Endocrine System- Definition, Glands, Hormones, Functions, Disorders

What is Endocrine System?

Definition of Endocrine System

An endocrine system is a group of ductless glands where the secretory cells diffuse the hormones directly into the bloodstream.

• The glands of the endocrine system are termed endocrine glands and are defined by the lack of ducts for the passage of their secretions.
• The endocrine system is considered to work together in coordination with the nervous system. However, the responses of the endocrine system are often slower, and the influence is much broader.
• The endocrine system also depends on the cardiovascular system for the distribution of their products. As a result, the endocrine glands are some of the most vascular tissues in the body.
• The hormones secreted by endocrine glands are effective in very small amounts; thus, the circulating levels of these hormones are typically low.
• The endocrine system also contains cells and tissues that are not exclusively classified as endocrine glands but contain cells that can secrete hormones.
• Some of such cells and tissues are the hypothalamus, thymus, pancreas, skin, heart, and adipose tissues.
• The study of the structure and function of the endocrine glands and the cells, along with the diagnosis and treatment of disorders involving the endocrine system, is termed endocrinology.
• Unlike most other systems in the body, the glands of the endocrine system are not grouped together and remain scattered throughout the body.
• Different hormone systems of the endocrine organs help in the regulation of all body functions like metabolism, growth, electrolyte balance, reproduction, and behavior.
• Most endocrine glands are regulated by a simple negative feedback mechanism, while the secretion is controlled by the nervous and immune systems.

Glands and Hormones of the Endocrine System

The endocrine system consists of the following glands;

A. Hypothalamus and Pituitary Gland

• The hypothalamus is a small organ situated in the bran below the thalamus, which controls the secretion of the pituitary gland.
• The hypothalamus is attached to the pituitary gland by a small stalk called the infundibulum, and it is considered the connecting link between the endocrine system and the nervous system.
• The cells of the hypothalamus alone secrete about nine different hormones, out of which seven hormones are involved in the regulation of the pituitary gland.
• The hormones produced in the hypothalamus are termed inhibiting or releasing hormones.
• The pituitary gland is a small pea-shaped organ measuring about 1-1.5 cm in diameter, which occurs in the hypophyseal fossa of the sphenoid bone in the skull.
• The pituitary gland can be differentiated into two anatomically and functionally separate parts; anterior pituitary and posterior pituitary.
• The anterior pituitary, also called adenohypophysis, is composed of epithelial cells and accounts for about 70% of the total pituitary gland.
• The anterior pituitary is supplied with a portal system that ensures the circulation of hormones produced by the gland.
• The secretion of the anterior pituitary is influenced by the releasing hormone produced by the hypothalamus. 
• The posterior pituitary is composed of neural tissue, which is triggered by an action potential that originates in the cell body of the hypothalamus.
• The hormones of the posterior pituitary are synthesized in the nerve cell bodies and are transported along the axons to be stored in the axon terminals. The nerve stimuli from the hypothalamus regulate exocytosis of the vesicles to release the hormones into the bloodstream.
• The overall secretion of hormones by the pituitary is regulated by a negative feedback mechanism.

Hormones of the hypothalamus and pituitary gland

1. Growth Hormone

• Growth hormone is the most abundant hormone secreted by the anterior pituitary that stimulates the growth of body cells.
• The hormones stimulate the division of all body cells, but the bone and skeletal muscles are the most easily influenced.
• The release of growth hormone by the anterior pituitary is influenced by the growth hormone-releasing hormone and growth hormone-inhibiting hormone secreted by the hypothalamus.
• Growth hormone also affects the metabolism of certain organs in the body like the liver, heart, and kidneys.

2. Thyroid-stimulating hormone

• The thyroid-stimulating hormone stimulates the growth and activity of thyroid hormone, which itself is stimulated by the thyrotrophin releasing hormone from the hypothalamus.
• Thyroid-stimulating hormone is responsible for the regulation of the secretion of thyroid hormones, thyroxine (T4) and tri-iodothyronine (T3).

3. Adrenocorticotropic hormone

• The synthesis and release of adrenocorticotrophic hormone (ACTH) are regulated by the corticotrophin-releasing hormone from the hypothalamus.
• This hormone influences the adrenal cortex, which releases steroid hormones like cortisol.
• The level of ACTH is the highest at 8 am, which then falls to the lowest at about 6 pm.

4. Prolactin

• Prolactin is a hormone secreted during pregnancy that stimulates the mammary glands to produce milk.
• The production of prolactin is stimulated by prolactin-releasing hormone and inhibited by the prolactin inhibiting hormone, both produced by the hypothalamus.
• Besides, the sucking on the mammary gland post-childbirth also influences the levels of prolactin.

5. Gonadotrophin

• Gonadotrophins or sex hormones are released just before puberty in response to the luteinizing hormone-releasing hormone, also called gonadotrophin-releasing hormone.
• The appropriate levels of these hormones are essential at puberty to promote the mature functioning of the reproductive organs.
• The sex hormones in males and females that are involved in the proper functioning of the reproductive system are follicle-stimulating hormone and luteinizing hormone.

6. Oxytocin

• Oxytocin is released by the pituitary during childbirth as these affect the uterine smooth muscles and the muscles of the lactating breasts.
• Oxytocin is released by the posterior pituitary during childbirth to stimulate the sensory stretch receptors of the uterine cervix.
• Similarly, the suckling also generates a sensory impulse that influences the release of oxytocin which then triggers the contraction of milk ducts and ejection of milk.

7. Antidiuretic Hormone (ADH)

• Antidiuretic hormone or vasopressin is released by the posterior pituitary and is involved in the regulation of urine output. 
• Vasopressin affects the permeability of the distal convoluted tubules and collecting tubules in the kidney.
• The release of ADH is regulated by the osmotic pressure of the blood circulating and the osmoreceptors in the hypothalamus.

B. Thyroid Gland

• The thyroid gland is a butterfly-shaped bilobed gland that occurs on either side of the trachea.
• The two lobes of the gland are connected by a narrow piece of tissue called the isthmus.
• The two lobes of the gland function as a unit and produce two different types of hormones. The gland itself is made up of numerous microscopic spherical follicles surrounded by capillaries.
• The follicles are composed of epithelial cells that secrete the hormones into the lumen. The hormones are synthesized in the form of large precursor molecules called thyroglobulin.
• The thyroid gland is the only endocrine gland that can store large quantities of its secretory products, lasting up to 100 days.

Hormones of the Thyroid Gland

Thyroxine and Tri-iodothyronine

• Thyroxine and tri-iodothyronine are the two hormones secreted by the thyroid hormone in response to the thyroid-stimulating hormone secreted by the pituitary gland.
• The hormones are named T3 and T4 to indicate the number of iodine atoms present in the molecule.
• Thyroid hormones increase the basal metabolic rate (BMR), which regulates the level of oxygen consumption under standard conditions.
• These are also involved in the proper functioning of the sodium-potassium pumps that are essential for the electrolyte balance in the body.
• The thyroid gland and thyroid hormones contain most of the iodine occurring in the body. The hormones are lipid-soluble and thus diffuse through the plasma membrane into the interstitial fluid and then to the blood.

C. Parathyroid Gland

• Parathyroid glands occur as four small glands embedded in the posterior side of either lobe of the thyroid gland.
• All four glands function as a unit producing parathyroid hormone (PTH) that targets the cells of the bones and kidneys.
• The primary function of the hormone is to regulate the levels of calcium, magnesium, and phosphates in the blood.
• The glands are surrounded by fine connective tissue capsules consisting of spherical cells arranged in columns.
• The endocrine cells of the parathyroid glands are termed principal or chief cells that are polygonal with round nuclei and acidophilic cytoplasm.

Hormones of the Parathyroid Gland

Parathyroid Hormone

• Parathyroid hormone is responsible for the regulation of blood calcium levels, which influences the absorption of calcium from the digestive system.
• Parathormone and calcitonin serve as complementary hormones in order to maintain the appropriate blood calcium level in the body.
• The maintenance of the calcium levels in the body is important as it the major constituent of structures like teeth and bones.

D. Adrenal Gland

• Adrenal glands occur at the upper surface of each kidney which can be further divided into two distinct parts with different structures and functions.
• The glands occur as flattened structures with a crescent moon shape, and the size varies with the age and physiological condition of the person.
• The two parts of the glands occur in concentric regions; adrenal medulla and adrenal cortex.
• The cortex and medulla of the adrenal glands are generally considered two different glands as these have different embryonic origins, functions, and morphological features.
• The adrenal cortex originates from the mesoderm, whereas the medulla originates from the neural crest.
• The adrenal cortex is further divided into three distinct zones, each of which secretes different hormones. 
• The zona glomerulosa secretes mineralocorticoids, the zona fasciculata secretes glucocorticoids and the zona reticularis secretes androgens.
• The adrenal medulla occurs towards the center of the gland that consists of hormone-secreting chromaffin cells.
• The two important hormones secreted by the adrenal medulla are epinephrine and norepinephrine.

Hormones of the Adrenal Glands

1. Mineralocorticoids

• The primary mineralocorticoid is aldosterone which is involved in maintaining water and electrolyte balance in the body.
• The steroid hormone stimulates the reabsorption of sodium and excretion of potassium in the kidneys via a negative feedback mechanism.
• The secretion of aldosterone by the glands is regulated by the blood potassium level.

2. Glucocorticoids

• Cortisol is the primary glucocorticoid secreted by the adrenal cortex, but small amounts of corticosterone and cortisone can also be produced.
• The secretion of the hormone is regulated by the hypothalamus and the anterior pituitary.
• Glucocorticoids have different metabolic effects that are primarily concerned with the catabolism of biomolecules like proteins and fats.

3. Androgens

• Androgens are also secreted by the adrenal cortex in males, but the concentration of those androgens is so low that their effects are almost insignificant.
• In females, however, androgens are responsible for sex drive and can also be converted into estrogens by other tissues.
• Adrenal androgens stimulate the growth of auxiliary and pubic hair in males and females.
• The exact mechanism of regulation of androgen secretion is not yet understood, but it has been assumed that ACTH secretion is the principal drive.

4. Epinephrine or Adrenaline and Norepinephrine or Noradrenaline

• Adrenaline or epinephrine has a significant effect on the functioning of the heart and other metabolic processes. 
• Norepinephrine is a postganglionic neurotransmitter of the sympathetic branch of the autonomic nervous system, which is then released into the blood.
• The primary function of these hormones is to regulate the sympathetic nervous system.
• In some cases, norepinephrine is converted into epinephrine within the chromaffin cells of the adrenal medulla.

E. Pancreas

• The endocrine part of the pancreas is the clusters of cells, called pancreatic islets or Islet of Langerhans.
• These occur in the form of a cluster of cells scattered throughout the gland. The hormones produced by the cells are released directly into the bloodstream.
• The pancreatic islets consist of three different types of cells; α cells, β cells, and δ cells. The three different cells secrete three distinct hormones. A minor fourth type of cell also occurs in the pancreas called the PP cells.
• A pancreas has more than 1 million islets, most of which are concentrated in the tail region of the gland.
• The origin of pancreatic islets is the same as pancreatic acinar tissues, the epithelial outgrowth from the endoderm. The activity of these islets is regulated by the blood glucose level.

Hormones of the Pancreatic Islets

1. Insulin

• Insulin is produced by the β cells, which are the most abundant cells of the gland. Insulin is a polypeptide that functions to lower the blood glucose level.
• Even though insulin is often associated with glucose levels in the blood, it also monitors the levels of amino acids and fatty acids.
• The secretion of insulin is regulated by the blood glucose level and some degree of parasympathetic stimulation.

2. Glucagon

• Glucagon is produced by the α cells, which are involved in increasing the blood glucose by different metabolic pathways.
• The release of glucagon is stimulated by low blood glucose levels and decreased insulin levels.

F. Pineal Gland

• The pineal gland is the small gland present at the roof of the third ventricle attached to the surface by a short stalk composed of nerves.
• The gland develops from the embryonic neuroectoderm and consists of modified neurons called pinealocytes.

Hormones of the Pineal Gland

Melatonin

• Melatonin is the primary hormone secreted by the pineal gland, which is regulated by daylight and darkness.
• The level of melatonin is highest at midnight, which decreases to its lowest around midday.
• Melatonin works to coordinate the circadian and diurnal rhythms by influencing the hypothalamus.
• It is also involved in the inhibition of the growth and development of sex organs by affecting the synthesis of gonadotropins.

G. Thymus

• The thymus is a lymphoid organ that is present in the crest between the lungs and is involved in the synthesis of white blood cells.
• The thymus gland is only active until puberty, after which it begins to slowly shrink in size and is eventually replaced with fat.
• The thymus is a part of the immune system, and the normal functioning of the gland is thus essential to protect the body against autoimmunity.
• Before puberty, the thymus gland is significant in the production of T lymphocytes that protect the body against different antigens.

Hormone of the Thymus Gland

Thymosin

• Thymosin is the most important hormone produced by the thymus gland, which influences the immune response as well as stimulates the secretion of the pituitary gland.
• The hormone is also involved in the process of activation and differentiation of T cells into different types.

H. Gonads

• Gonads are the endocrine glands that secrete steroid sex hormones like the ones secreted by the adrenal cortex.
• The male gonads are the testes, whereas the female gonads are the ovaries.
• The ovaries occur in pairs and are located in the abdominal cavity of females. In addition to the formation of the female gamete, ovaries also produce different hormones, primarily estrogens, and progesterone.
• In males, testes occur in an extra-abdominal skin pouch called the scrotum which, in addition to the sperms, produce different male sex hormones.
• The hormones produced by the male and female gonads are the same as those produced by the adrenal cortex; however, the concentration of the hormones is usually higher in the case of these gonads.

Hormones of the Gonads

1. Estrogen

• Estrogen is produced by the cells in the ovaries, which are essential for the maturation of the reproductive organs and the appearance of secondary sex characters in the females at puberty.
• Estrogens are analogous to male steroid testosterone. The level of estrogen in the body increases during puberty in order to promote oogenesis and follicle growth.
• The activity of estrogen results in the proper functioning of different reproductive parts like the uterine tubes, uterus, and vagina.

2. Progesterone

• Progesterone is produced by the corpus luteum during the menstrual cycle, which regulates the cycles and activates changes in the cervical mucosa.
• Most of the effects of the hormone are observed during pregnancy as it inhibits uterine mobility. The level of progesterone increases more than that of estrogen during pregnancy.

3. Testosterone

• Testosterone is produced by the testes, which stimulates the maturation of sex organs, their development, and the appearance of secondary sex characters.
• The release of testosterone is maintained by the release of gonadotrophin-releasing hormone produced by the hypothalamus.

Functions of Endocrine System

The following are some of the functions of the endocrine system

1. The most important function of the endocrine system is to maintain the homeostasis of the body in order to ensure that the biochemical processes of different organs operate in a stable setting.
2. The hormones of the endocrine system are responsible for balancing the growth and differentiation of the body cells in order to enable the development of the individual.
3. The endocrine system also enhances the ability of the body to respond to different stressful internal and external factors.
4. Since the endocrine system consists of the reproductive organs of the body, these are essential for the development of reproductive behavior in individuals.
5. The endocrine system also performs different integrative functions of balancing the functions of different systems.
6. Endocrine glands like the thyroid gland are essential for the proper working of different metabolic processes.
7. The endocrine system remains in a close association with the nervous system and thus helps in maintaining a balanced relationship.

Endocrine System Video Lecture By Professor Dave Explains

Diseases and Disorders of Endocrine System

Disorders in the endocrine system occur as a result of hypersecretion or hyposecretion of the hormones. 

A. Diseases and Disorders of the pituitary gland

Hypersecretion

• The hypersecretion of the anterior pituitary hormones can result in gigantism and acromegaly.
• These conditions are the result of over secretion of the growth hormone-releasing hormone by the hypothalamus.
• Gigantism can be observed in the form of large bones and increased height. Acromegaly is characterized by large extremities as a result of the thickening of the bones and soft tissues.

Hyposecretion

• The hyposecretion of the pituitary gland can result in a number of disorders like ischaemic necrosis, dwarfism, and Frohlich’s syndrome.
• Ischaemic necrosis is characterized by the deficient stimulation of target glands and hyposecretion of all the glands controlled by the pituitary gland.
• Dwarfism is a commonly understood disorder as a result of the deficiency of the growth hormone in childhood.
• The hyposecretion of the ADH can result in a rare condition called diabetes insipidus, characterized by the loss of reabsorption of water by the renal tubules in the kidney.

B. Diseases and disorders of the thyroid gland

Hypersecretion

• The hypersecretion of thyroid hormone is called hyperthyroidism as a result of increased levels of T3 and T4.
• Hyperthyroidism can result in a group of diseases like Grave’s disease and toxic nodular goiter.
• Grave’s disease accounts for about 75% of the total cases of hyperthyroidism. It is an autoimmune condition where antibodies function as the thyroid-stimulating hormone.
• In the toxic nodular goiter, one or more nodules of the gland are affected by the goiter and become activated to secrete T3 and T4.

Hyposecretion

• Hyposecretion of thyroid hormone is called hypothyroidism which is often observed in the form of a simple goiter.
• Simple goiter can be detected by the enlargement of the thyroid gland due to the lack of T4 and T3 hormones.
• It also affects different metabolic processes in the body and might require surgical removal of the excess thyroid tissue.

C. Diseases and disorders of the adrenal gland

Hypersecretion

• The hypersecretion of the cortisol by the adrenal cortex results in Cushing’s syndrome characterized by excessive tissue breakdown, adiposity of the face, diminished protein synthesis and osteoporosis.
• Similarly, the hypersecretion of mineralocorticoids like aldosterone affects kidney functions, resulting in excessive reabsorption of ions and water.

Hyposecretion

• The hyposecretion of glucocorticoids results in reduced gluconeogenesis, low blood glucose levels, and muscle weakness.
• The hyposecretion of the mineralocorticoids affects the function of the kidney and causes blood sodium deficiency and potassium excess.

D. Diseases and disorders of the parathyroid gland

Hypersecretion

• The hypersecretion of parathyroid hormone results in increased blood calcium levels and is often caused as a result of a benign parathyroid tumor.
• The effects of hyperparathyroidism include muscle weakness, anorexia, constipation, and formation of renal calculi.

Hyposecretion

• The hyposecretion of parathormone causes hypocalcemia but is much rarer than hyperparathyroidism.
• It is often observed in the form of tetany, anxiety, or paraesthesia. Tetany results in a painful spasm of the skeletal muscles in the form of bending inwards of the hands, arms and feet.

E. Diseases and disorders of the pancreatic islets

Diabetes Mellitus

• Diabetes mellitus is the most common endocrine disorder that often leads to a varying degree of variation of carbohydrate and fat metabolism.
• Diabetes mellitus is caused by the complete absence or deficiency of insulin which affects glucose metabolism.
• Diabetes mellitus is of two types; Type 1 diabetes mellitus and Type 2 diabetes mellitus.
• Type 1 diabetes mellitus occurs in children and can be life-threatening if the onset is sudden.
• Type 2 diabetes mellitus is the more common form of the disorder accounting for about 90% of the total cases.
• The onset of Type 2 diabetes mellitus is usually gradual and can go undetected for a long period of time.

References

1. Hall JE and Guyton AC. (2011) Textbook of Medical Physiology. Twelfth Edition. Elsevier Saunders.
2. Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
3. Marieb EN and Hoehn K. (2013) Human Anatomy and Physiology. Ninth Edition. Pearson Education, Inc. 
4. Rastogi SC. (2007) Essentials of Human Physiology. Fourth Edition. New Age International Limited. 
5. Hiller-Sturmhöfel, S, and A Bartke. “The endocrine system: an overview.” Alcohol health and research world vol. 22,3 (1998): 153-64.
6. Hiller-Sturmhöfel S, Bartke A. The endocrine system: an overview. Alcohol Health Res World. 1998;22(3):153-64. PMID: 15706790; PMCID: PMC6761896.
7. Emanuele, N, and M A Emanuele. “The endocrine system: alcohol alters critical hormonal balance.” Alcohol health and research world vol. 21,1 (1997): 53-64.
8. Johnstone C, Hendry C, Farley A, McLafferty E. Endocrine system: part 1. Nurs Stand. 2014 May 27;28(38):42-9. doi: 10.7748/ns.28.38.42.e7471. PMID: 24844520.