Chapter 112. Endocrine System

1. The endocrine system is a relatively slow-acting chemical communication system of glands that secrete hormones—chemical messengers that are produced in one tissue and travel through the bloodstream to affect other tissues, including the brain. The pituitary gland is considered the “master gland” of the endocrine system, but the pituitary is actually controlled by the hypothalamus, which is part of the brain.

2. The glands of the endocrine system are distributed throughout the body, from the head to the lower abdomen. The major glands are listed here. In the head are the pineal gland and pituitary gland. In the throat are the thyroid gland and parathyroid glands. In the chest is the thymus gland. In the abdomen are the adrenal glands, pancreas, and ovaries or testes.

3. Hormones from the endocrine system influence many aspects of our lives, including growth, reproduction, metabolism, mood, and reactions to stress. For example, during emergencies, the adrenal glands, following orders from the autonomic nervous system, release epinephrine and norepinephrine. These hormones provide a surge of energy by increasing heart rate, blood pressure, and blood sugar. During the normal daily routine of food consumption, fluctuations in blood sugar are controlled by the pancreas, which contains special cells that release insulin, a hormone that regulates the utilization and storage of blood sugar.

4. The thyroid gland influences body metabolism. Improper functioning of the thyroid gland can lead to dramatic weight gain or weight loss. The parathyroid glands (which are only visible on the back surface of the thyroid gland) are specialized for controlling the amount of calcium in the blood.

5. The thymus gland is very active during childhood, when it helps produce the white blood cells that attack viruses and cancer cells. After it has done its job, it shrinks in size and becomes less active.

6. The ovaries and testes produce a group of sex hormones. The primary female hormones are estradiol and progesterone, which produce the female sex characteristics such as breast development, as well as prepare the uterus for pregnancy. The primary male sex hormone is testosterone, which produces the male sex characteristics and plays a role in sex drive and aggressive behavior in both males and females.

7. The pineal gland, located deep in the center of the brain, produces a hormone called melatonin, which regulates the sleep-wake cycle. The pituitary gland is the endocrine system's most influential gland. It releases hormones that influence growth, and it regulates the release of hormones by other glands.

8. Compared with the speed at which messages move through the nervous system, messages carried by hormones in the endocrine system move more slowly, but their effects usually last longer and affect a wider range of tissues.