Endocrine system
... production in the testes. Another is luteinizing hormone (LH). In females, LH works in conjunction with FSH to regulate the female reproductive cycle and the secretion of female sex hormones. In males, LH controls the production of the male sex hormones. Other hormones produced in the anterior pituitary include growth hormone, which is responsible for normal body growth, and prolactin, which promotes milk production in female mammals. Its designation as the master gland notwithstanding, the anterior pituitary itself is regulated by substances called releasing hormones that are secreted by the hypothalamus, the part of the brain located directly above the pituitary. These hypothalamic hormones stimulate--or, in some cases, inhibit--the secretions of the anterior pituitary. The posterior pituitary stores and releases two hormones: oxytocin, which causes the uterus to contract during birth, and vasopressin, which acts on the kidneys to restrict the output of urine. These two hormones are actually produced by the hypothalamus, which is linked directly to the posterior pituitary. Other endocrine glands in vertebrates include the thyroid, parathyroids, adrenals, pancreas, and gonads (sex glands). The thyroid produces hormones that control metabolic rate and oxygen consumption. Hormones from the parathyroids are concerned with calcium concentration in the blood, and the pancreas releases insulin and glucagon, hormones that, respectively, lower and raise the blood-sugar level. Hormones from the adrenal cortex regulate glucose and sodium metabolism. Those secreted by the central portion (medulla) of the adrenals affect the heart and the circulatory and respiratory systems; these hormones are important in helping an individual cope with stress. The heart itself releases a hormone-- atrial natriuretic peptide--that helps regulate blood pressure, blood volume, and the salt and water balance within the blood. (see also Index: thyroid hormone, parathormone) The female sex hormones--the estrogens and progesterone--are produced by the ovaries. Together with FSH and LH, these hormones control the cyclical changes in the female reproductive system--the menstrual cycle in human females and the estrous cycle in other female mammals. The estrogens also are responsible for female sexual characteristics. Progesterone is concerned with the maintenance of pregnancy. Male sex hormones--known as androgens--include testosterone, which is secreted by the testes. Testosterone is responsible for the maintenance of male sexual characteristics. Hormone chemistry. Structurally, vertebrate hormones fall into two main classes. Those of the adrenal cortex and the sex organs are steroids, a major class of lipid compounds. Virtually all other known vertebrate hormones consist of amino acids. Most nonsteroidal hormones are composed of chains of amino acids--either short chains (polypeptides) or long chains (proteins). The hormones of the adrenal medulla, however, are composed of amino acid derivatives called amines, those of the thyroid of a single amino acid combined with atoms of iodine. It is believed that hormones achieve their effects on target tissues and organs through either of two mechanisms. The steroid hormones and the hormones of the thyroid can, as a result of their chemical structures, pass through cell membranes. These hormones apparently enter a target cell and combine with an intracellular receptor protein. The hormone-receptor complex then enters the cell's nucleus, where it apparently affects the activity of specific genes. Genes carry the cell's hereditary blueprint for protein synthesis, a...