Why are hormones released directly into the bloodstream?

Discover how the human endocrine system disperses hormones from glands through the bloodstream

Discover how the human endocrine system disperses hormones from glands through the bloodstream

Hormones secreted by the glands of the endocrine system are carried in the bloodstream to distant target cells.


NARRATOR: The basic units of this system are special kinds of glands. Unlike other glands, endocrine glands secrete their products, called hormones, directly into the bloodstream. The powerful hormones travel everywhere in the body, influencing every aspect of life in very specific ways.

Many parts of the body secrete hormones, but six major types of glands are of particular importance to adults. They are the islets of Langerhans in the pancreas, the thyroid and parathyroids, the adrenals, the gonads, and in the head, the pituitary.

Every major endocrine gland is a highly structured organ. Throughout each run the smallest blood vessels, capillaries. Hormones are secreted into the capillaries. Through a microscope, it is possible to see red blood cells as they move through the capillaries and veins. Hormones, however, are not actually visible. They are in solution in the blood plasma, just as salt [music in] is in solution in seawater and is invisible. The hormones enter the general circulation. Eventually, they filter back to the capillaries elsewhere in the body. Unlike the red blood cells, hormones travel freely through the capillary walls. Though they travel everywhere in the body, hormones only influence specific target cells. A hormone may have relatively few targets, affecting only a few kinds of cells.

A different hormone may affect nearly every cell in the body. Every target cell has a chemical receptor, which the hormone specifically fits as a key specifically fits a lock. The receptor may be on the surface of the cell, or inside it. A cell that is not a target for a particular hormone has no receptors for it and is not affected by it.