Endocrine Glands

Hypothalamic-Pituitary Axis

The hypothalamus, an endocrine organ, regulates the anterior pituitary gland and transports hormones along the posterior pituitary gland.

Learning Objectives

Describe the hormones released by the anterior posterior pituitary and their effects on the body

Key Takeaways

Key Points

  • The endocrine system is made up by a group of endocrine glands, including the pituitary glands which are responsible for the release of hormones relating to important bodily functions and regulations.
  • The hypothalamus instigates endocrine responses to environmental changes from messages received from the body and brain.
  • The hypothalamus synthesizes hormones and transports them to the posterior pituitary gland while also synthesizing and secreting regulatory hormones that control cells in the anterior pituitary gland.
  • The anterior pituitary gland, regulated by the hypothalamus, produces seven tropic hormones which control the functioning of other organs.
  • The posterior pituitary stores hormones produced by the hypothalamus and releases them into the blood stream; the gland does not actually produce any hormones.

Key Terms

  • adenohypophysis: the anterior lobe of the pituitary gland, producing and secreting several peptide hormones that regulate many physiological processes including stress, growth, and reproduction
  • hypophysis: another name for the pituitary gland, which is located at the base of the brain
  • hypothalamus: a region of the forebrain located below the thalamus that regulates body temperature, some metabolic processes and governs the autonomic nervous system
  • diencephalon: the region of the human brain, specifically the human forebrain, that includes the thalamus, the hypothalamus, the epithalamus, the prethalamus or subthalamus, and the pretectum
  • neurohypophysis: the posterior lobe of the pituitary gland, responsible for the release of oxytocin and antidiuretic hormone (ADH)

The Endocrine System

The endocrine system uses chemical signals to communicate and regulate the body’s physiology. The system releases hormones that act on target cells to regulate development, growth, energy metabolism, reproduction, and many behaviors. Endocrine glands contain no ducts; they release their secretions directly into the intercellular fluid or into the blood. The collection of these glands makes up the endocrine system. The main endocrine glands, which we will learn about in this section and in the following ones, are the pituitary (anterior and posterior), thyroid, parathyroid, adrenal (cortex and medulla), pancreas, and gonads.

Hypothalamic-Pituitary Axis

The hypothalamus in vertebrates integrates the endocrine and nervous systems. The hypothalamus is an endocrine organ located in the diencephalon of the brain. It receives input from the body and other brain areas, initiating endocrine responses to environmental changes. The hypothalamus acts as an endocrine organ as it synthesizes hormones and transports them along axons to the posterior pituitary gland. The hypothalamus also synthesizes and secretes regulatory hormones that control the endocrine cells in the anterior pituitary gland. In addition, it contains autonomic centers that control endocrine cells in the adrenal medulla via neuronal control.

The pituitary gland, sometimes called the hypophysis or “master gland,” is located at the base of the brain in the sella turcica, a groove of the sphenoid bone of the skull. It is attached to the hypothalamus via a stalk called the pituitary stalk (or infundibulum). The pituitary has two distinct regions: the anterior pituitary and the posterior pituitary. These regions secrete nine different peptide or protein hormones. The anterior portion of the pituitary gland is regulated by releasing or release-inhibiting hormones produced by the hypothalamus. The posterior pituitary receives signals via neurosecretory cells to release hormones produced by the hypothalamus.

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Pituitary gland: The pituitary gland is located at (a) the base of the brain and is (b) connected to the hypothalamus by the pituitary stalk.

Anterior Pituitary Gland

The anterior pituitary gland, or adenohypophysis, is surrounded by a capillary network that extends from the hypothalamus, down along the infundibulum, and to the anterior pituitary. This capillary network is a part of the hypophyseal portal system which carries substances from the hypothalamus to the anterior pituitary and hormones from the anterior pituitary into the circulatory system. A portal system carries blood from one capillary network to another; therefore, the hypophyseal portal system allows hormones produced by the hypothalamus to be carried directly to the anterior pituitary without first entering the circulatory system.

The anterior pituitary produces seven hormones: growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), melanin-stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). Anterior pituitary hormones are sometimes referred to as tropic hormones because they control the functioning of other organs. While these hormones are produced by the anterior pituitary, their production is controlled by regulatory hormones produced by the hypothalamus. These regulatory hormones can be releasing hormones or inhibiting hormones, causing more or less of the anterior pituitary hormones to be secreted. The regulatory hormones travel from the hypothalamus through the hypophyseal portal system to the anterior pituitary where they exert their effect. Negative feedback then regulates how much of these regulatory hormones are released and how much anterior pituitary hormone is secreted.

Posterior Pituitary Gland

The posterior pituitary is significantly different in structure from the anterior pituitary. It is a part of the brain, extending down from the hypothalamus, and contains mostly nerve fibers and neuroglial cells which support axons that extend from the hypothalamus to the posterior pituitary. The posterior pituitary and the infundibulum together are referred to as the neurohypophysis.

The antidiuretic hormone (ADH) (or vasopressin) and oxytocin are produced by neurons in the hypothalamus and transported within these axons along the infundibulum to the posterior pituitary. They are released into the circulatory system via neural signaling from the hypothalamus. These hormones are considered to be posterior pituitary hormones even though they are produced by the hypothalamus, since that is where they are released into the circulatory system. The posterior pituitary itself does not produce hormones, but instead stores hormones produced by the hypothalamus, releasing them into the blood stream. Vasopressin has two primary functions: to retain water in the body and to constrict blood vessels. The hormone regulates the body’s retention of water by acting to increase water absorption in the collecting ducts of the kidney nephron. Oxytocin, known as the bonding hormone, has roles in various behaviors, including orgasm, social recognition, pair bonding, anxiety, and maternal behaviors. The hormone has been found to be released in large amounts after distension of the cervix and uterus during labor, facilitating birth, maternal bonding, and lactation.

Thyroid Gland

The thyroid gland, the largest endocrine gland, is responsible for the production of the hormones T3, T4, and calcitonin.

Learning Objectives

Describe the hormones produced by the thyroid and explain how their production is regulated

Key Takeaways

Key Points

  • The thyroid gland is made up of thyroid follicles, which produce three main hormones.
  • T3 and T4 hormones increase the metabolic activity of the body‘s cells while calcitonin helps regulate calcium concentrations in body fluids.
  • T3 and T4 release is controlled by thyroid stimulating hormone; however, calcitonin release is controlled by calcium ion concentrations.

Key Terms

  • thyroglobulin: a globulin, produced by the thyroid gland, that has a role in the production of the thyroid hormones
  • thyroxine: a hormone (an iodine derivative of tyrosine), produced by the thyroid gland, that regulates cell metabolism and growth
  • thyrocalcitonin: a hormone, secreted by parenchymal cells, that regulates calcium and phosphate metabolism
  • triiodothyronine: the most powerful thyroid hormone, affecting almost every process in the body, including body temperature, growth, and heart rate

Thyroid Gland

The thyroid gland, one of the largest endocrine glands in the body, is located in the neck, just below the larynx and in front of the trachea. It is a butterfly-shaped gland with two lobes that are connected by the isthmus. It has a dark red color due to its extensive vascular system. When the thyroid swells due to dysfunction, it can be felt under the skin of the neck.

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Thyroid gland: The location of the thyroid gland is in the neck below the larynx and in front of the trachea; it is the largest endocrine gland in the body, producing T3, T4, and calcitonin.

The thyroid gland is made up of many spherical thyroid follicles which are lined with a simple cuboidal epithelium. These follicles contain a viscous fluid, called colloid, which stores the glycoprotein thyroglobulin. This glycoprotein is the precursor to the thyroid hormones. The follicles produce hormones that can be stored in the colloid or released into the surrounding capillary network for transport to the rest of the body via the circulatory system.

The thyroid gland produces the hormones T3 (triiodothyronine) and T4 (thyroxine). These hormones increase the metabolic activity of the body‘s cells. Follicle cells are stimulated to release stored T3 and T4 by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary. These thyroid hormones increase the rates of mitochondrial ATP production.

Another hormone produced by the thyroid gland, thyrocalcitonin, or calcitonin, decreases the concentration of calcium in the blood. Most of the calcium removed from the blood is stored in the bones. Calcitonin is produced by parafollicular cells of the thyroid, either releasing hormones or inhibiting hormones. The hormone’s release is not controlled by TSH, but instead is released when calcium ion concentrations in the blood rise. Calcitonin functions to help regulate calcium concentrations in body fluids. It acts in the bones to inhibit osteoclast activity and in the kidneys to stimulate excretion of calcium. The combination of these two events lowers body fluid levels of calcium.

Parathyroid Glands

Parathyroid glands produce parathyroid hormone, which is responsible for specific physiological responses in the body related to calcium.

Learning Objectives

Describe how the parathyroid glands regulate calcium levels in the blood

Key Takeaways

Key Points

  • Parathyroid glands are responsible for the regulation the body’s calcium and phosphorus levels by producing parathyroid hormone, which helps control calcium release.
  • Oxyphil cells and chief cells are two main types of cells that make up parathyroid tissue; chief cells make parathyroid hormone while the role of oxyphil cells remains unknown.
  • Parathyroid hormone is released into the bloodstream where it travels to target cells, binding to a receptor found on the target cells.
  • Parathyroid hormones help regulate calcium levels by increasing blood calcium concentrations when calcium ion levels fall below normal.

Key Terms

  • parathyroid hormone: a polypeptide hormone that is released by the chief cells of the parathyroid glands and is involved in raising the levels of calcium ions in the blood
  • calcitriol: the active metabolite 1,25-dihydroxycholecalciferol of vitamin D3 that is involved in the absorption of calcium
  • osteoclast: a large multinuclear cell associated with the resorption of bone
  • osteoblast: a mononucleate cell from which bone develops

Parathyroid Glands

The parathyroid glands are small endocrine glands that produce parathyroid hormone. Most people have four parathyroid glands; however, the number can vary from two to six. These glands are located on the posterior surface of the thyroid gland. Normally, there is a superior gland and an inferior gland associated with each of the thyroid’s two lobes. Each parathyroid gland is covered by connective tissue and contains many secretory cells that are associated with a capillary network. There are two major types of cells that make up parathyroid tissue: oxyphil cells and chief cells, the latter of which actually produce parathyroid hormone. The function of oxyphil cells is unknown.

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Parathyroid glands: The parathyroid glands are located on the posterior of the thyroid gland. The parathyroid glands produce parathyroid hormone (PTH) which increases blood calcium concentrations when calcium ion levels fall below normal.

One of the parathyroid glands’ most important functions is to regulate the body’s calcium and phosphorus levels. Another function of the parathyroid glands is to secrete parathyroid hormone, which causes the release of the calcium present in bone to extracellular fluid.

Parathyroid hormone (PTH), also known as parathormone, is released directly into the bloodstream, traveling to its target cells, which are often quite far away. It then binds to a receptor (found either inside or on the surface of the target cells). Receptors bind a specific hormone, resulting in a specific physiologic (normal) response of the body.

Parathyroid Glands and Calcium Regulation

PTH opposes the effect of thyrocalcitonin (or calcitonin ), a hormone produced by the thyroid gland that regulates calcium levels. It does this by removing calcium from its storage sites in bones and releasing it into the bloodstream. It also signals the kidneys to reabsorb more of this mineral, transporting it into the blood. PTH can also signal the small intestine to absorb calcium by transporting it from the diet into the blood. Calcium is important for metabolization to occur. Blood cannot clot without sufficient calcium. Skeletal muscles require this mineral in order to contract. A deficiency of PTH can lead to tetany, a condition characterized by muscle weakness due to lack of available calcium in the blood.

More specifically, PTH increases blood calcium concentrations when calcium ion levels fall below normal. First, PTH enhances reabsorption of calcium by the kidneys; it then stimulates osteoclast activity and inhibits osteoblast activity. Finally, PTH stimulates synthesis and secretion of calcitriol by the kidneys, which enhances Ca2+ absorption by the digestive system. PTH and calcitonin work in opposition to one another to maintain homeostatic calcium levels in body fluids.

Adrenal Glands

Adrenal glands are composed of the adrenal cortex and medulla; both produce hormones that control essential body functions and responses.

Learning Objectives

Distinguish between the hormones produced by the adrenal cortex and adrenal medulla and the functions they regulate

Key Takeaways

Key Points

  • The two major hormones produced by the adrenal cortex are the mineralocorticoids, which regulate the salt and water balance, and the glucocorticoids, which can regulate blood glucose and the body’s inflammatory response.
  • There are three main glucocorticoids: cortisol, corticosterone, and cortisone.
  • The adrenal medulla produces the hormones epinephrine and norepinephrine; these hormones regulate heart rate, breathing rate, cardiac muscle contractions, blood pressure, and blood glucose levels.

Key Terms

  • glucocorticoid: any of a group of steroid hormones, produced by the adrenal cortex, that are involved in metabolism and have anti-inflammatory properties
  • aldosterone: a mineralocorticoid hormone, secreted by the adrenal cortex, that regulates the balance of sodium and potassium in the body
  • epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress

Adrenal Glands

Adrenal glands are a pair of ductless glands located above the kidneys. Through hormonal secretions, the adrenal glands regulate many essential functions in the body, including biochemical balances that influence athletic training and general stress response. The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla, which secrete different hormones.

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Adrenal glands: Adrenal glands are located on top of the kidneys. These glands are composed of the adrenal cortex and the adrenal medulla.

Adrenal Cortex

The adrenal cortex is made up of layers of epithelial cells and associated capillary networks. These layers form three distinct regions: an outer zona glomerulosa that produces mineralocorticoids; a middle zona fasciculata that produces glucocorticoids; and an inner zona reticularis that produces androgens, which are sex hormones that promote masculinity. Androgens are produced in small amounts by the adrenal cortex in both males and females. They do not affect sexual characteristics and may supplement sex hormones released from the gonads.

The hormones made by the adrenal cortex supply long-term responses to stress. The two major hormones produced are the mineralocorticoids and the glucocorticoids. The mineralocorticoids regulate the salt and water balance, leading to the increase of blood volume and blood pressure. The main mineralocorticoid is aldosterone, which regulates the concentration of sodium ions in urine, sweat, pancreas, and saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels.

The glucocorticoids regulate increases in blood glucose and also reduce the body’s inflammatory response. The three main glucocorticoid hormones are cortisol, corticosterone, and cortisone. The glucocorticoids stimulate the synthesis of glucose and gluconeogenesis (converting a non-carbohydrate to glucose) by liver cells. They also promote the release of fatty acids from adipose tissue. These hormones increase blood glucose levels to maintain levels within a normal range between meals. Cortisol is one of the most active glucocorticoids. It usually reduces the effects of inflammation or swelling throughout the body. It also stimulates the production of glucose from fats and proteins, which is a process referred to as gluconeogenesis. Aldosterone is one example of a mineralcorticoid. It signals the tubules in the kidney nephrons to reabsorb sodium while secreting or eliminating potassium. If sodium levels are low in the blood, the kidney secretes more renin, an enzyme that stimulates the formation of angiotensin from a molecule made from the liver. Angiotensin stimulates aldosterone secretion. As a result, more sodium is reabsorbed as it enters the blood. Aldosterone, the major mineralcorticoid, stimulates the cells of the distal convoluted tubules of the kidneys to decrease re-absorption of potassium and increase re-absorption of sodium. This in turn leads to an increased re-absorption of chloride and water. These hormones, together with such hormones as insulin and glucagon, are important regulators of the ionic environment of the internal fluid.

Adrenal Medulla

The adrenal medulla contains large, irregularly-shaped cells that are closely associated with blood vessels. These cells are innervated by pre-ganglionic autonomic nerve fibers from the central nervous system.

The adrenal medulla contains two types of secretory cells: one that produces epinephrine (adrenaline) and another that produces norepinephrine (noradrenaline). Epinephrine is the primary adrenal medulla hormone, accounting for 75 to 80 percent of its secretions. Epinephrine and norepinephrine increase heart rate, breathing rate, cardiac muscle contractions, blood pressure, and blood glucose levels. They also accelerate the breakdown of glucose in skeletal muscles and stored fats in adipose tissue.

The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic nervous system. Secretion of these hormones is stimulated by acetylcholine release from pre-ganglionic sympathetic fibers innervating the adrenal medulla. These neural impulses originate from the hypothalamus in response to stress to prepare the body for the fight-or-flight response.

Pancreas

The pancreas produces digestive enzymes and hormones, which are important in blood sugar regulation and other body functions.

Learning Objectives

Describe the hormones produced by the pancreas and the functions they regulate

Key Takeaways

Key Points

  • The pancreas has both endocrine and exocrine functions; it is sometimes referred to as a heterocrine gland.
  • Glucagon and insulin are examples of hormones created by the pancreas, produced by an alpha or a beta cell type, respectively.
  • Both insulin and glucagon are responsible for the regulation of blood glucose levels in the body.

Key Terms

  • insulin: a polypeptide hormone that regulates carbohydrate metabolism
  • islets of Langerhans: regions in the pancreas that contain its endocrine cells
  • exocrine: producing external secretions that are released through a duct
  • glucagon: a hormone, produced by the pancreas, that opposes the action of insulin by stimulating the production of sugar

The Pancreas

The pancreas is an elongated organ that is located between the stomach and the proximal portion of the small intestine. It contains both exocrine cells that excrete digestive enzymes and endocrine cells that release hormones. It is sometimes referred to as a heterocrine gland because it has both endocrine and exocrine functions.

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Pancreas: The pancreas is found underneath the stomach and points toward the spleen. It is both an endocrine and exocrine gland.

As an endocrine gland, the pancreas produces several important hormones,such as insulin and glucagon, which are secreted into the bloodstream to regulate blood sugar levels, along with other activities throughout the body. As a digestive organ, the pancreas secretes pancreatic juice containing digestive enzymes that assist the absorption of nutrients and the digestion in the small intestine. Food particles are reduced to basic elements that can be absorbed by the intestine and used by the body. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.

The endocrine cells of the pancreas form clusters called pancreatic islets or the islets of Langerhans. The pancreatic islets contain two primary cell types: alpha cells, which produce the hormone glucagon, and beta cells, which produce the hormone insulin. These hormones are responsible for the regulation of blood glucose levels. As blood glucose levels decline, alpha cells release glucagon to raise the blood glucose levels by increasing rates of glycogen breakdown and glucose release by the liver. When blood glucose levels rise, such as after a meal, beta cells release insulin to lower blood glucose levels by increasing the rate of glucose uptake in most body cells, and by increasing glycogen synthesis in skeletal muscles and the liver.

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Islets of Langerhans: The islets of Langerhans are clusters of endocrine cells found in the pancreas; they stain lighter than surrounding cells. The alpha and beta cells produce glucagon and insulin, respectively.

Pineal Gland and Gonads

The pineal gland is responsible for melatonin production, while the gonads secrete hormones relating to sexual characteristic development.

Learning Objectives

Describe the effects of melatonin and gonad produced hormones in the body

Key Takeaways

Key Points

  • The pineal gland, a small endocrine gland in the brain, is responsible for producing a hormone involved in the regulation of biological rhythms, mainly circadian rhythms.
  • The gonads (the testes in males and ovaries in females) are responsible for the production of steroid hormones, such as testosterone, estrogen, and progesterone.
  • Testosterone regulates the development and function of the primary sex organs and secondary male characteristics in males, such as deepened voice pitch and body hair.
  • Estrogen and progesterone are responsible for the development of secondary sex characteristics and the preparation of the body for childbirth.

Key Terms

  • androgen: the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of masculine characteristics in vertebrates
  • melatonin: a hormone that is secreted by the pineal gland; it stimulates color change in the skin of reptiles and is involved in the sleep/wake and reproductive cycles in mammals
  • photoperiod: the normal duration of natural daylight experienced by an organism; daylength

Pineal Gland

The pineal gland is a small endocrine gland in the brain. It is located near the center of the brain, between the two hemispheres, tucked in a groove where the two rounded thalamic bodies join. The gland consists of two types of cells known as parenchymal and neuroglial cells.

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Nervous system endocrine glands: the pineal gland: The pineal gland, found in the brain, produces the hormone melatonin.

The main hormone produced and secreted by the pineal gland is melatonin. The rate of melatonin production is affected by the photoperiod. Collaterals from the visual pathways innervate the pineal gland. During the day photoperiod, little melatonin is produced; however, melatonin production increases during the dark photoperiod (night). In some mammals, melatonin has an inhibitory affect on reproductive functions by decreasing production and maturation of sperm, oocytes, and reproductive organs. Melatonin is an effective antioxidant, protecting the CNS from free radicals such as nitric oxide and hydrogen peroxide. Lastly, melatonin is involved in biological rhythms, particularly circadian rhythms such as the sleep-wake cycle and eating habits.

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Pineal gland: The pineal gland is an endocrine gland located in the middle of the brain. It is responsible for the production of melatonin, a hormone which acts as an antioxidant and is involved in the regulation of biological rhythms.

Gonads

The gonads are additional types of endocrine glands. They are the sex organs and include the male testes and female ovaries. Their main role is the production of steroid hormones. The testes produce androgens, which allow for the development of secondary sex characteristics and the production of sperm cells. Testosterone, the most prominent androgen in males, stimulates the development and functioning of the primary sex organs. It also stimulates the development and maintenance of secondary male characteristics, such as hair growth on the face and the deep pitch of the voice.

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Testes: The testes produce androgens, such as testosterone, which regulate primary sex organ development and function, as well as the development of secondary sex characteristics and the production of sperm cells.

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Endocrine system: gonads and their hormones: The gonads are the sex organs. Male testes produce androgens, while female ovaries produce estrogen and progesterone.

The ovaries produce hormones, such as estrogen and progesterone, which cause secondary sex characteristics and prepare the body for childbirth. Estrogen increases at the time of puberty, causing the growth of the uterus and vagina. Without estrogen, egg maturation would not occur. Estrogen is also responsible for secondary sex characteristics such as female body hair and fat distribution. Estrogen and progesterone are responsible for the development of the breast and for the uterine cycle. Progesterone is a female hormone secreted by the corpus luteum after ovulation during the second half of the menstrual cycle. It prepares the lining of the uterus for implantation of a fertilized egg and allows for complete shedding of the endometrium at the time of menstruation. In the event of pregnancy, the progesterone level remains stable beginning a week or so after conception.

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Ovaries: The ovaries produce estrogen and progesterone, which are hormones responsible for the development of sexual characteristics in females and the preparation of female bodies for pregnancy and childbirth.

Organs with Secondary Endocrine Functions

Several organs with specialized non-endocrine functions possess endocrine roles, such as hormone production and release.

Learning Objectives

Describe the role of organs with secondary endocrine functions

Key Takeaways

Key Points

  • Organs that also have some endocrine roles include the heart, kidneys, intestines, thymus, gonads, and adipose tissue.
  • Endocrine cells in the heart release the hormone atrial natriuretic peptide in response to increased blood volume.
  • The intestines contain endocrine cells that produce hormones that are responsible in aiding digestion, while the endocrine role of the kidneys includes the release of hormones specific to Na+ and water retention in the body; another kidney hormone (EPO) is involved in red blood cell formation.
  • Thymosin, involved in the development of the immune response, is a hormone produced by the thymus.
  • Adipose tissue is responsible for the production of leptin, which is generated in response to food intake.

Key Terms

  • atrial natriuretic peptide: a strong vasodilatory, peptide hormone, secreted by the cardiac muscle cells
  • thymosin: a polypeptide hormone, secreted by the thymus, that stimulates the development of T cells as part of the immune system
  • leptin: a protein hormone produced in adipose tissue; it plays a role in regulating appetite and metabolism
  • anorexigenic: creating or inducing a state of anorexia
  • orexigenic: that stimulates the appetite

Organs with Secondary Endocrine Functions

There are several organs whose primary functions are non-endocrine, but that also possess endocrine functions. These include the heart, kidneys, intestines, thymus, and adipose tissue.

The heart possesses endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These cells release the hormone atrial natriuretic peptide (ANP) in response to increased blood volume. High blood volume causes the cells to be stretched, resulting in hormone release. ANP acts on the kidneys to reduce the reabsorption of sodium (Na+), causing Na+ and water to be excreted in the urine. ANP also reduces the amounts of renin released by the kidneys and aldosterone released by the adrenal cortex, further preventing the retention of water. In this way, ANP causes a reduction in blood volume and blood pressure, while reducing the concentration of Na+ in the blood.

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ANP hormone structure: The hormone atrial natriuretic peptide (ANP), released in response to increased blood volume, is produced by endocrine cells in the heart.

The gastrointestinal tract produces several hormones that aid in digestion. The endocrine cells are located in the mucus of the GI tract throughout the stomach and small intestine. Some of the hormones produced include gastrin, secretin, and cholecystokinin, which are secreted in the presence of food. They also act on other organs, such as the pancreas, gallbladder, and liver, by triggering the release of gastric juices, which help to break down and digest food in the GI tract.

While the adrenal glands associated with the kidneys are major endocrine glands, the kidneys themselves also possess endocrine function. Renin, released in response to decreased blood volume or pressure, is part of the renin-angiotensin-aldosterone system that leads to the release of aldosterone. Aldosterone then causes the retention of Na+ and water, raising blood volume. The kidneys also release calcitriol, which aids in the absorption of calcium (Ca2+) and phosphate ions. Erythropoietin (EPO), a protein hormone produced by the kidney, triggers the formation of red blood cells in the bone marrow. EPO is released in response to low oxygen levels. Because red blood cells are oxygen carriers, increased production results in greater oxygen delivery throughout the body. EPO has been used by athletes to improve performance as greater oxygen delivery to muscle cells allows for greater endurance. Because red blood cells increase the viscosity of blood, artificially high levels of EPO can cause severe health risks.

The thymus is found behind the sternum. It is most prominent in infants, becoming smaller in size through adulthood. The thymus produces hormones referred to as thymosins, which contribute to the development of the immune response.

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Thymus: The thymus, located behind the sternum, produces the hormone thymosin.

Adipose tissue is a connective tissue found throughout the body. It produces the hormone leptin in response to food intake. Leptin increases the activity of anorexigenic neurons and decreases that of orexigenic neurons, producing a feeling of satiety after eating, thus affecting appetite and reducing the urge for further eating. Leptin is also associated with reproduction. It must be present for GnRH and gonadotropin synthesis to occur. Extremely thin females may enter puberty late; however, if adipose levels increase, more leptin will be produced, improving fertility.