The endocrine system is the body's network of nine glands and over 100 hormones that maintain and regulate numerous events throughout the body. The glandsof the endocrine system include the pituitary, thyroid, parathyroids, thymus, pancreas, pineal, adrenals, and ovaries or testes; in addition, the hypothalamus, in the brain, regulates the release of pituitary hormones. Each of these glands secrete hormones (chemical messengers) into the blood stream. Oncehormones enter the blood, they travel throughout the body and are detected byreceptors that recognize specific hormones. These receptors exist on targetcells and organs. Once a target site is bound by a particular hormone, a cascade of cellular events follows that culminates in the physiological responseto a particular hormone.
Most endocrine hormones are maintained at specific concentrations in the plasma, the non-cellular, liquid portion of the blood. Receptors at set locationsmonitor plasma hormonal levels and inform the gland responsible for producing that hormone if levels are too high or too low for a particular time of day, month, or other life period. When excess hormone is present, a negative feedback loop is initiated such that further hormone production is inhibited. Most hormones have this type of regulatory control. However, a few hormones operate on a positive feedback cycle such that high levels of the particular hormone will activate release of another hormone. With this type of feedback loop, the end result is usually that the second hormone released will eventuallydecrease the initial hormone's secretion. An example of positive feedback regulation occurs in the female menstrual cycle, where high levels of estrogenstimulate release of the pituitary hormone, luteinizing hormone (LH).
All hormones are influenced by numerous factors. The hypothalamus can releaseinhibitory or stimulatory hormones that determine pituitary function. And every physiological component that enters the circulation can effect some endocrine function. Overall, this system uses multiple bits of chemical information to hormonally maintain a biochemically balanced organism.
The pituitary gland has long been called "the master gland" because it secretes multiple hormones that, in turn, trigger the release of other hormones from other endocrine sites. The pituitary is roughly situated behind the nose and is anatomically separated into two distinct lobes, the anterior pituitary (AP) and the posterior pituitary (PP). The entire pituitary hangs by a thin piece of tissue, called the pituitary stalk, beneath the hypothalamus in the brain. The AP and PP are sometimes called the adenohypophysis and neurohypophysis, respectively.
The PP secretes two hormones, oxytocin and antidiuretic hormone (ADH), underdirection from the hypothalamus. AP cells are categorized according to the hormones that they secrete. The hormone-producing cells of the AP include: somatotrophs, corticotrophs, thyrotrophs, lactotrophs, and gonadotrophs. Somatotrophs secrete growth hormone; corticotrophs secrete adrenocorticotropic hormone (ACTH); thyrotrophs secrete thyroid stimulating hormone (TSH); lactotrophssecrete prolactin; and gonadotrophs secrete LH and follicle stimulatory hormone (FSH). Each of these hormones sequentially signals a response at a targetsite. While ACTH, TSH, LH, and FSH primarily stimulate other major endocrineglands, growth hormone and prolactin primarily coordinate an endocrine response directly on bones and mammary tissue, respectively.
The pineal gland is a small cone-shaped gland believed to function as a bodyclock. The pineal is located deep in the brain just below the rear-most portion of the corpus callosum (a thick stretch of nerves that connects the two sides of the brain). The pineal gland, also called the pineal body, has mystified scientists for centuries. The 17th-century philosopher Rene Descartes speculated that the pineal was the seat of the soul. However, its real function is somewhat less grandiose than that.
The pineal secretes the hormone melatonin, the level of which fluctuates on adaily basis, with the levels highest at night. Although its role is not wellunderstood, some scientists believe that melatonin helps to regulate other daily events. Exactly what controls melatonin levels is not well understood either; however, visual registration of light may regulate the cycle.
The thyroid is a butterfly-shaped gland that wraps around the back of the esophagus. The two lobes of the thyroid are connected by a band of tissue calledthe isthmus. An external covering of connective tissue separates each lobe into another 20-40 follicles. Between the follicles are numerous blood and lymph vessels in another connective tissue called stroma. The epithelial cells around the edge of the follicles produce the major thyroid hormones.
The major hormones produced by the thyroid are triiodothyronine (T3), thyroxine (T4), and calcitonin. T3 and T4 are iodine-rich molecules that fuel metabolism. The thyroid hormones play several important roles in growth, metabolism, and development. The thyroids of pregnant women often become enlarged in late pregnancy to accommodate metabolic requirements of both thewoman and the fetus.
Thyroid hormones accelerate metabolism in several ways. They promote normal growth of bones and increase growth hormone output. They increase the rate oflipid synthesis and mobilization. They increase cardiac output by increasingrate and strength of heart contractions. They can increase respiration, the number of red blood cells in the circulation, and the amount of oxygen carriedin the blood. In addition, they promote normal nervous system development including nerve branching.
While most people have four small parathyroid glands poised around the thyroid gland, about 14% of the population have one or two additional parathyroid glands. Because these oval glands are so small, the extra space occupied by extra glands does not seem to be a problem. The sole function of these glands is to regulate calcium levels in the body. Although this may seem like a simple task, the maintenance of specific calcium levels is critical. Calcium has numerous important bodily functions. Calcium makes up 2-3% of adult weight with roughly 99% of the calcium in bones. Calcium also plays a pivotal role in muscle contraction and neurotransmitter secretion.
In young children, the thymus extends into the neck and the chest, but afterpuberty, it begins to shrink. The size of the thymus in most adults is very small. Like some other endocrine glands, the thymus has two lobes connected bya stalk. The thymus secretes several hormones that promote the maturation ofdifferent cells of the immune system in young children. In addition, the thymus oversees the development and "education" of a particular type of immune system cell called a T lymphocyte, or T cell.
Although many details of thymal hormonal activity are not clear, at least four thymal products have been identified: thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin. Because AIDS is characterized by T cell depletion, some AIDS treatment approaches have tried administering tymosinto boost T cell production.
The pancreas is a large endocrine and exocrine gland situated below and behind the stomach in the lower abdomen. The pancreas is horizontally placed suchthat its larger end falls to the right and its narrower end to the left. Clusters of exocrine pancreatic cells called acini secrete digestive enzymes intothe stomach; endocrine cells secrete hormones responsible for maintaining blood glucose levels.
The endocrine cells of the pancreas are contained in the islets of Langerhanswhich are themselves embedded in a rich network of blood and lymph vessels.Insulin is secreted in response to high plasma glucose levels. Insulin facilitates glucose uptake into blood cells thus reducing plasma glucose levels. Glucagon has the opposite effect; low plasma glucose triggers the breakdown ofstored glucogen in the liver and glucose release into the blood. By balancingthese two hormones, the islets continually regulate circulating glucose levels.
One of the two adrenals sit atop each kidney and are divided into two distinct regions, the cortex and the medulla. The outer area makes up about 80% of each adrenal and is called the cortex. And the inner portion is called the medulla. The adrenals provide the body with important buffers against stress while helping it adapt to stressful situations.
Cells of the adrenal medulla, called chromaffin cells, secrete the hormones,epinephrine (adrenaline) and non-epinephrine (nor-adrenaline). Chromaffin cells are neuroendocrine cells which function like some nerve fibers of the sympathetic nervous system. However, these cells are endocrine, because the neurohormones that they release target distant organs. Although the effects of these two medullary hormones are the same whether they originate in the endocrine or the nervous system, endocrine hormonal effects are prolonged, because they are removed more slowly from blood than from a nerve terminal. Bothcortical and medullary hormones work together in emergencies, or stressful situations, to meet the physical demands of the moment.
The ovaries are located at the end of each fallopian tube in the female reproductive tract, and they produce the female reproductive hormones estrogen, progesterone, and relaxin. Although the fluctuation of these hormones is critical to the female menstrual cycle, they are initially triggered by a hormone from the hypothalamus, called a releasing factor, that enables gonadotrophs inthe pituitary to release LH and FSH that, in turn, regulate part of the menstrual cycle. All of these hormones work together as part of the endocrine system to ensure fertility. They are also important for the development of sexual characteristics during puberty.
The two testes are located in the scrotum, which hangs between the legs behind the penis. Most of the testes is devoted to sperm production, but the remaining cells, called Leydig cells, produce testosterone. Testosterone caries out two very important endocrine tasks in males: it facilitates sexual maturation, and it enables sperm to mature to a reproductively competent form. Healthy men remain capable of fertilizing an egg throughout their post-pubertal life. However, testosterone levels do show a gradual decline after about the ageof 40 with a total drop of around 20% by age 80.