Endocrine Glands

In humans, there are more than a dozen tissues and organs that produce hormones. These can be listed under two categories:

1. Exclusively endocrine: the pituitary, the thyroid, the parathyroid, thymus and the adrenals.
2. Partially endocrine: The pancreas, gastric and duodenal epithelium, the gonads (testis in males and ovary in females) and placenta in females.

1. Pituitary - the master gland

The pituitary gland (also called hypophysis) is a small projection (about the size of a pea) which hangs from the base of the mid-brain. It is connected to the hypothalamus of the brain by the pituitary stalk. The hypothalamus, although a part of the brain, also secretes some hormones.

The pituitary controls most other endocrine glands. It has two distinct parts: the anterior pituitary and the posterior pituitary.

2. Thyroid

Thyroid is a bilobed structure situated in the front region of the neck. It secretes two hormones - thyroxine and calcitonin. Thyroxine regulates basal metabolism i.e. the rate of cellular oxidation resulting in heat production. Calcitonin regulates the calcium and phosphate levels in the blood.

3. Parathyroids

These are two small pairs of glands wholly or partially embedded in the thyroid gland. Their secretion parathormone raises blood calcium level by stimulating release of calcium from bones.

4. Thymus

It is located at the base of neck. It produces some hormones involved in maturation of T lymphocytes. It begins to atrophy after puberty.

5. Adrenals

The adrenals are a pair of glands situated like caps one above each kidney. Each adrenal consists of two parts: a central medulla and a peripheral cortex.

6. Pancreas

Pancreas is an endocrine as well as an exocrine gland. It has special groups of cells called Islets of Langerhans, which consists of three kinds of cells - alpha cells producing the hormone glucagon, beta cells producing hormone insulin and gamma cells producing hormone somatostatin.

7. Gonads (Testis and Ovary)

Testes in males possess two kinds of cells: the sperm-producing germinal cells and the hormone-producing interstitial cells. The hormones produced are called androgens and the commonest one among them is testosterone.

The testosterone stimulates the development of the male characters during which the body at puberty starts developing facial hair, and their voice cracks and deepens.

Ovaries in females produce two kinds of hormones - estrogen and progesterone. Estrogen is secreted from the follicles of the ovary and stimulates the development of breasts and fat deposition on the hip in a mature woman. Estrogen prepares the wall of the uterus for receiving the fertilized egg.

Progesterone is secreted by the corpus luteum (follicle left after the release of ovum). It brings about the final changes in the uterus for the retention and growth of the foetus during pregnancy.

8. Placenta

Placenta of a pregnant woman produces certain hormones. One such hormone is human chorionic gonadotropin (HCG), which maintains the activity of corpus luteum in secreting progesterone continuously, when a women becomes pregnant.

9. Hormones from Stomach and Intestine

Gastrin is the hormone secreted by the mucus membrane of the pyloric end of the stomach. It stimulates the gastric glands to secrete gastric juice.

Secretin is the hormone secreted by the inner lining of the duodenum. It stimulates the production of pancreatic juice while the hormone cholecystokinin stimulates release of bile from gall bladder.