MT Training > Endocrine

The endocrine system consists of glands which release specific chemical substance called hormones directly into the bloodstream rather than into ducts leading to the exterior of the body. Those glands that send their chemical substances into ducts and out of the body are called exocrine glands. Examples of exocrine glands are sweat, mammary, mucous, salivary and lacrimal (tear) glands.

1. Endocrine Glands or Ductless Glands are

1)      Thyroid gland

2)      Parathyroid glands(four glands)

3)      Adrenal glands (one pair)

4)      Pancreas (islets of langerhans)

5)      Pituitary gland

6)      Ovaries in female (one pair)

7)      Testes in male (one pair)

Endocrine tissue (apart from the major glands) is reviewed in table-1-1.

Table 1-1, ENDOCRINE TISSUE (APART FROM MAJOR GLANDS): LOCATION SECRETION, AND ACTION

1. Thyroid Gland

The thyroid gland is composed of a right and a left lobe on either side of the trachea, just below a large piece of cartilage called the thyroid cartilage. The thyroid cartilage covers the larynx and produces the prominence on the neck known as the Adam’s apple. The isthmus of the thyroid gland is a narrow strip of glandular tissue that connects the two lobes on the ventral surface of the trachea.

Hormones secreted by the thyroid gland are

1.         thyroxine (T4)

2.         triiodothyronine (T3).

These hormones are synthesized in the thyroid gland from iodine, which is picked up from the blood circulating through the gland, and from an amino acid called tyrosine. T4 is much more concentrated in the blood. Whereas T3 is far more potent in affecting the metabolism of cells. Most thyroid hormone is bound to protein molecules as it travels in the bloodstream.

T4 and T3 are necessary in the body to maintain a normal level of metabolism in all body cells. Cells need oxygen to carry on metabolic processes, one aspect of which is the burning of food to release the energy stored within the food. Thyroid hormone aids cells in their O₂ uptake and thus supports the metabolic rate in the body. Injections of thyroid hormone will raise the metabolic rate. Whereas removal of the thyroid gland, diminishing thyroid hormone content in the body, will result in a lower metabolic rate, heat loss, and poor physical and mental development.

A more recently discovered hormone produced by the thyroid gland is called calcitonin (thyrocalcitionin). Calcitionin is secreted when calcium levels in the blood are high. It stimulates calcium to leave the blood and enter the bones, thus lowering blood calcium back to normal.

2. Parathyroid Glands

The parathyroid glands are four small oval bodies located on the dorsal aspect of the thyroid gland.

Parathyroid hormone :(PTH) is secreted by the parathyroid glands. This hormone (also known as Parathormone) mobilizes calcium (a mineral substance) from bones into the bloodstream, where calcium is necessary for the proper functioning of body tissue, especially muscles, Normally, calcium in the food we eat is absorbed from the intestine and carried, by the blood, to the bones, where it is stored. The adjustment of the level of calcium in the blood is a good example of the way hormones in general control the homeostasis (equilibrium or constancy in the internal environment) of the body. If there is a decrease in blood calcium (as in pregnancy or rickets, a vitamin D-deficiency disease), parathyroid hormones is secreted in larger amounts to cause calcium to leave the bones and enter the bloodstream. Thus, blood calcium levels are brought back to normal. Conversely, any situation of increase in calcium in the bloodstream, such as excess quantity of calcium or vitamin D in the diet, will lead to decreased parathyroid hormone secretion (calcium then leaves the blood to enter bones), decreasing blood calcium, so that homeostasis is again achieved.

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