Regulating Other Functions

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The thyroid gland, located in the throat region, produces several hormones (examples: thyroxine and triiodothyronine) that elevate the body's met abolic rate. The thyroid also secretes a hormone called calcitonin, which works antagonistically with the hormone parathormone produced by the adjacent parathyroid glands. When blood calcium is high (as in a condition called hypercalcemia), the calcitonin gene in thyroid cells begins producing the protein hormone calcitonin, which stimulates bone cells called osteoblasts to build more bone, thereby removing calcium from the bloodstream. Once blood calcium levels are back to normal, calcitonin production halts. In hypercal-cemia, the parathormone gene in parathyroid cells begins producing parathormone (also a protein hormone) that stimulates bone cells called osteo-clasts to break down bone, thereby restoring blood calcium levels but possibly contributing to osteoporosis (bones that are brittle because of calcium deficiency) and other bone-related disorders. Parathormone production is stopped once blood calcium levels are back to normal.

The islets of Langerhans in the pancreas secrete two antagonistic protein hormones, insulin and glucagon. In response to high glucose levels in the blood (as in hyperglycemia), genes in beta cells produce and secrete insulin, which directs body cells, especially liver cells, to absorb glucose and store it as a polysaccharide called glycogen. Insulin production will stop once blood glucose levels are reduced to normal. An insulin deficiency leads to prolonged hyperglycemia, a serious and often fatal disorder called diabetes melli-tus. When blood glucose levels are too low (as in hypoglycemia), genes in the alpha cells of the islets of Langerhans produce and secrete glucagon, which directs body cells to break down their gly-cogen reserves and begin releasing glucose back into the bloodstream until normal blood glucose levels are reached, upon which glucagon production ceases.

Further endocrine glands include the adrenal cortex, located on top of each kidney, which secretes three major classes of steroid hormones: the glucocorticoids such as cortisol, which controls fat and protein metabolism; the mineralocorticoids such as aldosterone, which controls blood sodium levels; and the androgens (male sex steroids). The adrenal medulla, located internally to the adrenal cortex, is derived from nervous tissue and secretes two hormones, epinephrine and norepinephrine, that double as excitatory neurotransmitters at nerve axon endings; chemical energy transmission between nerve cells occurs at synapses, or gaps, between adjacent neurons. Neurotransmit-ters are protein hormones that relay electrical impulses from one neuron (nerve cell) to another throughout the trillion-cell nervous systems of mammals. Other neurotransmitters include the excitatory acetylcholine and inhibitors glycine, enkephalin, and gamma-aminobutyric acid.

The kidney secretes the protein hormone erythropoietin when the blood has a low red blood cell level; erythropoietin stimulates the un-differentiated stem cells called hemocytoblasts in the red bone marrow of flat bones (ribs, sternum) to differentiate and develop into mature red blood cells. Platelet-derived growth factor (PDGF) is released from damaged blood vessels to activate platelet cells to begin blood clotting. Macrophage colony stimulatory factor and eosinophil chemo-tactic factor are two hormones that both activate and attract certain respective immune system cells to the site of an infection or allergic reaction. His-tamine is released from damaged tissue and causes blood vessel dilation, so that the vessels are more leaky, thus allowing hormones and other molecules to reach the injury site, eventually leading to the inflammation and itching associated with wound healing.

The hormone prostaglandin helps inflammation and contracts some smooth muscles located throughout the body; nerve growth factor stimulates the growth of sensory nerves throughout the body; and epidermal growth factor stimulates the growth of the epidermis, the outermost skin layer that is constantly being shed and replaced. Sunlight exposure to skin produces cholecalciferol, or vitamin D, which helps to stimulate bone growth and maintenance.

The hormone prolactin is responsible for stimulating milk production in mammals. (PhotoDisc)

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