The endocrine system is a widespread group of glands and organs that act as the body's control system for producing, storing, and secreting chemical substances called hormones.
The primary glands that compose the endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal, ovary, and testes. The pancreas, considered both an organ and a gland, is also part of the system. The thymus is sometimes considered an endocrine-system organ. Although not part of the endocrine system, other organs that secrete hormones are the heart, brain, lungs, kidneys, liver, skin, and placenta. The word “endocrine” means that in response to specific stimuli, the hormones produced by the glands are released into the bloodstream.
Disorders of the endocrine system often result from the over- or underproduction of a particular hormone. Too much or too little of a hormone can be harmful. The endocrine organs use a feedback mechanism to regulate hormone levels. It acts much like a household thermostat, increasing production of a specific hormone when it detects too little in the blood, or decreasing production when it detects too much or the right amount. Tight control of hormone levels is needed for the body to function properly. The endocrine organs secrete hormones directly into the bloodstream, where special proteins usually bind to them, helping to maintain them as they travel through the body.
Hormones are compounds produced by the endocrine glands. They generally control the growth, development, and metabolism of the body; the electrolyte composition of body fluids; and reproduction. The specific functions of the endocrine glands and pancreas are unique.
The pituitary is the master gland of the endocrine system. Located at the base of the brain, the gland, which is about the size of a marble, consists of two parts: anterior and posterior. The anterior pituitary produces hormones that either stimulate other glands (such as adrenal, testis, ovary, and thyroid) to produce targetgland hormones, or directly affect the target organs.
Three of these hormones—adrenocorticotropic hormone (ACTH), gonadotropins, and thyroid-stimulating hormone (TSH)—act on other glands. ACTH stimulates the adrenal cortex to produce corticosteroid hormones and small amounts of male and female sex hormones. Gonadotropins are two hormones that regulate the production of male and female sex hormones and the egg (ova) and sperm (spermatozoa) cells. TSH stimulates the thyroid gland to produce and release thyroid hormone.
Two hormones, vasopressin and oxytocin, are stored but not produced in the posterior pituitary. Vasopressin, also called arginine vasopressin (AVP), helps the body to conserve water by increasing reabsorption of water from the kidneys. Oxytocin stimulates contractions in the uterus during childbirth and activates milk ejection caused by an infant sucking on the breast.
The adrenals are small glands on top of the kidneys. The adrenals have two parts: an outer layer called the cortex and an inner layer called the medulla. The adrenal cortex produces a variety of hormones called corticosteroids, including hydrocortisone (cortisol), which helps increase blood glucose levels. It also reduces the amount of glucose absorbed by muscles and adipose tissue. Another function of cortisol is to protect the body from the adverse affects of stress, including emotional and physical trauma.
The adrenal medulla produces adrenaline and noradrenaline, substances that increase the heart rate and blood pressure during times of stress. Their action is referred to as the “fight-or-flight” response.
The thyroid gland is composed of two sections in front of the windpipe and below the voice box. It produces two hormones, thyroxine (T4) and triiodothyronine (T3), which together are called the thyroid hormones. They help regulate growth and development and help in childhood brain development. The thyroid also contains cells that produce the hormone calcitonin, which helps to maintain normal calcium levels in the blood.
The parathyroid complex is composed of four small glands, each the size of a pea and located on one of the four corners of the thyroid gland. They secrete parathyroid hormone, which regulates calcium levels in the blood.
The pancreas is located in the upper abdomen, just behind and below the stomach. It has two functions: to produce various enzymes that aid in digestion and to produce insulin and glucagon, hormones that are key to the body's management of glucose (sugar) in the blood.
The primary purpose of insulin is to lower blood-glucose levels in the body. It helps form glycogen, proteins, and lipids, which are stored in the body (usually in the liver, muscles, and adipose tissue) to be used for energy. Glucagon increases blood-glucose levels, an action opposite to that of insulin. A strict balance between the glucagon and insulin is required to maintain proper blood-sugar levels.
Located deep inside the brain, the hypothalamus maintains direct control of the pituitary gland. It acts as the central “control room” of the endocrine system, directing the activities of the other parts of the system. The hypothalamus controls eating and drinking, sexual behavior, blood pressure, heart rate, body temperature, emotions, and the sleeping/waking cycle. When the brain receives information indicating that hormonal changes are needed somewhere in the body, the hypothalamus secretes chemicals that either stimulate or suppress hormone production in the pituitary gland.
The pineal is located in the center of the brain. This gland secretes melatonin, a hormone that helps regulate the sleeping/waking cycle. Disturbances in the production of melatonin cause jet lag, experienced by many long-distance travelers. Melatonin also influences development of the male and female sex glands. The thymus processes lymphocytes in infants and is partly responsible for immune system development.
The ovaries and testes, also called the sex glands, produce cells and hormones essential to reproduction and development of the body, including male and female sex characteristics. The three types of sex hormones are estrogens, progestogens, and androgens (including testosterone).
The main role of estrogens is to coordinate development and function of the female genitalia and breasts. Estrogens are also associated with the start of the menstrual bleeding cycle. Estrogen production in the ovaries ceases during menopause. Estrogen is also produced in men (by the testes), though at lower levels than occur in women.
The primary androgen produced in the testes is the steroid testosterone. While mainly associated with male development, testosterone is produced in small amounts in women by the ovaries. During pregnancy, testosterone helps to develop the internal and external male sex organs. In males, testosterone promotes the growth of the sex organs and develops or stimulates male characteristics, such as deepening voice; growth of facial, pubic, and other body hair; and muscle growth and strength. In adult males, testosterone maintains the masculine characteristics and sexual potency and regulates sperm production.
There are two basic classes of endocrine conditions: problems associated with hormone-production levels, and problems caused by tissues that are unable to respond to hormones. Hormone-production disorders are broken into two groups: insufficient hormone production, called hypofunction, and too much hormone production, called hyperproduction. Endocrinesystem disorders include the following:
A wide variety of symptoms can indicate a hormonal imbalance in the body A specific group of symptoms, however, may give an initial indication of a problem in the endocrine system. Excessive thirst, frequent urination, and unexplained weight loss are classic signs of diabetes mellitus, the most common endocrine disorder.
Many primary care physicians still treat endocrine problems, especially diabetes, themselves. However, the primary care doctor often makes a preliminary diagnosis and then refers the patient to an endocrinesystem specialist, called an endocrinologist.
As previously discussed, the endocrine system is responsible for hormone activity throughout the body. Hormone secretion moves from various endocrine glands to the organs, controlling many important physiologic functions.
Effective hormone control is crucial, and exercise aids in regulating and balancing hormones by assisting in the release and reception of hormones for optimal balance of these substances. Physiologically, exercise serves to strengthen endocrine receptor sites on their target organ cells, as well.
Effects of the endocrine system are associated with the “fight-or-flight” response, so it is important to note that regular exercise increases blood circulation throughout the body, bringing oxygen to the brain and muscles while increasing the release of endorphins (naturally occurring opiates). Endorphins are associated with positive emotion (mood) and increased energy. Regular, systematic exercise and good nutritional practices enhance not only the endocrine system, but the cardiovascular and musculoskeletal systems as well.
Exercise-induced hormonal changes have positive effects on temperament, mental mood, concentration, and libido. In addition, these hormonal changes result in reduced anxiety and a decreased sensitivity to pain.
As exercise boosts the number of hormones circulating throughout the body, engaging in regular physical activity along with maintaining a healthy diet can help minimize weight gain, improve weight loss, build lean muscle, enhance muscle tone, and improve overall mental well-being. An active, balanced endocrine system is essential to good health.
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Endocrine Society, 2055 L St. NW, Ste. 600, Washington, DC, 20036, (202) 971-3636, (888) 363-6274, Fax: (202) 736-9705, email@example.com, http://www.endocrine.org/ .
Ken R. Wells
Revised by Laura Jean Cataldo, RN, EdD