Endocrine system includes endocrine glands and their secretions (hormones).
Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
HUMAN ENDOCRINE (DUCTLESS) GLANDS
8. Pancreas (Islets of Langerhans)
9. Gonads (Testis & Ovary)
Neurosecretory cells (nuclei) of hypothalamus secrete the following types of hormones:
§ Releasing hormones: Stimulate secretion of pituitary hormones. E.g. gonadotropin releasing hormone (GnRH) stimulates release of gonadotrophins from pituitary.
§ Inhibiting hormones: Inhibit secretion of pituitary hormones. E.g. Somatostatin inhibits release of growth hormone from pituitary.
§ Oxytocin & vasopressin: These are transported axonally and stored in pituitary. (See pituitary gland).
2. PITUITARY GLAND
- Smallest endocrine gland.
- It is located in a bony cavity called sella tursica and is attached to hypothalamus by a stalk.
- It is divided into Adenohypophysis & Neurohypophysis.
It has 2 parts: Pars distalis and Pars intermedia.
i. Pars distalis (Anterior pituitary): It produces
¨ Somatotropin or Growth hormone (GH): For body growth. Over-secretion of GH causes Gigantism (abnormal growth). Hyposecretion of GH causes Dwarfism (stunted growth).
¨ Prolactin (PRL): Regulates growth of mammary glands and formation of milk.
¨ Thyroid stimulating hormone (TSH): Stimulates secretion of thyroid hormones from thyroid gland.
¨ Adrenocorticotrophic hormone (ACTH): Stimulates the synthesis and secretion of steroid hormones called glucocorticoids from the adrenal cortex.
¨ Luteinizing hormone (LH): Stimulate gonadal activity. In males, it stimulates the synthesis and secretion of androgens from testis. In females, it induces ovulation and maintains the corpus luteum.
¨ Follicle stimulating hormone (FSH): Stimulates gonadal activity. In males, FSH & androgens regulate spermatogenesis. In females, FSH stimulates the growth and development of the ovarian follicles.
ii. Pars intermedia: In human, it is almost merged with pars distalis. It produces Melanocyte stimulating hormone (MSH). It acts on melanocytes to regulate pigmentation of skin.
Store Oxytocin & Vasopressin (hormones of hypothalamus).
i. Oxytocin: Contracts the smooth muscles. In females, it stimulates contraction of uterus at the time of child birth, and milk ejection from the mammary gland.
ii. Vasopressin or Anti-diuretic hormone (ADH): Stimulates reabsorption of water and electrolytes by DCT of kidney and thereby reduces diuresis (loss of water through urine).
3. PINEAL GLAND
It is located on dorsal side of forebrain. Secretes melatonin.
Functions of melatonin:
· Regulates diurnal (24-hour) rhythm of body. E.g. maintenance of sleep-wake cycle, body temperature etc.
· Influences metabolism, pigmentation & menstrual cycle.
· Influences our defense capability.
4. THYROID GLAND
- Largest endocrine gland. It includes 2 lobes located on either side of the trachea. The lobes are interconnected with isthmus (a connective tissue).
- Thyroid gland is composed of follicles & stromal tissues.
Follicular cells secrete the following hormones:
§ Thyroxin (tetraiodothyronine, T4) & Triiodothyronine (T3): Their functions are
- Regulation of basal metabolic rate (BMR).
- Support RBC formation.
- Control metabolism of carbohydrates, proteins & fats.
- Maintenance of water and electrolyte balance.
§ Thyrocalcitonin (TCT): A protein hormone. It regulates (lowers) the blood calcium levels.
Iodine is essential for normal hormone synthesis in thyroid.
- Enlargement of thyroid gland due to deficiency of iodine.
- In adult women, it causes irregular menstrual cycle.
- Hypothyroidism during pregnancy affects the baby causing stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism etc.
Hyperthyroidism (Exophthalmic goiter):
- Abnormal increase of thyroid hormones resulting in adverse effects on the physiological activities.
- It is caused due to development of the nodules or the cancer of thyroid gland.
5. PARATHYROID GLAND
4 parathyroid glands are present on back side of the thyroid gland, one pair each in the two lobes of thyroid gland. They secrete Parathyroid hormone (PTH) – a peptide hormone.
Functions of parathyroid hormone:
· Increases Ca2+ level in blood (hypercalcaemic hormone).
· It stimulates the bone resorption (demineralization).
· Stimulates the reabsorption of Ca2+ by the renal tubules and increases Ca2+ absorption from the digested food.
· Along with TCT, it helps in calcium balance in the body.
It is located on dorsal side of the heart and aorta. It secretes Thymosins (peptide hormones). Thymus is degenerated in old individuals. So production of thymosins decreases. As a result, immune responses of old persons become weak.
Functions of thymosins:
· Differentiation of T-lymphocytes, which provide cell-mediated immunity.
· Promote production of antibodies for humoral immunity.
7. ADRENAL GLAND
It has 2 parts: Adrenal cortex & Adrenal medulla.
a. Adrenal cortex
It has 3 layers: inner zona reticularis, middle zona fasciculata & outer zona glomerulosa.
It Produces corticoid hormones such as
o Glucocorticoids (mainly cortisol):
- Involved in carbohydrate metabolism.
- Stimulate gluconeogenesis, lipolysis and proteolysis.
- Inhibit cellular uptake and utilization of amino acids.
- Maintain cardiovascular system and kidney functions.
- Cortisol stimulates RBC production.
- Produces anti-inflammatory reactions and suppress immune response.
o Mineralocorticoids (mainly aldosterone):
- Regulate the water (body fluid volume), electrolytic balance, osmotic pressure and blood pressure.
- Aldosterone stimulates the reabsorption of Na+ & water from renal tubules and excretion of K+ and PO43- ions.
o Androgenic corticoids: Role in growth of axial hair, pubic hair and facial hair during puberty.
b. Adrenal medulla
- Produce catecholamine hormones such as Adrenaline (epinephrine) & Noradrenaline (norepinephrine).
- They are rapidly secreted in response to any stress emergency situations so called emergency hormones (hormones of Fight or Flight).
- These increase alertness, pupilary dilation, piloerection (rising of hairs), sweating, heartbeat, heart contraction and rate of respiration. Stimulate the breakdown of glycogen thereby increase glucose in blood. Also stimulate the breakdown of lipids and proteins.
8. PANCREAS (ISLETS OF LANGERHANS)
- A composite (Heterocrine) gland i.e. exocrine + endocrine.
- Islets of Langerhans are the endocrine part. There are about 1-2 million Islets (1-2% of pancreatic tissue).
- α cells and β cells in the islets secrete peptide hormones such as Glucagon and Insulin respectively. They maintain Glucose homeostasis in blood.
§ Glucagon: Hyperglycemic factor. They
- Maintain normal glucose level.
- Acts on hepatocytes and stimulates glycogenolysis resulting in an increased blood sugar (hyperglycemia).
- Stimulates gluconeogenesis.
- Reduces the cellular glucose uptake and utilization.
§ Insulin: Hypoglycemic factor. They
- Regulate glucose homeostasis.
- Acts on hepatocytes and adipocytes and enhances cellular glucose uptake and utilization. So glucose from blood rapidly moves to hepatocytes and adipocytes. Thus blood glucose level decreases (hypoglycemia).
- Stimulates glycogenesis (glucose converts to glycogen).
Prolonged hyperglycemia leads to Diabetes mellitus (loss of glucose through urine and formation of harmful compounds like ketone bodies). Treatment is insulin therapy.
9. GONADS (TESTIS & OVARY)
- A pair of testis (male primary sex organ) is present in the scrotal sac. Testis is formed of seminiferous tubules and interstitial (stromal) tissues.
- Leydig (interstitial) cells in the inter-tubular spaces produce hormones called androgens (mainly testosterone). They
o Regulate the development, maturation and functions of the accessory sex organs like epididymis, vas deferens, seminal vesicles, prostate gland, urethra etc.
o Have role in spermatogenesis.
o Stimulate male sexual behavior (libido), growth of muscles, hairs, aggressiveness, low pitch voice etc.
o Help in anabolism of protein and carbohydrate.
- A pair of ovaries (female primary sex organ) is located in the abdomen.
- Ovary is formed of ovarian follicles and stromal tissues.
After ovulation the ruptured follicles form a structure called Corpus luteum.
- Ovarian follicles produce Estrogen and Corpus luteum produce Progesterone.
- Estrogen (a steroid hormone): It stimulates
o Growth and activities of female secondary sex organs.
o Development of ovarian follicles.
o Female secondary sex characters (e.g. high pitch voice) and sexual behavior.
o Development of Mammary gland.
- Progesterone (a steroid hormone): It
o Supports pregnancy.
o Acts on mammary glands to stimulate formation of alveoli (sacs to store milk) and milk secretion.
HORMONES OF HEART, KIDNEY & GASTROINTESTINAL TRACT
1. Atrial wall of heart: Produce a peptide hormone called Atrial natriuretic factor (ANF). When blood pressure increases, ANF causes the dilation of blood vessels and thereby reduces the BP.
2. JGA of kidney: Produce Erythropoietin (peptide hormone). Stimulates erythropoiesis.
3. Gastro-intestinal tract: Produce peptide hormones like
o Gastrin: Acts on gastric glands and stimulates the secretion of HCl and pepsinogen.
o Secretin: Acts on exocrine pancreas and stimulates secretion of water and bicarbonate ions.
o Cholecystokinin (CCK): Acts on both pancreas and gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively.
o Gastric inhibitory peptide (GIP): Inhibits gastric secretion.
Several other non-endocrine tissues secrete hormones called growth factors. These are essential for the normal growth of tissues and their repairing or regeneration.
Based on the chemical nature, hormones are various types:
a. Peptide, polypeptide, protein hormones: E.g. insulin, glucagon, pituitary hormones, hypothalamic hormones etc.
b. Steroids: E.g. cortisol, testosterone, estradiol & progesterone.
c. Iodothyronines (thyroid hormones).
d. Amino-acid derivatives: E.g. Adrenaline, nor-adrenaline.
MECHANISM OF HORMONE ACTION
- Hormones produce their effects by binding to the specific proteins (hormone receptors) located in the target tissues.
- Hormone receptors include membrane-bound receptors and intracellular receptors (mostly nuclear receptors).
- A hormone binds to its receptor to form hormone-receptor complex. Each receptor is specific to one hormone only.
- Hormone-receptor complex formation leads to certain biochemical changes in the target tissue and thereby metabolism and physiological functions are regulated.
- Hormones which interact with membrane-bound receptors normally do not enter the target cell, but generate second messengers (e.g. cyclic AMP, IP3, Ca2+ etc.) which in turn regulate cellular metabolism.
- Hormones which interact with intracellular receptors (e.g. steroid hormones, iodothyronines) mostly regulate gene expression or chromosome function by the interaction of hormone-receptor complex with the genome. Cumulative biochemical actions result in physiological and developmental effects.