ENDOCRINE SYSTEM: FUNCTIONS, PARTS, DISEASES - ANATOMY AND PHYSIOLOGY - 2025

The endocrine system is a collection of ductless glands and tissues that produce a wide variety of secretions called hormones, which are released into the blood and distributed throughout the body through the circulatory system.

Hormones are chemical substances, effective at very low concentrations (micromolar or less than micromolar), produced by non-nerve endocrine cells, or by neurons, that regulate the functioning of nearby or distant populations of cells within the body.

CAMILALUGOZAMORA

Hormones are secreted directly into the extracellular fluid that surrounds endocrine cells. From there, they spread into the blood capillaries and then to the rest of the body.

There are also some chemical substances that, although they act like hormones, remain in the tissue in which they are produced (paracrine substances), or influence the very cells that secrete them (autocrine substances).

Endocrinology is the study of the physiological functions, pathology and evolution of hormones and, by extension, of autocrine and paracrine substances.

The endocrine system is dispersed throughout much of the body. Its components can consist of discrete endocrine organs, or be part of organs that also have non-endocrine functions.

The endocrine system is involved in the regulation of almost all physiological processes in the body. During animal evolution, the increase in physiological complexity has been accompanied by the morphological and functional diversification of the endocrine system.

Features

Hormones coordinate almost all the physiological activities of the body, which can be grouped into: 1) metabolism; 2) growth; 3) reproduction.

Metabolism can be defined as the sum of all chemical reactions in the body. In a very general way, it can be subdivided into: a) water and electrolyte metabolism; b) energy metabolism.

CAMILALUGOZAMORA

Hormones regulate the absorption, storage and excretion of water and electrolytes, maintaining a constant ionic environment.

They also regulate the flow of organic substrates, enabling appropriate concentrations of ATP within cells. For example, many hormones facilitate the digestion and absorption of food. Insulin causes glucose to be stored as glycogen.

Growth is the result of the interaction of metabolism with mitosis. Growth hormone, among others, regulates this process.

Reproduction is the result of the interaction of metabolism with meiosis and mitosis. Steroid hormones and gonadotropins promote gametogenesis. Relaxin and oxytocin stimulate lactation.

Chemical nature of hormones

Hormones belong to three chemical categories: 1) peptides and proteins; 2) amines (modified amino acids); 3) lipids (mainly steroids).

Peptides and proteins include the most abundant and versatile hormones. They vary in number of amino acids, from short peptides (thyrotropin-releasing hormone, antidiuretic hormone), to proteins of various sizes (prolactin, follicle-stimulating hormone, chorionic gonadotropin).

Amines include hormones derived from aromatic amino acids (tryptophan, phenylalanine, tyrosine).

Lipids include hormones derived from cholesterol, alcohols, and ketones. Hormones derived from alcohols have names ending in "ol" (eg, estradiol). Hormones derived from ketones have names ending in "one" (eg, aldosterone).

Hydrophobic hormones are difficult to store because they penetrate the cell membranes of the glands, therefore, they are synthesized when needed. In addition, for their diffusion in the body, they require transporter proteins endowed with hydrophobic regions. Its half-life is long.

Hydrophilic hormones can be stored to be rapidly secreted when needed. They are freely transported in serum. Because they cannot penetrate cell membranes, they must interact with cell surface receptors that generate a secondary signal that acts within the target cell. Its half-life is short.

How does it work?

It all begins with the synthesis of a hormone, which can be (peptides and amines) or not (lipid hormones) stored in the endocrine gland.

The hormone is released into the bloodstream, in which it travels to the target tissues and cells in the free state (this is the case of peptides and amines, except for thyroid hormone), or bound to transport proteins (this is the case of lipids and thyroid hormone).

Upon reaching its destination, the hormone binds to receptors (proteins) located on target cells that specifically recognize it.

Electrically charged hormones (peptides and neurotransmitters) bind to membrane receptors, causing a conformational change of other membrane proteins, which activate intracellular enzymes that catalyze the synthesis of secondary messengers that activate phosphorylating enzymes.

Hormones without electrical charge (eg, steroids and thyroid hormone) bind intracellularly to cytoplasmic or nuclear receptors, directly affecting the expression of genes in the cell.

The hormone (unchanged or degraded) then leaves the target cells, being transported through the bloodstream to the liver or kidneys, where it is excreted in the bile or urine.

Parts

The human endocrine system consists of nine glands (or pairs of glands), in alphabetical order: 1) adrenal (cortex and medulla); 2) ovaries; 3) endocrine pancreas; 4) parathyroid; 5) pineal; 6) pituitary (anterior and posterior); 7) testicles; 8) thymus; 9) thyroid.

In addition, this system includes six tissues that produce hormones, in alphabetical order: 10) heart; 11) liver; 12) kidneys; 13) central nervous system, specifically the hypothalamus; 14) adipose tissue; 15) gastrointestinal tract.

Adrenal glands

There are two adrenal glands, one on the left kidney and one on the right. They measure 5 cm in length and weigh 5 g. They are yellowish due to their high cholesterol content. Each adrenal gland has a cortex (outer region) and a medulla (inner region).

The cortex has three layers: 1) zona glomerulosa (secretes mineralocorticoids, mainly aldosterone); 2) zona fasciculata (secretes glucocorticoids, mainly cortisol); 3) zona reticularis (secretes adrenal androgens). Cholesterol is the precursor lipid for all hormones produced by the cortex.

The functioning of the cortex is controlled mainly by adrenocorticotropic hormone, secreted by the anterior pituitary. Mineralocorticoid secretion is independently controlled by several factors in the blood, the most important of which is angiotensin II, which is a peptide formed by the action of renin.

The medulla is part of the sympathetic nervous system, which activates the fight and flight responses of the individual. It secretes catecholamines (adrenaline = epinephrine; noradrenaline = norepinephrine).

Hormones of the adrenal glands

Aldosterone. It is a steroid. Regulates blood pressure, increasing extracellular volume. In turn, it is regulated by a mechanism known as the renin-angiotensin-aldosterone system.

Cortisol. It is a steroid. Facilitates hepatic gluconeogenesis (glucose production). Inhibits glucose uptake by extrahepatic tissues. Inhibits protein synthesis. Reduces inflammation. Its secretion increases during periods of psychological and physiological stress.

Adrenal androgens. They are steroids. They include dehydroepiandrosterone and androstenedione. They promote sexual maturation and libido. In women, together with those of the ovaries, they are the main androgens.

Adrenaline and noradrenaline. They are modified amino acids (monoamines derived from phenylalanine and tyrosine). They increase the heart rate. They raise blood pressure by vasoconstriction. They increase the concentration of circulating glucose, promoting gluconeogenesis in the liver. They increase pulmonary ventilation due to bronchodilation.

Ovaries

Women have two ovaries in the pelvic cavity, one on each side of the uterus. The ovaries are almond shaped and about 4 cm long.

They contain the ovarian follicles that give rise to mature eggs and secrete female sex hormones (estrogens and progesterone). They also secrete small amounts of androgens.

Hormones of the ovaries

Estrogens (estradiol, estrone, estriol). They are steroids. They occur in the corpus luteum (corpus luteum) and in developing follicles. They inhibit the excessive development of follicles. They promote the development of the female sexual organs (puberty). They determine the female pattern of body fat distribution.

Progestins. They are steroids. They occur in the corpus luteum. They maintain the endometrium. They thicken vaginal secretions. They prepare the mammary glands for lactation.

Androgens (mainly testosterone). They are steroids. They are produced in the follicles. They promote bone mineralization.

Pancreas

The pancreas is an elongated gland 12-15 cm in length, located in the abdomen, behind the stomach and in front of the spine, between the curve of the duodenum and the spleen. It secretes enzymes (amylase, lipase, proteases) that are transported through the pancreatic duct to the duodenum.

The pancreas also has endocrine functions. Pancreatic hormones (insulin and glucagon) are produced in the islets of Langerhans, which are small plates of irregularly shaped endocrine tissue, covered by dense networks of capillaries, dispersed in the non-endocrine parenchyma of the gland.

Hormones of the endocrine pancreas

Insulin. It is a peptide. It promotes growth. It reduces the level of blood glucose after a meal and promotes the storage of this sugar in the tissues. Increases the synthesis of proteins and lipids. Glucose represents the main stimulus for its secretion.

Glucagon. It is a peptide. It is gradually released after a meal. It acts mainly in the liver, generating glucose by glycogenolysis. In the same organ, it induces the production of glucose from compounds that are not carbohydrates (gluconeogenesis). Outside of the liver, it promotes the production of ketone bodies. It is inhibited by insulin.

Parathyroid

The parathyroid glands (two pairs, one upper, one lower) are located in the nape, behind the thyroid gland. They are yellow or brown in color. Each is somewhat smaller than a pea in size, weighing 30–50 mg. They produce the parathyroid hormone that stabilizes the blood level of calcium and phosphate, allowing the function of nerves and muscles.

The top pair is generally in the same position. The inferior pair (15-20% of people) is sometimes in an ectopic position, for example, embedded in the thyroid gland, or in the chest cavity between the sternum and the spinal column. The lack of between one and three of the four parathyroid glands (5% of people) has no detectable clinical effects.

Parathyroid hormone

Parathyroid hormone. It is a peptide. By its action, the bones release calcium and phosphate, and the kidneys reabsorb calcium and prevent the reabsorption of phosphate from the urine. In addition, it promotes the renal activation of vitamin D, facilitating the intestinal absorption of calcium.

Parathyroid hormone is a hypercalcemic factor, that is, it causes an elevation of the plasma calcium level. When the parathyroid gland detects low levels of calcium, it releases the hormone by exocytosis.

Pituitary

The pituitary gland, or pituitary gland, although small (0.5 cm in diameter), is sometimes called the master gland because it controls the rest of the endocrine system. Anatomically and functionally, it is divided into: 1) anterior pituitary (or lobe) gland, also called the adenohypophysis; 2) posterior pituitary (or lobe) gland, also called the neurohypophysis.

The pituitary gland is housed in the pituitary fossa, in the lower part of the skull, on the sella turcica (sella turcica) of the sphenoid. The posterior pituitary is in contact with the anterior one in front and with the hypothalamus behind. The anterior pituitary produces six hormones (all peptides). The posterior stores and releases hormones from the hypothalamus.

Hormones of the anterior pituitary

Adrenocorticotrophic hormone. It acts on the adrenal cortex. Increases the secretion of corticosteroids.

Growth hormone. It acts on hepatocytes and fat cells. Promotes growth and regulates metabolism.

Thyroid stimulating hormone. It acts on the thyroid gland. Stimulates the secretion of thyroxine and triiodothyronine.

Follicle stimulating hormone. It acts on the ovaries and testicles. In the former, it fulfills the function indicated by its name. In the second, it stimulates spermatogenesis.

Luteinizing hormone. It acts on the ovaries and testicles. Increases the secretion of sex hormones.

Prolactin. It acts on the mammary glands. Stimulates milk production. This hormone is also produced by the hypothalamus, the placenta, the uterus, and the mammary glands themselves.

Testicles

The testes are a pair of male reproductive organs that produce androgens and sperm. They are ovoid in shape. They are found outside the body cavity, between the legs, in a sac called the scrotum, made up of skin, muscles, and connective tissue.

Sperm are produced in the seminiferous tubules, while androgens are produced in Leydig cells, located in the space between these tubules. LDL cholesterol is absorbed by these cells, serving as a precursor for testosterone.

Male sex hormones, also present in women, are called androgens. Testosterone is the most important androgen. Other androgens include dehydroepiandrosterone, androstenedione, and dihydrotestosterone.

Hormones from the testicles

Testosterone. It is a steroid. It leads to puberty. Develops and maintains male sexual characteristics. Increase muscle strength. Promotes libido. It is necessary for an erection.

Dihydrotestosterone. It is a steroid. It is an active metabolite of testosterone. It occurs in the testicles, prostate, and skin. It is essential for the embryonic development of the male reproductive organs.

Thyroid

It is a highly vascularized gland shaped like a butterfly (bilobed) located at the nape of the neck. It runs between the fifth cervical vertebra and the first thoracic vertebra.

Its two lobes are connected by a middle isthmus that is at the level of the second and third rings of the trachea. It weighs 25-30 g. It is surrounded by a fine, fibrous tissue called a capsule.

It produces hormones that regulate the metabolic rate and have effects on most cells in the body.

Thyroid hormones

Tri-iodothyronine (T ₃) and thyroxine (T ₄). They are modified amino acids. T ₄ is a prohormone that needs to be converted to T ₃ to take effect (T ₃ is the active form).

T ₃ promotes the metabolism of carbohydrates, proteins and lipids. Increases cardiac activity, peripheral vasodilation, oxygen consumption and heat production. Regulates development. Promotes tissue growth. It influences the nervous system, increasing mental and physical alertness. It is essential for reproduction.

Calcitonin. It is a peptide. It reduces the concentration of calcium in the blood by opposing the action of the parathyroid hormone.

Hypothalamus

FerPortillo

It is a structure the size of an almond located behind the eyes, just below the thalamus. It is part of the autonomic nervous system. At the same time it is an endocrine tissue. It controls the pituitary, which is an endocrine gland.

It consists of neurons and neuroendocrine cells. The latter receive neuronal signals and release hormones into the blood.

Hormones of the hypothalamus

Dopamine. It is a modified amino acid. It is released by the anterior pituitary. Inhibits prolactin secretion.

Antidiuretic hormone. It is a peptide. It is released by the posterior pituitary. It promotes the renal reabsorption of water.

Corticotropin-releasing hormone. It is a peptide. It is released by the anterior pituitary. It induces the secretion of adrenocorticotrophic hormone.

Gonadotropin-releasing hormone. It is a peptide. It is released by the anterior pituitary. It stimulates the secretion of luteinizing hormone and follicle-stimulating hormone.

Growth hormone releasing hormone. It is a peptide. It is released by the anterior pituitary. It induces the secretion of growth hormone.

Thyrotrophin-releasing hormone. It is a peptide. It is released by the anterior pituitary. It induces the secretion of thyroid stimulating hormone.

Oxytocin. It is a peptide. It is released by the posterior pituitary. It stimulates uterine contractions and facilitates the production of breast milk.

Somatostatin. It is a peptide. It is released by the anterior pituitary. Inhibits the secretion of growth hormone.

Gastrointestinal tract

The walls of the small and large intestines contain numerous endocrine cells that produce hormones that facilitate digestion and glucose homeostasis.

Endocrine cells in the small intestine secrete incretin hormones that decrease appetite and intestinal motility, and increase insulin secretion, in response to food. The secretion of these hormones is directly dependent on the concentration of glucose.

The incretin hormones are glucagon-like peptide 1 and gastric inhibitory polypeptide. The non-incretin hormones secreted by the intestine are gastrin, vasoactive intestinal peptide, and ghrelin.

Hormones of the gastrointestinal tract

Glucagon-like peptide 1. It is derived from glucagon precursors. It is released in response to food intake. Increases insulin secretion. Reduces gastric emptying. It sends a signal of satiety to the hypothalamus. It is secreted by specialized cells in the small and large intestines.

Gastric inhibitory polypeptide. It increases the secretion of insulin by the pancreas. It is secreted by specialized cells in the small intestine.

Gastrine. It is a peptide. Its secretion is stimulated by the dilation, due to food, of the intestinal wall. Stimulates the secretion of gastric acid by the stomach. Increases gastric motility.

Vasoactive intestinal peptide. It is produced throughout the digestive tract, in the pancreas, and in the central nervous system. It has neuroendocrine effects. It causes vasodilation, slowing the flow of blood in the intestine. Contract the smooth muscles of the intestine. Increases the secretion of water and electrolytes by the epithelial cells of the intestine.

Ghrelin. It is a peptide. It is produced by the stomach and intestinal wall in response to fasting. It transmits the hunger signal to the hypothalamus.

Other endocrine glands and tissues

Pineal gland (epiphysis). it formed the primitive pineal eye. It is a pineapple-shaped neuroendocrine structure (hence its name), located under the brain. It secretes melatonin, a hormone that controls the circadian rhythm.

Scam. It is located behind the sternum and in front of the trachea and consists of two lobes. In infants, it weighs about 40 g and is essential for immunogenesis. After puberty regresses. It secretes thymosin, a hormone that stimulates the production of T cells.

The heart secretes the atrial natriuretic hormone, which reduces blood pressure by promoting the excretion of sodium and water.

The liver secretes the insulin-like growth factors IGF-I (children and adults) and IGF-II (fetus). These hormones have mitogenic effects on many tissues. For example, they stimulate bone proliferation and collagen synthesis by osteoblasts.

The kidneys secrete three hormones: 1) erythropoietin, which acts on the bone marrow, stimulating the production of red blood cells; 2) renin, which produces angiotensin in the blood; 3) 1,25-dihydroxycholecalciferol, which acts on the small intestine, stimulating the absorption of calcium.

The adipose tissue secretes leptin, a hormone that acts on the brain, reducing appetite.

Comparison with the nervous system

Animals function as integrated organisms, in which their cells act in a coordinated and harmonious way. This requires intercellular communication between distant body regions, which is carried out jointly by the endocrine and nervous systems, each specialized for different activities and response times.

In both systems, cell-to-cell communication involves the delivery of a chemical messenger by a signaling cell to a target cell.

In the endocrine system, a chemical messenger (hormone) that travels a long distance in the bloodstream is sent by a secretory endocrine tissue (signal cells) to a receptor endocrine or non-endocrine tissue (target cells).

In the nervous system, an electrical signal (nerve impulse) that travels a long distance within a neuron (signal cell) is transferred to a neighboring postsynaptic cell (target cell) mediated by a neurotransmitter (chemical messenger).

The endocrine system controls extensive and long-lasting physiological activities, such as growth processes, which can last for years. The nervous system coordinates precise and short-lived physiological responses, such as reflexes, that take milliseconds to perform.

Both systems interact in many ways. For example, certain populations of neurons secrete hormones called neurohormones.

Major diseases

Thyroid

Hyperthyroidism. Excess thyroid hormones in the blood. It is primary if it is due to thyroid disease. It is secondary if it is due to pathology of the pituitary. Causes increased appetite, weight loss, heat intolerance, sweating, rapid heart rate, fatigue, and bulging eyes. In severe cases there is goiter (lump in the neck due to an enlarged thyroid).

Hypothyroidism. Thyroid hormone deficiency in the blood. It is characterized by slowed metabolism, bradycardia, muscle weakness, cramps, dry skin, hair loss, throaty voice, and weight gain. If it is present at birth it causes cretinism. There may be goiter.

Endocrine pancreas

Gestational diabetes. It develops during pregnancy. It is due to insulin resistance caused by increases in the concentration of growth hormone, placental prolactin, progesterone, or cortisol. It affects 2–3% of pregnant women.

Diabetes mellitus. Insufficient production of insulin by the pancreas, or resistance of the tissues to insulin. Type 1 (insulin-dependence) is due to the destruction of cells in the pancreas and develops in childhood or adolescence. Type 2 (non-insulin dependence) develops gradually with age. It is due to insufficient insulin production.

Pituitary

Acromegaly. Overproduction of growth hormone due to pathologies of the pituitary. There is an abnormal growth, progressive with age, of the head, face, hands, feet and internal organs. If it develops before puberty it produces gigantism.

Hypopituitarism. Hormone deficiency caused by damage (tumors, surgery, radiation therapy) to the anterior pituitary gland. It leads to atrophy of the thyroid and adrenal glands, as well as the gonads.

Cushing syndrome. Excess corticosteroid hormones due to pituitary pathology or medication. It is characterized by a round face (full moon), central obesity, abnormal stretch marks, hypertension, acne, osteoporosis, susceptibility to infection, peptic ulcers, female baldness, depression, insomnia, paranoia, and euphoria.

Adrenal glands

Addison's disease. Also called primary adrenal insufficiency. It is due to the almost total destruction of the adrenal cortex by various pathologies, such as aotoinmumnes processes. It causes weight loss, anemia, pigmentation abnormalities, severe tooth decay, stiffness of the cartilage of the ear, fatigue and hypotension.

Conn syndrome. It is due to excess aldosterone caused by a tumor or adrenal hyperplasia.

It can also be caused by heart or liver failure, which reduces blood flow through the kidneys, leading to an overproduction of renin and angiotensin. Symptoms are sodium retention and potassium loss, hypertension, thirst, and fatigue.

References

1. Barrett, KE, Brooks, HL, Barman, SM, Yuan, JX-J. 2019. Ganong's review of medical physiology. McGraw-Hill, New York.
2. Bolander, FF Jr. 2004. Molecular endocrinology. Elsevier, Amsterdam.
3. Boron, WF, Boulpaep, EL 2017. Medical physiology. Elsevier, Philadelphia.
4. Fox, T., Vaidya, B., Brooke, A. 2015. Endocrinology. Medical, London.
5. Hall, JE 2016. Guyton and Hall textbook of medical physiology. Elsevier, Philadelphia.
6. Hill, RW, Wyse, GA, Anderson, M. 2012. Animal physiology. Sinauer Associates, Sunderland.
7. Hinson, J., Raven, P., Chew, S. 2007. The endocrine system: basic science and clinical conditions. Churchill Livingstone, Edinburgh.
8. Kay, I. 1998. Introduction to animal physiology. Bios, Oxford.
9. Kleine, B., Rossmanith, WG 2016. Hormones and the endocrine system: textbook of endocrinology. Springer, Cham.
10. Kraemer, WJ, Rogol, AD 2005. The endocrine system in sports and exercise. Blackwell, Malden.
11. Moyes, CD, Schulte, PM 2014. Principles of animal physiology. Pearson, Essex.
12. Neal, JM 2016. How the endocrine system works. Wiley, Hoboken.
13. Norris, DO 2007. Vertebrate endocrinology. Elsevier, Amsterdam.
14. Rushton, L. 2009. The endocrine system. Infobase, New York.
15. Sherwood, L., Klandorf, H., Yancey, PH 2013. Animal physiology: from genes to organisms. Brooks / Cole, Belmont.