>> The endocrine system is a collection of cells, tissues, and organs responsible for the production and secretion of chemical signals called hormones. An endocrine gland is an organ that secretes hormones directly into the bloodstream. All hormones act on target cells remote from their point of origin. The hypothalamus plays a key role in regulating the endocrine system. It's a region in the brain that contains specialized neurons called neurosecretory cells that secrete hormones. Some of these cells project into the posterior region of the adjacent pituitary gland, where the hormones are released into the blood stream. Other hypothalamic neurons release inhibitory hormones or releasing hormones into the blood stream that feeds the anterior region of the pituitary gland. These hypothalamic hormones cause the anterior pituitary to withhold or release its own sets of hormones. The pituitary gland releases a great variety of hormones, including growth hormone (GH), which has growth-stimulating effects on a variety of cell types throughout the body and hormones that target the thyroid gland, adrenal gland, kidneys, and reproductive organs. The thyroid gland produces hormones such as thyroxine, which regulates the body's metabolic rate, and calcitonin, which decreases the blood calcium level. The parathyroid glands, which are imbedded in the thyroid gland, secrete parathyroid hormone (PTH), which increases the blood calcium level. PTH and calcitonin work together to keep the blood calcium level close to the set point. The adrenal glands sit atop the kidneys. They secrete hormones such as epinephrine (which is involved in short-term stress responses), cortisol (which is involved in long-term stress responses), and aldosterone (which controls the reabsorption of sodium ions by the kidneys). In addition to their function in producing urine, the kidneys are endocrine glands. When blood oxygen levels are low, the kidneys release the hormone erythropoietin, or EPO, which increases the body's synthesis of red blood cells. The pancreas is an endocrine gland that produces the hormones insulin and glucagon, which work together to maintain homeostatic levels of glucose in the blood. The testes produce sperm but also act as the major source of the male sex hormones, especially testosterone, which induces and maintains the male's secondary sex characteristics beginning at puberty. The ovaries, likewise, produce the female gametes, but also function as endocrine glands. They produce estrogens, which induce and maintain the female's secondary sex characteristics.