What is Endocrine System? Hormones of the hypothalamus and pituitary gland

What is Endocrine System?

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

  • The glands of the endocrine system are called 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 usually slower and the influence is much wider.
  • The endocrine system also depends on the cardiovascular system for the distribution of its products. As a result, the endocrine glands are some of the most vascular tissues in the body.
  • The hormones secreted by the endocrine glands are effective in very small amounts; therefore, the circulating levels of these hormones are usually low.
  • The endocrine system also contains cells and tissues that are not exclusively classified as endocrine glands, but instead contain cells that can secrete hormones.
  • Some of these 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 cells, along with the diagnosis and treatment of disorders that involve the endocrine system, is called endocrinology.
  • Unlike most other systems in the body, the glands of the endocrine system are not clustered and remain dispersed throughout the body.
  • The different hormonal systems of the endocrine organs help in the regulation of all bodily functions such as metabolism, growth, electrolyte balance, reproduction, and behavior.
  • Most endocrine glands are regulated by a simple negative feedback mechanism, while secretion is controlled by the nervous and immune systems.

Glands and Hormones of the Endocrine System

The endocrine system consists of the following glands;

Hypothalamus and pituitary gland

  • The hypothalamus is a small organ located in the brain 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 is considered the connecting link between the endocrine system and the nervous system.
  • The hypothalamus cells alone secrete about nine different hormones, of which seven hormones are involved in the regulation of the pituitary gland.
  • The hormones produced in the hypothalamus are called inhibitory or releasing hormones.
  • The pituitary gland is a small pea-shaped organ measuring approximately 1-1.5 cm in diameter, located in the pituitary 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 the adenohypophysis, is made up of epithelial cells and represents approximately 70% of the total pituitary gland.
  • The anterior pituitary has 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.
  • Hormones from the posterior pituitary are synthesized in nerve cell bodies and transported along axons to be stored at axon terminals. Nerve stimuli from the hypothalamus regulate the exocytosis of the vesicles to release hormones into the bloodstream.
  • The general secretion of hormones by the pituitary is regulated by a negative feedback mechanism.

Hormones of the hypothalamus and pituitary gland

Growth Hormone

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

2. Thyroid-stimulating hormone

  • Thyroid-stimulating hormone stimulates the growth and activity of thyroid hormone, which in turn is stimulated by a thyrotropin-releasing hormone from the hypothalamus.
  • Thyroid-stimulating hormone is responsible for regulating the secretion of thyroid hormones, thyroxine (T4) and triiodothyronine (T3).

3. Adrenocorticotropic hormone

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

4. Prolactin

  • Prolactin is a hormone secreted during pregnancy that stimulates the mammary glands to produce milk.
  • Prolactin production is stimulated by prolactin-releasing hormone and inhibited by prolactin-inhibiting hormone, both produced by the hypothalamus.
  • In addition, the suction of the mammary gland after delivery also influences prolactin levels.

5. Gonadotropin

  • Gonadotropins or sex hormones are released just before puberty in response to luteinizing hormone-releasing hormone, also called gonadotropin-releasing hormone.
  • Adequate levels of these hormones are essential at puberty to promote the mature functioning of the reproductive organs.
  • The sex hormones in men and women that are involved in the proper functioning of the reproductive system are follicle-stimulating hormone and luteinizing hormone.

6. Oxytocin

  • The pituitary releases oxytocin during childbirth, as they affect the smooth muscles of the uterus and the muscles of the lactating breasts.
  • Oxytocin is released from the posterior pituitary during labor to stimulate the sensory stretch receptors of the cervix.
  • Similarly, sucking also generates a sensory impulse that influences the release of oxytocin which then triggers the contraction of the milk ducts and the expulsion of milk.

7. Antidiuretic hormone (ADH)

  • Antidiuretic hormone or vasopressin is released by the posterior pituitary and participates in the regulation of urine production.
  • 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 circulating blood and osmoreceptors in the hypothalamus.

Thyroid gland

  • The thyroid gland is a butterfly-shaped, bilobed gland found on both sides of the windpipe.
  • The two lobes of the gland are connected by a narrow piece of tissue called the isthmus.
  • The two lobes of the gland work as one 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 made up of epithelial cells that secrete hormones into the lumen. Hormones are synthesized in the form of large precursor molecules called thyroglobulin.
  • The thyroid gland is the only endocrine gland that can store large amounts of its secretory products, lasting up to 100 days.

Hormones of the thyroid gland

Thyroxine and Tri-iodothyronine

  • Thyroxine and triiodothyronine are the two hormones secreted by the thyroid hormone in response to the thyroid-stimulating hormone secreted by the pituitary gland.
  • The hormones are called 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 and potassium pumps that are essential for the balance of electrolytes in the body.
  • The thyroid gland and thyroid hormones contain most of the iodine present in the body. Hormones are fat-soluble and therefore diffuse through the plasma membrane into the interstitial fluid and then into the blood.

Parathyroid gland

  • The parathyroid glands are formed as four small glands embedded in the backside of each lobe of the thyroid gland.
  • The four glands function as a parathyroid hormone (PTH) producing unit that targets cells in the bones and kidneys.
  • The main 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 called principal or principal cells which are polygonal with round nuclei and acidophilic cytoplasm.

Hormones of the parathyroid gland

Parathyroid hormone

  • Parathyroid hormone is responsible for the regulation of calcium levels in the blood, which influences the absorption of calcium from the digestive system.
  • Parathormone and calcitonin serve as complementary hormones to maintain the proper level of calcium in the blood in the body.
  • Maintaining calcium levels in the body is important, as it is the main component of structures such as teeth and bones.

Adrenal gland

  • The adrenal glands are located on the upper surface of each kidney, which can be divided into two distinct parts with different structures and functions.
  • The glands appear as flattened structures shaped like a crescent moon and the size varies with the age and physiological condition of the person.
  • The two parts of the glands are in concentric regions; adrenal medulla and adrenal cortex.
  • The cortex and medulla of the adrenal glands are generally considered two different glands as they have different embryonic origins, functions, and morphological characteristics.
  • The adrenal cortex originates from the mesoderm, while the medulla originates from the neural crest.
  • The adrenal cortex is 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 is located towards the center of the gland that consists of hormone-secreting chromaffin cells.
  • The two important hormones secreted by the adrenal medulla are adrenaline and norepinephrine.

Hormones of the adrenal glands

1. Mineralocorticoids

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

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 mainly related to the catabolism of biomolecules such as proteins and fats.

3. Androgens

  • Androgens are also secreted by the adrenal cortex in men, but the concentration of these androgens is so low that their effects are almost negligible.
  • In women, however, androgens are responsible for sexual desire and can also be converted to estrogens by other tissues.
  • Adrenal androgens stimulate the growth of pubic and auxiliary hair in men and women.
  • The exact mechanism of regulation of androgen secretion is not yet understood, but it has been assumed that ACTH secretion is the main drive.

4. Epinephrine or adrenaline and norepinephrine or norepinephrine

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

E. Pancreas

  • The endocrine part of the pancreas is a group of cells called pancreatic islets or islets of Langerhans.
  • These occur in the form of a group 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 different hormones. A fourth minor type of cell called PP cells is also found in the pancreas.
  • A pancreas has more than 1 million islets, most of which are concentrated in the tail region of the gland.
  • The origin of the pancreatic islets is the same as that of the pancreatic acinar tissues, the epithelial growth of the endoderm. The activity of these islets is regulated by the level of glucose in the blood.

Hormones of the pancreatic islets

1. Insulin

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

2. Glucagon

  • Glucagon is produced by α cells, which are involved in raising blood glucose through different metabolic pathways.
  • Glucagon release is stimulated by low blood glucose levels and decreased insulin levels.

F. Pineal gland

  • The pineal gland is the small gland present in the roof of the third ventricle attached to the surface by a short stem made up of nerves.
  • The gland develops from the embryonic neuroectoderm and consists of modified neurons called pinealocytes.
  • Pineal gland hormones

Melatonin

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

G. Thymus

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

A hormone of the Thymus Gland

Thymosin

  • Thymosin is the most important hormone produced by the thymus gland, influencing the immune response and stimulating secretion from the pituitary gland.
  • The hormone is also involved in the process of activating and differentiating T cells into different types.

H. Gonads

  • The gonads are the endocrine glands that secrete steroid sex hormones such as those secreted by the adrenal cortex.
  • The male gonads are the testes, while 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, the ovaries also produce different hormones, mainly estrogens, and progesterone.
  • In men, the testicles are located in an extra-abdominal bag of skin called the scrotum that, in addition to sperm, produces 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 hormone concentration is usually higher in the case of these gonads.

Gonad hormones

1. Estrogen

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

2. Progesterone

  • Progesterone is produced by the corpus luteum during the menstrual cycle, which regulates cycles and activates changes in the cervical mucosa.
  • Most of the effects of the hormone are seen 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 the sexual organs, their development, and the appearance of secondary sexual characteristics.
  • Testosterone release is maintained by the release of gonadotropin-releasing hormone produced by the hypothalamus.

Endocrine system functions

The following are some of the functions of the endocrine system.

  • The most important function of the endocrine system is to maintain the homeostasis of the body to ensure that the biochemical processes of different organs operate in a stable environment.
  • The hormones of the endocrine system are responsible for balancing the growth and differentiation of cells in the body to allow the development of the individual.
  • The endocrine system also improves the body’s ability to respond to different internal and external stressors.
  • Since the endocrine system is made up of the body’s reproductive organs, these are essential for the development of reproductive behavior in individuals.
  • The endocrine system also performs different integrative functions of balancing the functions of different systems.
  • Endocrine glands such as the thyroid are essential for the proper functioning of different metabolic processes.
  • The endocrine system remains in close association with the nervous system and therefore helps to maintain a balanced relationship.

Diseases of Endocrine System

Endocrine system disorders occur as a result of hypersecretion or under-secretion of hormones.

 Diseases and disorders of the pituitary gland

Hypersecretion

  • Hypersecretion of anterior pituitary hormones can lead to gigantism and acromegaly.
  • These conditions are the result of excessive secretion of growth hormone-releasing hormone by the hypothalamus.
  • Gigantism can be seen 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

  • Hyposecretion of the pituitary gland can result in several disorders such as ischemic necrosis, dwarfism, and Frohlich syndrome.
  • Ischemic necrosis is characterized by poor stimulation of the target glands and hyposecretion of all glands controlled by the pituitary gland.
  • Dwarfism is a disorder commonly understood as a result of growth hormone deficiency in childhood.
  • The hyposecretion of ADH can result in a rare condition called diabetes insipidus, characterized by the loss of water reabsorption by the renal tubules in the kidney.

B. Diseases and disorders of the thyroid gland. 

Hypersecretion

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

Hyposecretion

  • The hyposecretion of thyroid hormone is called hypothyroidism, which is often seen in the form of simple goiter.
  • Simple goiter can be detected by an enlarged thyroid gland due to a lack of T4 and T3 hormones.
  • It also affects different metabolic processes in the body and may require surgical removal of excess thyroid tissue.
  • C. Diseases and disorders of the adrenal gland.

Hypersecretion

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

Hyposecretion

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

D. Diseases and disorders of the parathyroid gland.

Hypersecretion

  • Hypersecretion of parathyroid hormone results in increased blood calcium levels and is often due to a benign parathyroid tumor.
  • The effects of hyperparathyroidism include muscle weakness, anorexia, constipation, and kidney stone formation.

Hyposecretion

  • Hyposecretion of parathyroid hormone causes hypocalcemia but is much rarer than hyperparathyroidism.
  • It is often seen in the form of tetany, anxiety, or paresthesia. Tetany produces a painful spasm of the skeletal muscles in the form of an inward bending of the hands, arms, and feet.

E. Diseases and disorders of the pancreatic islets.

Mellitus diabetes

  • Diabetes mellitus is the most common endocrine disorder that often leads to varying degrees of variation in carbohydrate and fat metabolism.
  • Diabetes mellitus is caused by the absence or total deficiency of insulin that 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 most common form of the disorder, accounting for approximately 90% of all cases.
  • The onset of type 2 diabetes mellitus is usually gradual and can go unnoticed for a long period.

ref- https://microbenotes.com/endocrine-system/

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