Endocrine glands are tissues that secrete substances called hormones into the bloodstream. These glands are located in several places in the body and secrete hormones that influence an enormous range of bodily functions and processes. Endocrine glands include the hypothalamus, pituitary, adrenals, parathyroid, pineal, thyroid, islets of Langerhans in the pancreas, ovaries and testes. The uterus and placenta of a pregnant woman also are considered part of the endocrine system.
Collectively, endocrine glands regulate bodily processes that occur slowly. Examples of such functions include metabolism, cell growth and puberty. In contrast, the nervous system controls functions that occur rapidly, such as movement. The nervous system and endocrine system have different overall functions, but they are not entirely distinct systems, and they do work in conjunction with one another to regulate the body’s processes. These two systems are linked by a part of the brain called the hypothalamus.
Endocrine glands influence the way the body works by producing hormones. These molecules act as messengers that tell various cell types what to do and when to do it. There are many types of hormones, but most cell types can interact with only a limited number of them. Each endocrine gland produces a few specific hormones and secretes them as needed in response to hormonal signals produced by other endocrine glands.
The most important of the endocrine glands is the pituitary gland. Considered the “master gland” of the endocrine system, it secretes hormones that regulate the activity of the thyroid, ovaries, testes and adrenal glands. The pituitary gland also produces hormones that control the growth of body tissues, initiate uterine contractions during labor and lactation in breastfeeding women, regulate the body’s ability to feel pain and help the body balance water levels. Also in the brain is the pineal gland. This gland secretes melatonin, which is involved in regulating the sleep-wake cycle.
The thyroid and parathyroid endocrine glands, located in the neck, regulate metabolism and calcium balance. The thyroid produces hormones that control the pace at which cells burn fats and other fuels for energy, as well as the pace of other chemical reactions in the body. The parathyroid glands produce a hormone that controls the level of calcium available in the bloodstream.
The adrenal glands, located on top of the kidneys, produce two important types of hormones: epinephrine and corticosteroids. Epinephrine, or adrenaline, is produced in response to stress and has the effect of increasing blood pressure and heart rate. Corticosteroids are important in several body processes, including the stress response, immune function and sexual function.
Two important hormones are produced in the pancreas: glucagon and insulin. These hormones regulate blood sugar levels, stored energy levels and the conversion of sugar or stored energy into usable energy to fuel cellular chemical reactions. Both hormones are produced in an area of the pancreas called the islets of Langerhans.
Sex hormones are produced by two endocrine glands: the testes in men and the ovaries in women. In men, the testes produce androgens such as testosterone and control body changes that occur during puberty, as well as sperm production. In women, the ovaries produce the hormones estrogen and progesterone, controlling body development during puberty as well as controlling the menstrual cycle.