Glands are aggregates of specialized cells that secrete or excrete chemical substances which are used elsewhere in the body. Glands carry out regulatory, digestive, reproductive, and other functions in the body. A gland may be an independent structure or may be incorporated into another, larger, structure that has still other functions. In addition, a gland can be endocrine, secreting its hormones directly into the blood stream without a duct; or it can be exocrine, secreting its products through a duct into the digestive tract, onto the skin, or other target areas.
The two adrenal glands, one atop each kidney, are endocrine glands that secrete various hormones, including epinephrine (adrenalin) corticosteroids, and mineralocorticoids that are part of the body's response to stressful situations.
The islets of Langerhans of the pancreas are endocrine glands that secrete the hormones insulin and glucagon, which lower and raise the levels of blood glucose (sugar). The pancreas, too, is an exocrine gland, for it also secretes digestive enzymes (pancreatic juice) through ducts which lead into the duodenum of the small intestine. Other endocrine glands include the thyroid gland, the parathyroid glands, the testes and the ovaries, the thymus gland, and the pituitary gland.
Other exocrine glands include the lacrimal glands, which manufacture and secrete tears, the salivary glands, which secrete saliva, the liver, which manufactures and secretes bile, the mammary glands, which manufacture and secrete milk, and the eccrine sweat glands of the skin, which secrete sweat to regulate body temperature. The kidneys are glands in that the juxtaglomerular cells of the nephrons secrete renin, which helps to regulate blood pressure.
Glands increase or decrease their activities in response to changes in body temperature, salinity, temperature, and other stimuli, most of which are coordinated by control centers in the brain.