Useful Notes on the 3 Major Endocrine Glands in Human Body

Major endocrine glands in our body are

i. Thyroid gland

ii. Parathyroid glands (four in humans)

iii. Thymus gland

iv. Hypothalamus gland

v. Pancreas (pancreatic islets clusters)

vi. Pituitary gland (anterior and posterior)

vii. Pineal gland

viii. Adrenals

ix. Gonads

In addition, endocrine cells of stomach, small intestine, liver, kidney, heart, placenta, and skin also secrete hormones.

1. Thyroid Gland:

The thyroid gland is a large endocrine gland located in the neck region just below the larynx in front of the trachea or windpipe. It has two lateral lobes one on either side of trachea. The two lobes are connected by a narrow mass of tissue called isthmus.

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The thyroid gland has a rich blood supply. Thus, thyroid gland can deliver large amount of hormones in a short period of time, if necessary. Four small round parathyroid glands are embedded in the posterior surface of the thyroid gland.

Thyroxine:

i. It regulates basal metabolism by stimulating rate of cellular oxidation resulting into energy production and maintenance of body temperature;

ii. It regulates growth and development of the body, ossification of bones and mental development;

iii. It regulates activities of the nervous system.

Undersecretion (hyposecretion) as well as oversecretion (hypersecretion) of thyroxine results in an abnormal growth of the body.

Undersecretion (Hypothyroidism):

Undersecretion of thyroxine may cause,

Simple Goitre:

In this condition, the thyroid gland of adults enlarges and becomes visible as a swelling in the neck. Insufficient secretion of thyroxine or insufficient amount of iodine in diet may cause simple goitre. Simple goitre is commonly found in people living in hilly areas because the soil of hilly areas is deficient in iodine. Thus, the food crops grown there have less of iodine content.

Cretinism:

This is caused due to defective development or early atrophy (degeneration) of thyroid gland. This condition is observed in children. Children suffering from cretinism have stunted growth, short club-like fingers, deformed bones and teeth. Their abdomen becomes pot-bellied and skin becomes rough, dry with scanty hair growth. Mental retardation of various degrees is also observed.

Myxoedema:

The hypothyroidism in adults causes myxoedema. In this condition, facial tissues swell and look puffy. Other symptoms include slow heart rate, low body temperature, sensitivity to cold, dry hair and skin, muscular weakness and general lethargy.

Oversecretion (Hyperthyroidism):

Oversecretion of thyroxine may cause exophthalmic goitre. A person suffering from this disorder shows increased metabolic rate, rapid heartbeat, protruding eyes and short breathing rate.

Calcitonin (Not included in the syllabus):

Calcitonin is the another hormone secreted by thyroid gland. It

i. Regulates blood calcium and phosphate levels in the blood;

ii. Facilitates absorption of calcium released by bones.

Parathyroid:

Parathyroid gland secretes parathormone which promotes movement of calcium ions from the bones to the blood and causes ossification.

2. Adrenal Glands:

In our body, two adrenal glands are present, one on top of each kidney; hence, they are also called suprarenal glands. Each adrenal gland has following two parts:

i. An outer adrenal cortex, and

ii. An inner adrenal medulla.

Adrenal Cortex:

Adrenal cortex secretes many hormones. They may be divided into the following main categories glucocorticoids and mineralocorticoids.

Glucocorticoids:

Glucocorticoids are group of hormones such as Cortisol, corticosterone and cortisone. Of the three, Cortisol is the major hormone. Glucocorticoids,

i. Regulate the metabolism of protein, fats and carbohydrates in the body;

ii. Regulate the blood-sugar level and ensure energy supply to the body;

iii. Adapt the body to external stress such as severe heat or cold, infections and burns, etc.

iv. Certain cortical hormones act as sex-hormone causing premature sexual maturity in children.

Mineralocorticoids (Aldosterone):

Aldosterone is major mineralocorticoid secreted by adrenal cortex. Aldosterone,

i. Controls reabsorption of sodium in urinary tubules and maintains Na⁺ and K⁺ ratio in the extracellular and intracellular fluids;

ii. Regulates salt-water balance in the body.

Adrenal Medulla:

Adrenal medulla secretes two major hormones— adrenalin (also known as epinephrine) and nor-adrenalin (nor-epinephrine). Adrenalin accounts for almost 80 per cent of the total secretion of the adrenal medulla.

Both adrenalin and nor-adrenalin, together control emotions, fear, anger, blood pressure, heartbeat, respiration and relaxation of smooth muscles. Adrenalin is also known as emergency hormone as it prepares body for fight or flight situation. It increases heart beat, increases blood supply to muscles and decreases blood supply to visceral organs. We have high levels of adrenalin

3. Pancreas:

Pancreas is a compound gland located posterior to the stomach and attached to the duodenal loop in the abdominal region. It secretes both digestive juice as well as hormones. Pancreas is also referred as exoendocrine gland. It has two parts—

(i) An exocrine (duct) part, which produces digestive juices, and

(ii) Endocrine (ductless) part, which secretes hormones.

Its endocrine part contains hormone secreting cells called Islets of Langerhans, which are scattered in the entire gland (islets: little islands). The Islets of Langerhans in pancreas contain beta, alpha and delta cells that secrete insulin, glucagon and somatostatin hormones, respectively.

Insulin:

Insulin is secreted by beta cells of Islets of Langerhans.

i. It regulates blood sugar level by regulating conversion of glucose into glycogen.

Whenever there is increase in blood glucose, insulin is secreted which induces uptake of glucose and glucose is burnt or stored as glycogen. This reduces the blood glucose level.

ii. It stimulates deposition of extra glucose as glycogen in the liver and muscles.

Glucagon:

Glucagon is secreted by alpha cells of Islets of Langerhans.

i. It stimulates glycogen break down and some carbohydrates back to glucose;

ii. It increases sugar level in the blood.