endocrine system (ĕn`dəkrĭn), body control system composed of a group of glands that maintain a stable internal environment by producing chemical regulatory substances called hormones hormone, secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue.
..... Click the link for more information. . The endocrine system includes the pituitary gland pituitary gland, small oval endocrine gland that lies at the base of the brain . It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system ).
..... Click the link for more information. , thyroid gland thyroid gland, endocrine gland, situated in the neck, that secretes hormones necessary for growth and proper metabolism. It consists of two lobes connected by a narrow segment called the isthmus. The lobes lie on either side of the trachea, the isthmus in front of it.
..... Click the link for more information. , parathyroid glands parathyroid glands (pâr'əthī`roid)
..... Click the link for more information. , adrenal gland adrenal gland (ədrēn`əl) or suprarenal gland
..... Click the link for more information. , pancreas pancreas (păn`krēəs), glandular organ that secretes digestive enzymes and hormones.
..... Click the link for more information. , ovaries ovary, ductless gland of the female in which the ova (female reproductive cells) are produced. In vertebrate animals the ovary also secretes the sex hormones estrogen and progesterone , which control the development of the sexual organs and the secondary sexual
..... Click the link for more information. , and testes (see testis testis (tĕs`tĭs) or testicle
..... Click the link for more information. ). The thymus gland thymus gland (thī`məs), mass of glandular tissue located in the neck or chest of most vertebrate animals.
..... Click the link for more information. , pineal gland pineal gland (pĭn`eəl), small organ (about the size of a pea) situated in the brain .
..... Click the link for more information. , and kidney (see urinary system urinary system, group of organs of the body concerned with excretion of urine, that is, water and the waste products of metabolism. In humans, the kidneys are two small organs situated near the vertebral column at the small of the back, the left lying somewhat higher
..... Click the link for more information. ) are also sometimes considered endocrine organs.

The endocrine glands appear unique in that the hormones they produce do not pass through tubes or ducts. The hormones are secreted directly into the internal environment, where they are transmitted via the bloodstream or by diffusion and act at distant points in the body. In contrast, other glands including sweat glands, salivary glands, and glands of the gastrointestinal system secrete the substances they produce through ducts, and those substances are used in the vicinity of the gland.

The regulation of body functions by the endocrine system depends on the existence of specific receptor cells in target organs that respond in specialized ways to the minute quantities of the hormonal messengers. Some endocrine hormones, such as thyroxine thyroxine (thīrŏk`sēn), substance secreted by the thyroid gland .
..... Click the link for more information. from the thyroid gland, affect nearly all body cells; others, such as progesterone progesterone (prōjĕs`tərōn')
..... Click the link for more information. from the female ovary, which regulates the uterine lining, affect only a single organ. The amounts of hormones are maintained by feedback mechanisms that depend on interactions between the endocrine glands, the blood levels of the various hormones, and activities of the target organ. Hormones act by regulating cell metabolism. By accelerating, slowing, or maintaining enzyme activity in receptor cells, hormones control growth and development, metabolic rate, sexual rhythms, and reproduction.

Pituitary Control

The master gland, i.e., the gland that regulates many of the other endocrine glands, is the pituitary, located at the base of the brain. Also called the hypophysis, the pituitary secretes at least five hormones that directly affect the other endocrine glands. It secretes thyrotropin thyrotropin (thī'rätrō`pĭn) or thyroid-stimulating hormone
..... Click the link for more information. , which manages thyroid gland activity, adrenocorticotropic hormone adrenocorticotropic hormone (ədrē`nōkôr'təkōtrŏp`ĭk)
..... Click the link for more information. (ACTH), which regulates activity of the adrenal cortex, and three gonadotropic hormones gonadotropic hormone (gō'nădətrŏp`ĭk) or gonadotropin,
..... Click the link for more information. , follicle-stimulating hormone (FSH), luteinizing hormone (LH), and luteotropic hormone (LTH), all of which control the growth and reproductive activities of the sex glands. The pituitary also produces substances that do not act directly on other endocrine glands: somatotropic hormone, or growth hormone growth hormone or somatotropin (sōmăt'ətrō`pən)
..... Click the link for more information. , which controls growth in all tissues; antidiuretic hormone antidiuretic hormone (ăn'tēdīy
..... Click the link for more information. (ADH), which controls the rate of water excretion in the urine; oxytocin, which stimulates uterine contraction and helps regulate milk production by the breasts; and melanocyte-stimulating hormone, which regulates the activity of the melanocytes, or pigment-producing cells.

Adrenal Gland

The adrenal gland is another endocrine gland regulated by the pituitary. The adrenal cortex, the outer part of each of the two adrenal glands, produces aldosterone aldosterone (ăl'dōstĭrōn`), steroid secreted by the cortex of the adrenal gland.
..... Click the link for more information. , cortisol cortisol (kôr`tĭsôl') or hydrocortisone,
..... Click the link for more information. , and other steroids steroids, class of lipids having a particular molecular ring structure called the cyclopentanoperhydro-phenanthrene ring system. Steroids differ from one another in the structure of various side chains and additional rings.
..... Click the link for more information. . These substances regulate salt concentration in body fluids and glucose, fat, and protein metabolism. The inner portion of the gland, the adrenal medulla, secretes epinephrine epinephrine (ĕp'ənĕf`rīn), hormone important to the body's metabolism, also known as adrenaline.
..... Click the link for more information. (adrenaline) and norepinephrine, substances connected with the autonomic nervous system that help the body to respond to danger or stress.

The Thyroid Gland

The thyroid, located below the larynx and partially surrounding the trachea, produces thyroxine, which controls the metabolic rate of most body cells, and calcitonin, which is responsible for maintaining proper calcium serum levels in the body.

The Sex Hormones

The testes produce the male sex hormone testosterone testosterone (tĕstŏs`tərōn), principal androgen, or male sex hormone .
..... Click the link for more information. , which controls the development of the male sex organs as well as secondary sex characteristics. The pituitary hormone LH regulates testosterone production, and FSH initiates sperm formation in the testes. In females, FSH, LH, and LTH are integrated into the complex monthly cycles of ovulation, production of the hormones estrogen estrogen (ĕs`trəjən)
..... Click the link for more information. and progesterone by the ovaries and corpus luteum, and menstruation; LTH also contributes to lactation. Estrogen controls growth of the sex organs and breasts and regulates secondary sex characteristics. The most important function of progesterone is to prepare the uterine lining for implantation of a fertilized egg.

Other Endocrine Glands

The other endocrine glands are not directly controlled by the pituitary. The four parathyroid glands, located behind the thyroid, secrete a hormone that regulates calcium and phosphate metabolism. The endocrine portion of the pancreas, called the islets of Langerhans, secretes insulin insulin, hormone secreted by the β cells of the islets of Langerhans, specific groups of cells in the pancreas . Insufficiency of insulin in the body results in diabetes . Insulin was one of the first products to be manufactured using genetic engineering .
..... Click the link for more information. , which regulates the level of sugar (glucose) in the blood and glucagon glucagon (gl
..... Click the link for more information. , which raises blood sugar level. The thymus, sometimes considered another endocrine gland, processes lymphocytes in newborn animals, seeding the lymph nodes and other lymph tissues; it is partly responsible for the development of the organism's immune system (see immunity immunity, ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
..... Click the link for more information. ). The kidney is sometimes considered an endocrine gland because it secretes the hormone renin which, with other substances, regulates blood pressure. The kidney produces a glycoprotein called erythropoietin, which stimulates red blood cell production. The pineal gland produces a substance called melatonin, which helps regulate the body's internal clock.

The Hypothalamus

Physiological processes are under nervous system as well as endocrine control and a gland adjacent to the pituitary, called the hypothalamus hypothalamus (hī'pəthăl`əməs)
..... Click the link for more information. , mediates between the two systems. The hypothalamus secretes pituitary-regulating substances in response to nervous system stimuli including smell, taste, pain, and emotions. Thus, stress, cold, heat, and other stimuli release CRF, or adrenocorticotropic hormone-releasing factor, from the hypothalamus, causing ACTH to be produced by the pituitary, which in turn stimulates the production of the adrenal hormone cortisol. Similar chemical regulatory mechanisms operate in the regulation of the sex and thyroid hormones. Hypothalamic activity is also regulated by other body substances, e.g., cortisol inhibits the production of hypothalamic CRF.

endocrine system

Major glands of the human endocrine system. The hypothalamus stimulates the pituitary gland and …

Group of ductless glands that secrete hormones necessary for normal growth and development, reproduction, and homeostasis. The major endocrine glands are the hypothalamus, pituitary, pineal, thyroid, parathyroids, adrenals, islets of Langerhans in the pancreas, ovaries, and testes. Secretion is regulated either by regulators in a gland that detect high or low levels of a chemical and inhibit or stimulate secretion or by a complex mechanism involving the hypothalamus and the pituitary. Tumours that produce hormones can throw off this balance. Diseases of the endocrine system result from over- or underproduction of a hormone or an abnormal response to a hormone.