The endocrine system consists of a group of glands and organs that regulate and control various body functions by producing and secreting hormones. Hormones are chemical substances that affect the activity of another part of the body. In essence, hormones serve as messengers, controlling and coordinating activities throughout the body. Endocrine glands release their hormones directly into the bloodstream (as opposed to exocrine glands, which release hormones or other substances into a duct).
The individual organs that make up the endocrine system have different and often unrelated functions. Doctors who specialize in disorders of the endocrine system are known as endocrinologists. Many endocrinologists further subspecialize in the functions and disorders of specific glands.
The major glands of the endocrine system, each of which produces one or more specific hormones, are the
The hypothalamus (a small region of the brain that connects to the pituitary gland) secretes several hormones that control the pituitary gland. The pituitary gland is sometimes called the master gland because it controls the functions of many other endocrine glands.
During pregnancy, the placenta also acts as an endocrine gland in addition to its other functions.
Not all organs that secrete hormones or hormonelike substances are considered part of the endocrine system. For example, the kidneys produce the hormone renin to help control blood pressure and the hormone erythropoietin to stimulate the bone marrow to produce red blood cells. In addition, the digestive tract produces a variety of hormones that control digestion, affect insulin secretion from the pancreas, and alter behaviors, such as those associated with hunger. Fat (adipose) tissue also produces hormones that regulate metabolism and appetite.
Additionally, the term "gland" does not necessarily mean that the organ is part of the endocrine system. For example, sweat glands, salivary glands, glands in mucus membranes, and mammary glands are called exocrine glands, because they secrete substances other than hormones and because they secrete the substances into ducts, not directly into the bloodstream. The pancreas is both an endocrine and exocrine gland.