signalling molecule used. In increasing order of range, the four signalling strategies are:

• autocrine signalling,
• juxtacrine signalling,
• paracrine signalling, and
• endocrine signalling

The type of signalling used in the endocrine system is, as the name suggests, endocrine signalling. A brief overview of each of these is provided below.

Autocrine Signalling

Autocrine signalling is used by cells to effect changes within themselves. The cell releases a signalling molecule, such as a hormone or cytokine. This signalling molecule then binds to receptors on the same cell, triggering changes within that cell. This form of signalling is employed by cells of the immune system, amongst others.

Cytokines are a type of protein that exert an effect on our immune system!

Fig. 1 – A diagram showing the process of autocrine signalling

Juxtacrine Signalling

Juxtacrine signalling is used by cells to effect changes directly in neighbouring cells. In this mode of signalling, nothing is released from the signalling cell. Instead, proteins on the signalling cell interact with receptor proteins on or in the receiving cell.

Fig. 2 – An overview of juxtacrine signalling

There are three sub-types of juxtacrine signalling. In one type, surface proteins on the signaller cell interact with surface proteins on the receiver cell. The second features a protein in the extracellular matrix secreted by one cell binding to the receptor within another. The third has the signalling molecule move through channels connecting the cytoplasm of the two cells. A comically named example of this is the bride of sevenless proteins interaction with its corresponding receptor protein, sevenless.

Paracrine Signalling

Paracrine signalling is used by cells to effect changes within cells in the local area. The signalling cell releases paracrine factors into the surrounding area. These factors then spread into the surrounding area. Diffusion of the paracrine factors creates a gradient, with the concentration lowering the further from the signalling cell you get. The stronger the concentration of paracrine factors a cell is exposed to, the larger its response. This type of signalling is used for example within synapses.

Fig. 3 – An overview of paracrine signalling, showing the diffusion gradient that causes more distant cells to be exposed to lower concentrations of the signal molecule

Paracrine factors are small proteins produced by cells to signal other nearby cells. They trigger different changes when cells are exposed to different concentrations created by diffusion.

Endocrine Signalling

Endocrine signalling is the type of cell signalling used by the endocrine system. Endocrine signalling relies on the use of chemical signalling molecules called hormones.

Hormones are small signalling molecules that bind to receptor proteins expressed by different cells to trigger a response and help regulate physiological activities.

This form of signalling can act on cells throughout the body, outside of the local area, a significant increase in communication range when compared to the above signalling methods. This range is achieved by the gland secreting the hormone into the bloodstream, rather than the immediate extracellular area. The bloodstream then carries the hormone around the body, ready for it to interact with relevant receptors on its target.

Fig. 4 – A diagram showing how endocrine signalling works

Hormones within the human body belong to five classes, determined by the compounds they are formed from. Protein hormones are, as the name suggests, proteins. Amino acid hormones are modified amino acids, steroid hormones are formed from cholesterol, eicosanoids are formed from lipids and gas hormones are gases that exert a biological effect, such as nitric oxide.

The Function Of The Endocrine System

The endocrine system regulates virtually all factors within the body. Anything in which some degree of homeostasis is required is regulated by a part of the endocrine system such as blood glucose levels or metabolic rates. The endocrine system regulates factors within the body directly, through the action of hormones, or indirectly, through knock-on effects of hormone interactions.

Homeostasis is the maintenance of steady-state conditions in a biological system. Check out the article Blood glucose concentration to learn how the endocrine system helps regulate our blood glucose levels!

Because of this wide range of impacts, the endocrine system is involved in many diseases, ranging from metabolic disorders to cancer. The study of the role the endocrine system plays in disease is known as endocrinology. Diseases caused by the endocrine system generally stem from altered levels of hormone release, inappropriately timed hormone release, the gland being missing or damaged, or the gland being overgrown and therefore overactive.

The Nervous System vs the Endocrine System

While the nervous system and the endocrine system both allow signals to be carried throughout the body, this is where their similarity ends. Nerve cells conduct electrical signals around the body. They do use chemical messengers, but only over incredibly short distances at special junctions between cells known as synapses, rather than long-distance via the bloodstream like the endocrine system does. They also generally act over a very short time period, but the signals travel quickly. This is the opposite of the endocrine signal, where due to the requirement for hormones to circulate in the blood, the response is much slower but more long-lasting as it acts until the hormones are broken down.

Read our article The Human Nervous System to learn more about this important regulatory system!

Diagram Of The Endocrine System

The diagram below shows the key glands of the endocrine system including the ovaries, testes, pineal gland, adrenal gland, thyroid, parathyroid, pituitary gland, hypothalamus and pancreas, each of which is discussed in depth after.

Fig. 5 – A diagram showing the key components of the endocrine system

Structure of the Endocrine System

The glands of the endocrine system differ in structure quite significantly from other glands, which make up the exocrine glands. Exocrine glands, such as those found in the digestive tract or the sweat glands, secrete their products outside of the body. In order to do this, they must have a duct to carry the product to the required area. As they don’t secrete their product into the blood, they have few blood vessels. Endocrine glands in contrast generally do not have ducts as their product diffuses into the bloodstream. This need for access to the bloodstream also means they have a lot more blood vessels than exocrine glands.

The Endocrine System’s Glands

There are nine main components that form the endocrine system, these are the ovaries, testes, pineal body, adrenal gland, thyroid, parathyroid, pituitary gland, pancreas, and hypothalamus. A brief overview of each of these is given below.

The Ovaries

The ovaries are the site of egg maturation and release, however, they also possess a significant role in hormonal control, both ovulation and many other systems. Hormones produced by the ovaries include oestrogen, inhibin and progesterone. Dysfunctions in the hormonal secretion of the ovaries can lead to several issues, one of the most common of which is Polycystic Ovary Syndrome.

PCOS or Polycystic Ovary Syndrome is a condition where the ovaries produce an excess of male sex hormones. Its name stems from the characteristic fluid-filled follicles which are overly enlarged in the ovaries.

Testes

The testes are the male equivalent to the ovaries, producing sperm cells instead of eggs. Like the ovaries, as well as producing gametes, they produce the hormone testosterone. Lowered hormone production by the testes, termed hypoandrogenism, leads to an array of symptoms such as genital shrinkage, low muscle mass, and frail bones amongst other symptoms.

Pineal Body

The pineal body, also known as the pineal gland, is a small gland located within the brain. It regulates the circadian rhythm via the production of melatonin, also known as the sleep hormone. Dysregulation of melatonin production can lead to a corresponding dysregulation of the circadian rhythm and reproductive cycles.

Circadian rhythm is a natural internal process which repeats a regular set of changes approximately over 24 hours, which is adjusted by external factors known as zeitgebers, meaning time givers in German. Some examples of these are temperature and daylight exposure.

Adrenal Gland

The adrenal glands are located at the top of each kidney. Like most glands of the endocrine system, they produce an array of hormones including steroid hormones which regulate blood pressure and salt levels and the metabolism of many compounds like androgens. These are usually converted to testosterone and DHT or estrogens, depending on the gender of their owner. The other hormones produced by the adrenal glands, for which they are named, are adrenaline and noradrenaline. These are responsible for the fight or flight response and act to increase blood pressure, heart and breathing rate, and blood sugar levels.

Thyroid

The thyroid is located in front of the windpipe within the neck. It produces several hormones, collectively known as thyroid hormones. Two variants of thyroid hormones are produced, numbered according to the number of iodine within their structure. T3 is the active form, and T4 its prohormone, with these being secreted in a 1:14 ratio. T4 is converted to T3 by deiodinases located throughout the body. Thyroid hormones act to increase metabolism throughout the body, increase respiratory and heart rate, and regulate brain development and function.

One of the most common disorders involving the thyroid is hypothyroidism, which causes decreased levels of thyroid hormone production. This is often caused by Hashimoto’s thyroiditis, a condition where the immune system attacks the thyroid.

Parathyroid

The parathyroid glands are small glands that sit behind the thyroid. They are responsible for the regulation of blood calcium levels through the production of parathyroid hormone. This induces calcium release from the bones, along with making the kidneys increase vitamin D production and calcium ion retention. Vitamin D acts to raise calcium levels by increasing absorption within the gut. Over or underproduction of the parathyroid hormones can lead to disordered calcium levels within the blood.

Pancreas

The pancreas is an organ within the digestive tract. It is responsible for regulating blood sugar levels by producing the hormones glucagon and insulin. Glucagon is released from alpha cells in the pancreas and raises blood sugar. Insulin is produced by beta cells in the pancreas and lowers blood sugar levels. These work together in a pair of negative feedback loops, which maintain a narrow range of blood sugar levels. Disorders of the pancreas most commonly lead to diabetes mellitus.

Diabetes Mellitus is a disorder characterised by abnormally elevated blood glucose levels, triggered by improper production of or response to insulin, the hormone which normally lowers blood sugar levels by removing glucose from the blood and sequestering it in glycogen.

Check out our article Diabetes to learn more about this prevalent disease!

Hypothalamus

The hypothalamus is the control centre of the endocrine system. It takes input from around the body and triggers the release of hormones from the pituitary gland. It does this by producing a specialised kind of hormone known as a neurohormone. These then travel down the hypophyseal system to the pituitary where they control the release of hormones from the pituitary.

Pituitary Gland

The pituitary is a gland that sits at the base of the hypothalamus within the brain. It secretes a wide array of hormones, many of which regulate other glands around the body. Actions that are controlled by a series of glands and hormones are referred to as an axis, such as the hypothalamic-pituitary-adrenal axis, which regulates various systems throughout the body, including digestion, immunity, mood, libido and metabolism. Because of its role in regulating the activity of other glands, the pituitary is often referred to as the master gland. Hormones released by the pituitary include but are not limited to, human growth hormone, thyroid-stimulating hormone, luteinising hormone, follicle-stimulating hormone, anti-diuretic hormone and oxytocin. Due to the large number of systems impacted by the pituitary, many different diseases can arise from pituitary disorders.

Endocrine System – Key takeaways

• The endocrine system uses hormones produced by glands to convey signals to locations far around the body.
• Hormones may be steroids, proteins, lipids, amino acids or gases and circulate through the bloodstream.
• The endocrine system is slower to respond than the nervous system, however, the response stays active much longer.
• Endocrine glands don’t have ducts to carry their product to another area, but they are heavily vascular so they can secrete their products into the blood.
• The main glands of the endocrine system include the ovaries, testes, pineal gland, adrenal gland, thyroid, parathyroid, pituitary gland, hypothalamus and pancreas.

An Overview of the Endocrine System

Learning Objectives

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

• Distinguish the types of intercellular communication, their importance, mechanisms, and effects
• Identify the major organs and tissues of the endocrine system and their location in the body

Neural and Endocrine Signaling

The nervous system uses two types of intercellular communication—electrical and chemical signaling—either by the direct action of an electrical potential, or in the latter case, through the action of chemical neurotransmitters such as serotonin or norepinephrine. Neurotransmitters act locally and rapidly. When an electrical signal in the form of an action potential arrives at the synaptic terminal, they diffuse across the synaptic cleft (the gap between a sending neuron and a receiving neuron or muscle cell). Once the neurotransmitters interact (bind) with receptors on the receiving (post-synaptic) cell, the receptor stimulation is transduced into a response such as continued electrical signaling or modification of cellular response. The target cell responds within milliseconds of receiving the chemical “message”; this response then ceases very quickly once the neural signaling ends. In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition.In contrast, the endocrine system uses just one method of communication: chemical signaling. These signals are sent by the endocrine organs, which secrete chemicals—the hormone—into the extracellular fluid. Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, inducing a characteristic response. As a result, endocrine signaling requires more time than neural signaling to prompt a response in target cells, though the precise amount of time varies with different hormones. For example, the hormones released when you are confronted with a dangerous or frightening situation, called the fight-or-flight response, occur by the release of adrenal hormones—epinephrine and norepinephrine—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones.

Practice Question

Watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine?

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In addition, endocrine signaling is typically less specific than neural signaling. The same hormone may play a role in a variety of different physiological processes depending on the target cells involved. For example, the hormone oxytocin promotes uterine contractions in women in labor. It is also important in breastfeeding, and may be involved in the sexual response and in feelings of emotional attachment in both males and females.

In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction (Table 1). So how does the fight-or-flight response that was mentioned earlier happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary.

The Following Chart compares the Endocrine and Nervous Systems.

Structures of the Endocrine System

The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function. The endocrine gland is the major player in this system. The primary function of these ductless glands is to secrete their hormones directly into the surrounding fluid. The interstitial fluid and the blood vessels then transport the hormones throughout the body. The endocrine system includes the pituitary, thyroid, parathyroid, adrenal, and pineal glands (Figure 1). Some of these glands have both endocrine and non-endocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, female ovaries, and male testes are other organs that contain cells with endocrine function. Moreover, adipose tissue has long been known to produce hormones, and recent research has revealed that even bone tissue has endocrine functions.

Figure 1. Endocrine glands and cells are located throughout the body and play an important role in homeostasis.

The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through ducts. Examples of exocrine glands include the sebaceous and sweat glands of the skin. As just noted, the pancreas also has an exocrine function: most of its cells secrete pancreatic juice through the pancreatic and accessory ducts to the lumen of the small intestine.

Other Types of Chemical Signaling

In endocrine signaling, hormones secreted into the extracellular fluid diffuse into the blood or lymph, and can then travel great distances throughout the body. In contrast, autocrine signaling takes place within the same cell. An autocrine (auto– = “self”) is a chemical that elicits a response in the same cell that secreted it. Interleukin-1, or IL-1, is a signaling molecule that plays an important role in inflammatory response. The cells that secrete IL-1 have receptors on their cell surface that bind these molecules, resulting in autocrine signaling.

Local intercellular communication is the province of the paracrine, also called a paracrine factor, which is a chemical that induces a response in neighboring cells. Although paracrines may enter the bloodstream, their concentration is generally too low to elicit a response from distant tissues. A familiar example to those with asthma is histamine, a paracrine that is released by immune cells in the bronchial tree. Histamine causes the smooth muscle cells of the bronchi to constrict, narrowing the airways. Another example is the neurotransmitters of the nervous system, which act only locally within the synaptic cleft.

Careers Connections: Endocrinologist

Endocrinology is a specialty in the field of medicine that focuses on the treatment of endocrine system disorders. Endocrinologists—medical doctors who specialize in this field—are experts in treating diseases associated with hormonal systems, ranging from thyroid disease to diabetes mellitus. Endocrine surgeons treat endocrine disease through the removal, or resection, of the affected endocrine gland.

Patients who are referred to endocrinologists may have signs and symptoms or blood test results that suggest excessive or impaired functioning of an endocrine gland or endocrine cells. The endocrinologist may order additional blood tests to determine whether the patient’s hormonal levels are abnormal, or they may stimulate or suppress the function of the suspect endocrine gland and then have blood taken for analysis. Treatment varies according to the diagnosis. Some endocrine disorders, such as type 2 diabetes, may respond to lifestyle changes such as modest weight loss, adoption of a healthy diet, and regular physical activity. Other disorders may require medication, such as hormone replacement, and routine monitoring by the endocrinologist. These include disorders of the pituitary gland that can affect growth and disorders of the thyroid gland that can result in a variety of metabolic problems.

Some patients experience health problems as a result of the normal decline in hormones that can accompany aging. These patients can consult with an endocrinologist to weigh the risks and benefits of hormone replacement therapy intended to boost their natural levels of reproductive hormones.

In addition to treating patients, endocrinologists may be involved in research to improve the understanding of endocrine system disorders and develop new treatments for these diseases.

Chapter Review

The endocrine system consists of cells, tissues, and organs that secrete hormones critical to homeostasis. The body coordinates its functions through two major types of communication: neural and endocrine. Neural communication includes both electrical and chemical signaling between neurons and target cells. Endocrine communication involves chemical signaling via the release of hormones into the extracellular fluid. From there, hormones diffuse into the bloodstream and may travel to distant body regions, where they elicit a response in target cells. Endocrine glands are ductless glands that secrete hormones. Many organs of the body with other primary functions—such as the heart, stomach, and kidneys—also have hormone-secreting cells.

Critical Thinking Questions

1. Describe several main differences in the communication methods used by the endocrine system and the nervous system.
2. Compare and contrast endocrine and exocrine glands.
3. True or false: Neurotransmitters are a special class of paracrines. Explain your answer.

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Glossary

autocrine: chemical signal that elicits a response in the same cell that secreted it

endocrine gland: tissue or organ that secretes hormones into the blood and lymph without ducts such that they may be transported to organs distant from the site of secretion

endocrine system: cells, tissues, and organs that secrete hormones as a primary or secondary function and play an integral role in normal bodily processes

exocrine system: cells, tissues, and organs that secrete substances directly to target tissues via glandular ducts

hormone: secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body

paracrine: chemical signal that elicits a response in neighboring cells; also called paracrine factor

Endocrine system – Medical Association “ONA”

The endocrine system ensures the stable operation of the human body in a constantly changing environment, its protection from stress.

The endocrine system includes the following main organs: hypothalamus, pituitary gland, thyroid gland, mammary gland, adrenal glands, pancreas, ovaries (testes in men), gallbladder and appendix. ES organs are glands that provide growth, physiological development, reproductive functions of a person, as well as the course of the most important processes of balancing the activity of the whole organism under continuously changing external influences. Management of all this work and control over the correctness of its implementation is carried out in the body by hormones, which are produced by the glands of the ES. Each hormone is responsible for its own organs and functions, but works in a delicate balance with hormones produced by other glands. Depending on the function being performed at the moment, a different balance of hormones is required. Its failure causes disruption of the corresponding systems, organs, the flow of necessary reactions, which leads to the appearance of abnormalities in the body, and then diseases. Therefore, regular monitoring of the hormonal background is the key to the timely detection of violations and their curability. Another feature of ES should be noted: the interconnection and interdependence of its glands is so great that a violation found in one of the organs almost automatically means the presence of problems in others – after all, there is a hormonal failure.

Endocrine glands (EG) do not have excretory ducts, and the substances they secrete (hormones) enter directly into the blood and lymph. By sending hormones into the blood, EJs create a communication system and control the work of literally every cell in the body. Hormones provide its chemical balance, give the cells an indication of how to act, given their abilities and capabilities. The purpose of the EJ is to ensure the well-being of the whole organism, the balance in the work of internal organs, regardless of changes in the external environment, to protect the body from the destructive effects of stress.

Hypothalamus. Regulates the temperature reaction, establishes the correct ratio between heat release and heat transfer, controls the pituitary gland.

Pituitary gland. Manages the operation of the entire system. Incoming and outgoing messages are coordinated and make the ES work efficiently and harmoniously. The pituitary gland controls the body’s chemical balance by influencing most of the body’s chemical processes (for example, regulating water-salt and fat metabolism).

There is a constant need for the body to adapt to external changes, and this is also part of the task of the pituitary gland. The human body grows, wears out, and the pituitary gland is engaged in ensuring growth and repair. In addition, it provides reproductive function.

The pituitary gland performs its functions with the help of hormones that stimulate other endocrine glands. Those, in turn, secrete hormones that affect the pituitary gland itself and the nervous system. What happens in the body when the production of only one hormone is disrupted can be seen in the example of growth hormone. If it is produced more than the norm, a person becomes a victim of gigantism, if it is less than the norm, he remains a dwarf.

The pituitary gland is called the conductor of the ES, and sometimes its “brain”.

Thyroid. If the pituitary gland is the conductor of ES, then the thyroid gland (TG) is its metronome. It kind of sets the pace for all the cells of the body. The main function of the thyroid gland is to control the proper metabolism, the absorption of oxygen. It can speed up the metabolism if it increases the amount of hormones sent to the blood stream, or slow it down by reducing their amount. However, this does not happen, since its work is under the control of thyroid-stimulating hormone secreted by the pituitary gland. And that, in turn, manages the thyroid hormone, which provides feedback to the pituitary gland. That is, between the pituitary gland and the thyroid gland, there is a self-regulating mechanism of balance and control, working in a certain cycle.

Normal thyroid function promotes growth, puberty, childbearing, mental development, emotional balance, vitality.

Adrenal glands. Their bark is actually a gland, also called adrenal. It produces adrenaline, which increases vascular tone and blood pressure. The adrenal cortex affects the performance of the body, its resistance to stress. Women’s adrenal glands also help the ovaries – they also produce sex hormones. Moreover, during menopause, when the ovaries stop their production.

Pancreas. It produces insulin, which enhances the process of burning glycogen in muscles, which proceeds with the release of energy.

Gallbladder. During stress, it releases bile, as a result of which the peristalsis of the small intestine increases, the rotting masses are quickly pushed into the large intestine, into the rectum. There is an accelerated release of toxins, thereby preventing their entry into the blood.

Appendix. At the moment of stress, the release of the hormone occurs, as a result of which the peristalsis of the large intestine increases and the rotting masses are released through the rectum (often this process is called “bear disease”).

Ovaries. In the broadest sense, the ovaries provide a woman with femininity. They perform this function with the help of two main hormones: estrogen and progesterone. Progesterone promotes fertility, while estrogen provides other signs of femininity, including the complex function of the menstrual cycle. And when the ovaries reduce its secretion, menopause occurs.

Hormones. This is a Greek word meaning “I excite,” “I set into action.” These are a kind of chemical messages, ordering the actions of certain organs. The hormone does not explain to the cells what kind of work they should do, but only determines how much and how quickly it should be done. Each hormone has its own controlled organs, but circulating throughout the body, it affects others, having a specific and general effect.

The pituitary gland produces three main hormones: gonadotropic (stimulates the ovaries and sex glands) and mammotropic or prolactin (affects the mammary glands, promotes the formation of milk after the birth of a child) and thyrotropic.

The thyroid gland produces thyroid hormones. Inside the body there are constant chemical processes. With their help, food and oxygen are processed and transformed into living matter, heat and energy are consumed, unnecessary residues are thrown away, and what wears out is reconstructed. Various chemical processes take place during respiration, digestion, in the work of muscles, in the secretion of glands. This is the metabolism for which the thyroid hormones are responsible.

The main adrenal hormone is adrenaline. Its functions can be reduced to three concepts: life, work, stress. It is involved in many metabolic processes, including fluid and salt balance, immunological reactions.

Along with progesterone and estrogen, the ovaries produce androgen, which ensures the balance of female and male sex hormones in the female body.

Which nervous system regulates the work of the endocrine system

The human body is a complex mechanism. Its cells, tissues and organs must work harmoniously and harmoniously. This condition is ensured by the work of two signaling systems: endocrine and nervous. Their interrelation provides several important conditions:

• homeostasis – the body’s ability to maintain constant characteristics;
• adaptation – the ability to change some factors of the internal environment depending on changes in external conditions;
• cell growth;
• reproduction.

The nervous system is a collection of organs that provide innervation to all organs and tissues. Its central section includes the brain and spinal cord, and the peripheral section includes the nerves. Information is captured by receptors, then moves in the form of impulses through nerve cells and reaches the brain. It provides a quick reaction in the form of muscle movement or other response to a stimulus. Also, the nervous system regulates the work of the endocrine system, controlling the intensity of hormone production.

The endocrine system is a collection of glands that secrete hormones into the blood. It includes the hypothalamus, pituitary gland, as well as peripheral glands: thyroid, pancreas, genital, adrenal glands. Hormones are biologically active substances that combine with the cells of various organs and can change their work, speed up or slow down biochemical processes in the body.

To understand which nervous system regulates the work of the endocrine system, you need to track the relationship. It is called “neuroendocrine regulation” and consists in controlling the production of hormones by the endocrine glands. This process is ensured by the work of several structures: the hypothalamus, neurotransmitter hormones, and the adrenal medulla.

The role of the hypothalamus

The hypothalamus is a small area of ​​the diencephalon, which is considered the center of neuroendocrine regulation. It is connected with other parts of the nervous system, the brain and spinal cord. Together with the pituitary gland, it forms the hypothalamic-pituitary system and regulates the intensity of the production of its hormones.

The hypothalamus receives signals from the following structures:

• basal nuclei (ganglia) – accumulations of gray matter in the white matter of the brain;
• spinal cord;
• parts of the brain: oblong, middle, thalamus, as well as some parts of the cerebral hemispheres.

The hypothalamus is the center that accumulates data from the whole organism, as well as from the external environment. Nerve cells of the hypothalamus are able to produce several types of neuroendocrine transmitters – biologically active substances that affect the intensity of the synthesis of tropic pituitary hormones:

1. Liberins – a group of compounds that stimulate hormonal synthesis. So, somatoliberin increases the production of somatotropic growth hormone, thyreoliberin – thyrotropic, gonadoliberin – luteinizing and follicle-stimulating hormones.
2. Statins are substances that suppress the production of tropic hormones by the pituitary gland. There are varieties such as somatostatin, prolactostatin, melanostatin.
3. Oxytocin and vasopressin are hormones produced by the hypothalamus but stored in the posterior pituitary gland. The first increases during childbirth and causes contraction of the muscular wall of the uterus, but also performs other functions. Vasopressin regulates water metabolism, increases vascular tone.

Hormones of the hypothalamus enter the pituitary gland through the bloodstream and influence its functions there. Statins and liberins do not always act strictly selectively. So, somatostatin can suppress the production of not only somatotropin, but also thyrotropic hormone, insulin and prolactin.

Nervous regulation of the adrenal glands

Adrenal glands are paired glands that in humans are located in the area of ​​the upper pole of the kidneys. In their structure, two components are distinguished: cortical and medulla. The cortex performs an endocrine function and produces hormones into the blood, and the medulla is an intermediate link between the nervous and endocrine systems.

One of the functions of the adrenal medulla is the production of catecholamines. This is a group of biologically active compounds that includes epinephrine and norepinephrine. They are activated to the maximum in stressful situations, when it is urgent to bring the body into tone, and trigger a number of changes:

• increased heart rate;
• increased vascular tone;
• increase in blood pressure;
• expansion of the bronchial lumen;
• inhibition of the digestive tract and a decrease in the secretion of its glands;
• dilated pupils;
• increased activity of sweat glands.

Adrenal medulla has a similar structure with nervous tissues, since during fetal development it is formed from identical rudiments. Histologically, the cells in this area are deformed neurons of the sympathetic autonomic nervous system, which then transformed into endocrine cells. They are activated under the influence of sympathetic nerve fibers. As a result of their irritation, adrenaline and norepinephrine are released into the bloodstream.

Catecholamines are considered “stress hormones” because their concentration increases in uncomfortable conditions for the body. They are activated during times of pain, exposure to cold, exercise, and muscle fatigue. Also, their increase can be caused by stress, vivid emotions, prolonged mental stress and other factors. The work of the adrenal medulla is controlled by structures such as the cerebral cortex and medulla oblongata, as well as the hypothalamus.

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In the process of neuroendocrine regulation, a two-way connection is observed. The organs of the endocrine system are under the control of nervous structures that stimulate or inhibit the synthesis of biologically active compounds. However, hormones also affect the central and peripheral parts of the nervous system. Thus, the thyroid gland secretes substances that act directly on the brain, bypassing the complex blood-brain barrier.