Chapter XXIII

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The endocrine system encompasses a number of glands that secrete hormones important for a wide variety of body functions. As you have learned, “endocrine” means “secreting .” This refers to secretion directly into blood or lymph vessels, as opposed to “” glands that secrete substances “outward,” namely through a duct to a different body part or outside of the body altogether (e.g., in perspiration). For this reason, endocrine glands are sometimes called “ductless.” (Note that some glands typically categorized as endocrine technically do not secrete directly into the blood or lymph.) The word “gland” itself comes ultimately from the Latin glans (“”) of which the diminutive form glandulae was used in antiquity to describe the glands of the throat. “Hormone” comes from the Greek horme, “”; this name is given on account of the fact that these chemicals “set in motion” or regulate some activity in an organ, including in some cases the regulation of the production of other hormones.

Several endocrine glands are found in the brain. The gland gets its name from its tiny "pine-cone" shape. Located in the diencephalon, the pineal gland secretes melatonin and is involved in the regulation of sleep, mood, puberty, and ovarian cycles. The gland is found in the base of the brain and secretes hormones that regulate the functioning of the thyroid gland, gonads, adrenal cortex, and other endocrine organs. Its name is actually a misnomer, coming from the Latin for “mucus,” as it was originally thought to produce nasal mucus. It is also known as the “” (lit. "under-growth"); the specific portion responsible for hormone secretion is called the “adrenohypophysis.” The ("under the thalamus") also has secretory functions that regulate the production of other hormones.

Other glands include the thyroid gland, located under the thyroid (“-shaped”) cartilage in the neck, which forms the “Adam’s apple.” Among other things, the thyroid produces and stores hormones that are critical for growth rate and other metabolic functions. The production of thyroid hormones is itself regulated by the pituitary gland. Nearby are the ("next to the thyroid") glands, normally four in number and located at the back of the thyroid gland, although there is variation in this arrangement; these glands produce a hormone that serves as a major regulator of calcium and phosphorous metabolism. The pancreas (see p. 224) contributes in similar fashion to the digestive process in both an exocrine capacity (by secreting digestive enzymes into the duodenum) and an endocrine capacity (by producing insulin, which metabolizes carbohydrates and fats).

The sex organs, testes in males and ovaries in females, are also considered part of the endocrine system because of the hormones they secrete: androgens (esp. testosterone) for males, estrogens and progesterone for females. These hormones play an important role in the respective sexual development of both genders. Androgens are also produced by the glands, located as the name suggests “next to the kidney.” These glands also produce steroid hormones, in addition to serving in various other capacities.