Endocrine system: glands

A gland is an organ that makes and releases a substance. The body has two kinds. Exocrine glands, such as sweat, oil, and salivary glands, release their products through tubes called ducts, to the skin or into a body cavity. Endocrine glands have no ducts. They release hormones straight into the blood. Today you will meet the major endocrine glands.

The hypothalamus is a small region at the base of the brain. It is part of the nervous system, but it also controls the endocrine system, making it the link between the two. Directly beneath it hangs the pituitary gland, about the size of a pea. It makes hormones such as growth hormone, and it also releases hormones that tell other glands what to do, which is why it is nicknamed the "master gland." Even so, the pituitary takes its orders from the hypothalamus.

The thyroid gland sits in the front of the neck, shaped like a butterfly. It releases hormones that set the speed at which your cells use energy, and it needs iodine to make them. Four tiny parathyroid glands sit behind the thyroid and release a hormone that raises blood calcium by drawing it from bones, which you learned about earlier in the unit. The two adrenal glands sit on top of the kidneys. They release adrenaline for emergencies and cortisol, a hormone that helps your body respond to stress.

The pancreas lies behind the stomach, and it is special because it is both. As an exocrine gland, it sends digestive enzymes through a duct into the small intestine. As an endocrine gland, it releases insulin, which lowers blood sugar, and glucagon, which raises it. The ovaries in females and the testes in males release the sex hormones estrogen and testosterone. The pineal gland in the brain releases melatonin, which helps regulate sleep.

Glands often work in chains. For example, the hypothalamus signals the pituitary, the pituitary signals the thyroid, and the thyroid hormones then act on cells throughout the body. When thyroid hormone levels are high enough, they signal the hypothalamus and pituitary to slow down, which is feedback that keeps levels steady.

A common mistake is thinking all glands are endocrine; sweat glands are not. Another is believing the pituitary is in charge of everything. It is the master of many glands, but the hypothalamus directs it, and feedback from hormones in the blood keeps it in check. Next: the circulatory system, which carries hormones to their targets.