Other Endocrine Glands

There are several other glands in the endocrine system, including the thymus gland, the pineal gland, and the parathyroid glands. There are also specialized endocrine cells in the brain, stomach, small intestine, liver, and other organs. The major endocrine glands and their functions are listed in Table 50-2.

Thymus Gland

The thymus (THIE-muhs) gland, located beneath the sternum and between the lungs, plays a role in the development of the immune system. The thymus gland secretes thymosin (THIE-moh-sin), an amino acid-based hormone that stimulates maturation of T cells, which help defend the body from pathogens.

Pineal Gland

The pineal (PIEN-ee-uhl) gland is located near the base of the brain, as shown in Figure 50-8. It secretes the hormone melatonin. Melatonin (mel-uh-TOH-nin) concentrations increase sharply at night and decrease dramatically during the day. This cyclic release of melatonin suggests that it helps regulate sleep patterns.

Parathyroid Glands

As Figure 50-9 shows, the four parathyroid glands are embedded in the back of the thyroid gland, two in each lobe. These glands secrete parathyroid hormone, which stimulates the transfer of calcium ions from the bones to the blood. Thus, parathyroid hormone has the opposite effect of calcitonin. A proper balance of calcium ions is necessary for cell division, muscle contraction, blood clotting, and neural signaling.

Digestive Cells

Endocrine cells within the walls of some digestive organs also secrete a variety of hormones that control digestive processes. For example, when food is eaten, endocrine cells in the stomach lining secrete gastrin (GAS-trin), a hormone that stimulates other stomach cells to release digestive enzymes and hydrochloric acid. Endocrine cells of the small intestine release secretin (si-KREE-tin), a hormone that stimulates the release of various digestive fluids from the pancreas.

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