THE ENDOCRINE SYSTEM

The endocrine system—the other communication system in the body—is made up of endocrine glands that produce hormones, chemical substances released into the bloodstream to guide such processes as metabolism, growth, and sexual development. Hormones are also involved in regulating emotional life.
The Thyroid Gland
The thyroid gland secretes thyroxin, a hormone that can reduce concentration and lead to irritability when the thyroid is overactive, and cause drowsiness and a sluggish metabolism when the thyroid is underactive.
The Parathyroid Glands
Within the thyroid are four tiny pea-shaped organs, the parathyroids, that secrete parathormone to control and balance the levels of calcium and phosphate in the blood and tissue fluids. This, in turn, affects the excitability of the nervous system.
The Pineal Gland
The pineal gland is a pea-sized gland that apparently responds to exposure to light and regulates activity levels over the course of the day.
The Pancreas
The pancreas lies in a curve between the stomach and the small intestine and controls the level of sugar in the blood by secreting insulin and glucagon.
The Pituitary Gland
The pituitary gland produces the largest number of different hormones and therefore has the widest range of effects on the body's functions. The posterior pituitary is controlled by the nervous system. It produces two hormones: vasopressin, which causes blood pressure to rise and regulates the amount of water in the body's cells, and oxytocin, which causes the uterus to contract during childbirth and lactation to begin. The anterior pituitary, often called the "master gland," responds to chemical messages from the bloodstream to produce numerous hormones that trigger the action of other endocrine glands.
The Gonads
These reproductive glands—the testes in males and the ovaries in females, and, to a lesser extent, the adrenal glands—secrete androgens (including testosterone) and estrogens.
The Adrenal Glands
The two adrenal glands are located above the kidneys. Each has two parts: an outer covering, the adrenal cortex, and an inner core, the adrenal medulla. Both influence the body's responses to stress. For example, in response to a stressful situation, the pituitary gland may release beta endorphin and ACTH, which, in turn, prompt the adrenal cortex to release hormones. Meanwhile, the autonomic nervous system stimulates the adrenal medulla to secrete hormones such as epinephrine into the bloodstream.

The glands of the endocrine system.