Chemical Control And Integration

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Diagrammatic representation of hormones produced in Human body

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Diagrammatic representation of the various endocrine hormones produced by the glands and tissues in a human body.

Hormones in animals

Hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts.
e.g. Insulin is a hormone produced by the beta-cells of the pancreas, It acts on every cell in the body and stimulates glucose uptake by the cells.
Types of hormones based on chemical nature
  1. The hormones which are secreted by endocrine glands are classified into three groups based on their chemical nature;
  2. Protein/peptide hormones (e.g. Insulin)
  3. Steroid hormones (e.g. estrogen)
  4. Amino acid derivative hormone (e.g. Thyroxine)

Properties of Hormones

  1. The hormones, irrespective of their chemical nature (protein or steroid or amino acid derivatives), have certain common properties.
  2. Hormones are produced by specialized glands and are secreted directly into the bloodstream.
  3. Hormones exert their effects not where they are locally produced, but on other tissues of the body.
  4. Hormones are transported by the bloodstream from the endocrine cells to serve as a chemical messenger.
  5. Hormones, unlike enzymes, do not catalyze any metabolic reactions
  6. They act on target cells or organs by regulating the rates of specific metabolic reactions.
  7. Hormones are required in very small amounts and their effect may be excitatory or inhibitory depending upon their concentration and the physiological state of the responding tissue.

Hormones of the hypothalamus

The hypothalamus produces various hormones which act on pituitary gland stimulating it to release hormones or stop/inhibit certain hormone secretions. The various releasing and inhibiting hormones produced by hypothalamus are 

  1. Adrenocorticotropin releasing hormone (ARH)
  2. Thyrotropin releasing hormone (TRH)
  3. Growth hormone releasing hormone (GHRH)
  4. Growth hormone inhibitory hormone (GHIH)
  5. Gonadotropin releasing hormone (GnRH)
  6. Prolactin releasing hormone (PRH)
  7. Prolactin inhibiting hormone (PIH)
  8. MSH releasing hormone (MSHRH)
  9. MSH inhibiting hormone (MSHIH)

Pituitary gland- Detailed structure

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The pituitary gland is the 'master gland' located at the base of the brain. It is attached to the hypothalamus with an infundibulum (stalk). The structure of pituitary can be anatomically divided into 2; Adenohypophysis and Neurohypophysis
  1. It consists of 3 portions; pars distalis, pars intermedia and pars tuberalis.
  2. Pars distalis region of pituitary is referred to as anterior pituitary.
  3. Pars intermedia region is almost merged with pars distalis in humans.
  4. Pars tuberalis wraps the pituitary stalk with a sheath of cells.
  5. The anterior pituitary produces various hormones (e.g. Growth hormone)
  1. It is also called pars nervosa/ posterior pituitary gland.
  2. A major portion of the posterior pituitary gland consists of unmyelinated axons from hypothalamic neurosecretory neurons.
  3. It stores the hormones (oxytocin and vasopressin) produced by the neurons of the hypothalamus.

Hormones of the Thyroid gland

The follicular cells of the thyroid gland synthesize two thyroid hormones; Tetraiodothyronine (T-Thyroxine) and triiodothyronine (T) and the C-cells of thyroid gland produces Thyrocalcitonin. The function of these hormones are as follows

  1. Thyroxine is the main hormone (80% in thyroxine form) secreted into the bloodstream by the thyroid gland. 
  2. It is the inactive form and most of it is converted to an active form called as triiodothyronine (T) by organs such as the liver and kidneys.
  3. Both T and T are synthesized from tyrosine and iodine.
  4. The thyroid hormones control the metabolism of carbohydrates, protein and fats. They play an important role in regulating the Basal Metabolic Rate (BMR)
  5. They are also important for maintenance of water and electrolyte balance. 
  6. Thyroid hormones also regulate the RBC production.
  7. The thyrocalcitonin maintains the blood calcium levels and promotes the bone formation.

Disorders of Thyroid gland

The two most common disorders associated with the thyroid gland are hyperthyroidism and hypothyroidism. 
  • It is a condition in which the thyroid is overactive, i.e., overproduction (higher than normal) of thyroid hormones.
  • It is a condition in which the thyroid is underactive, i.e., underproduction (lower than normal) of thyroid hormones.

Hormones of the Pancreas

The hormones produced by the endocrine part of the pancreas and their functions are:
  1. It is a polypeptide hormone.
  2. It is secreted by the pancreas when the glucose level in the blood drops.
  3. It increases the blood glucose levels by initiating glycogenolysis (glycogen degradation) and stimulating gluconeogenesis (synthesis of glucose from non-carbohydrate molecules) in the liver cells.
  4. Since it increases the blood glucose levels it is a hyperglycemic (hyper-more, glycemic-sugar levels) hormone.
  5. It also initiates lipolysis (degradation of lipid) in the adipose tissue.
  1. It is also a polypeptide hormone.
  2. It has antagonistic (opposite) effects to the hormone glucagon.
  3. It is released when the glucose level in the blood raises.
  4. It lowers blood sugar levels in different ways; increasing the uptake of glucose by all the cells in the body, initiating glycogenesis (glycogen synthesis) in the liver and muscle.
  5. It initiates lipogenesis (synthesis of lipids/fat) and storage of fat in adipose tissue and inhibits gluconeogenesis.
  6. It is a hypoglycemic (hypo=less) hormone as it reduces the sugar levels in the blood.
  1. Also known as growth hormone-inhibiting hormone (GHIH).
  2. It acts locally on the Islets of Langerhans and inhibits the secretion of both insulin and glucagon.
  3. It also reduces gastric secretions and inhibits the release of gastrointestinal hormones.
Pancreatic Polypeptide
  1. It is a polypeptide secreted by the cells of the pancreas.
  2. Protein is a strong stimulus for its production.
  3. It affects gastric and biliary secretion and motility of the GI tract.

Hormones of the Adrenal gland

The adrenal cortex and the adrenal medulla regions of the kidneys secrete several hormones that have a wide range functions in the body.
Hormones of the Adrenal Cortex

  1. The hormones produced by the adrenal cortex are called the corticoids
  2. Two types of corticoids are produced; Mineralocorticoids and Glucocorticoids.
  3. Mineralocorticoids regulate water and electrolyte balance.
  4. Glucocorticoids regulate the carbohydrate metabolism.
  5. Aldosterone is the main mineralocorticoid produced by the adrenal cortex.
  6. Aldosterone acts by stimulating the reabsorption of water and sodium and also causes excretion of potassium and phosphate ions. 
  7. Cortisol, cortisone and corticosterone are the glucocorticoids produced by the adrenal cortex.
  8. Glucocorticoids control the blood glucose levels by stimulating gluconeogenesis, lipolysis and proteolysis. 
  9. Cortisol also has anti-inflammatory activity and has the ability to suppress the immune response.
  10. Androgens are also produced by the adrenal cortex and they are responsible for the growth of facial hair, pubic hair etc., 
Hormones of the Adrenal medulla

  1. The medulla produces the catecholamines which include the hormones epinephrine (adrenaline) and norepinephrine (noradrenaline)
  2. These are released under stress conditions and are called the emergency hormones or hormones of fight or flight.
  3. Both the hormones stimulate glycogenolysis, lipolysis and proteolysis.
  4. They increase the heartbeat and rate of respiration

Hormones of the Ovary

The ovaries secrete two types of female sex hormones called the estrogens and the progestins. 
  1. In non-pregnant females, they are predominantly secreted by the ovaries. However, during pregnancy, the placenta secretes a lot of estrogens.
  2. The three types of estrogens are; -estradiol, estrone and estriol.
  3. The most active and the principal estrogen produced by the ovaries is the  -estradiol. 
  4. They play an important role in the growth and development of ovarian follicles and breast
  5. They are responsible for the development of secondary sexual characteristics (e.g. high pitch voice)
    They inhibit osteoclastic activity and stimulate bone growth. Osteoporosis in menopausal women is because of reduced estrogen levels.
  6. They also influence the texture of the skin and female sexual behaviour
Progestins (Progesterone)
  1. The progestins are secreted by the corpus luteum, a structure that is formed from the ruptured follicle (after ovulation/ later half of the ovarian cycle).
  2. Most important progestin is the progesterone. Small amounts of 17--hydroxyprogesterone is also secreted along with progesterone.
  3. Placenta also secretes large amounts of progesterone during pregnancy.
  4. The progestins mainly have 2 functions; prepare the uterus for pregnancy and prepare breasts for lactation.
  5. Progesterone acts on mammary glands and stimulates the formation of milk storing sacs called alveoli and milk secretion.

Hormones of the Testis

The testis produces various male hormones collectively called as the androgens.

  1. The androgens include; testosterone, dihydrotestosterone and androstenedione.
  2. Testosterone is considered to be the primary hormone as it is produced abundantly when compared to others.
  3. However, dihydrotestosterone is the more active form. The testosterone is ultimately converted to the active form in the target tissues.
  4. Androgens regulate the development, maturation and function of the male sex organs (e.g. epididymis, seminal vesicles etc.,)
  5. They play an important role in spermatogenesis and male sexual behaviour
  6. They also promote protein and carbohydrate synthesis.

Hormones of the kidneys

The kidneys secrete 3 different types of hormones; erythropoietin, renin and calcitriol (1,25- dihydroxycholecalciferol). 
  1. Erythropoietin and renin are peptide hormones secreted by the juxtaglomerular cells of the kidney
  2. Calcitriol is a steroid hormone synthesized by the cells of the proximal tubules of the nephrons.
  1. Its production is stimulated by hypoxia (insufficient oxygen supply to the tissues).
  2. It stimulates the formation of RBC from the haematopoietic stem cells in the bone marrow.
  1. Decreased blood pressure, low sodium levels are the stimuli for the production of renin from the kidney.
  2. The renin secreted has enzymatic activity and is an important part of the renin-angiotensin-aldosterone system (RAAS).
  3. The RAAS plays an important role in regulating cardiac output and arterial pressure.
  4. The main function of renin in the RAAS system is the conversion of angiotensinogen to angiotensin I (proteolytic activity).
Calcitriol (1,25- dihydroxycholecalciferol)
  1. It is the active form of vitamin D.
  2. It stimulates the formation of calcium-binding protein thereby increasing the intestinal absorption of calcium.
  3. Thus, it plays an important role in the mineralization of the bone (bone formation).