Hormones - Mechanisms Of Action, The Hypothalamus, The Pituitary Gland, The Thyroid Gland, The Parathyroid Glands - Major hormones
word concentrations secreted molecules
Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream from where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis. The word hormone comes from the Greek word, hormon, to stir up, and indeed excitation is characteristic of the adrenaline and the sex hormones. Most hormones produce an effect on specific target tissues that are sited at some distance from the gland secreting the hormone. Although small plasma concentrations of most hormones are always present, surges in secretion trigger specific responses at one or more targets. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.
The concentrations of several important biological building blocks such as amino acids are regulated by more than one hormones. For example, both calcitonin and parathyroid hormone (PTH) influence blood calcium levels directly, and other hormones affect calcium levels indirectly via other pathways.
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Hormones elicit a response at their target tissue, target organ, or target cell type through receptors. Receptors are molecular complexes which specifically recognize another molecule-in this case, a particular hormone. When the hormone is bound by its receptor, the receptor is usually altered in some way that it sends a secondary message through the cell to do something in response. Hormones that…
Hormones secreted by the hypothalamus modulate other hormones. The major hormones secreted by the hypothalamus are corticotrophin releasing hormone (CRH), thyroid stimulating hormone releasing hormone (TRH), follicle stimulating hormone releasing hormone (FSHRH), luteinizing hormone releasing hormone (LRH), and growth hormone releasing hormone (GHRH). CRH targets the adrenal glands. It triggers th…
The pituitary has long been called the master gland because of the vast extent of its activity. It lies deep in the brain just behind the nose. The pituitary is divided into anterior and posterior regions with the anterior portion comprising about 75% of the total gland. The posterior region secretes the peptide hormones vasopressin, also called anti-diuretic hormone (ADH), and oxytocin. Both are …
The thyroid lies under the larynx and synthesizes two hormones, thyroxine and tri-iodothyronine. This gland takes up iodine from the blood and has the highest iodine level in the body. The iodine is incorporated into the thyroid hormones. Thyroxine has four iodine atoms and is called T4. Tri-iodothyronine has three iodine atoms and is called T3. Both T3 and T4 function to increase the metabolic ra…
The parathyroid glands are attached to the bottom of the thyroid gland. They secrete the polypeptide parathyroid hormone (PTH) which plays a crucial role in monitoring blood calcium and phosphate levels. About 99% of the body's calcium is in the bones, and 85% of the magnesium is also found in bone. Low blood levels of calcium stimulate PTH release into the bloodstream in two steps. Initial…
The two adrenal glands, one on top of each kidney, each have two distinct regions. The outer region (the medulla) produces adrenaline and noradrenaline and is under the control of the sympathetic nervous system. The inner region (the cortex) produces a number of steroid hormones. The cortical steroid hormones include mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and g…
The pancreas folds under the stomach, secretes the hormones insulin, glucagon, and somatostatin. About 70% of the pancreatic hormone-secreting cells are called beta cells and secrete insulin; another 22%, or so, are called alpha cells and secrete glucagon. The remaining gamma cells secrete somatostatin, also known as growth hormone inhibiting hormone (GHIH). The alpha, beta, and gamma cells compri…
The female reproductive hormones arise from the hypothalamus, the anterior pituitary, and the ovaries. Although detectable amounts of the steroid hormone estrogen are present during fetal development, at puberty estrogen levels rise to initiate secondary sexual characteristics. Gonadotropin releasing hormone (GRH) is released by the hypothalamus to stimulate pituitary release of LH and FSH. LH and…
Male reproductive hormones come from the hypothalamus, the anterior pituitary, and the testes. As in females, GRH is released from the hypothalamus which stimulates LH and FSH release from the pituitary. In males, LH and FSH facilitate spermatogenesis. The steroid hormone testosterone is secreted from the testes and can be detected in early embryonic development up until shortly after birth. Testo…
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