Hormones are biologically active substances that regulate metabolism and physiological functions of the human body. The term "hormone" was introduced in medical practice by William Bayliss (William Maddock Bayliss, 18601924) and Ernest Starling (Ernest Henry Starling, 1866-1927). Hormones are released by internal secretion glands.
The major glands of the endocrine system are the hypothalamus, pituitary gland, and peripheral glands: thyroid gland, parathyroid gland, ovaries, testes, adrenal cortex, and pancreatic islets. Peripheral endocrine glands release hormones directly into the systemic bloodstream. Once hormones reach target tissues via the bloodstream, hormones bind to specific receptors, thus causing respective metabolic processes. The hypothalamus and pituitary gland also exert a regulatory effect on peripheral target tissues. In addition to that, the hypothalamus-pituitary system, through the negative feed-back mechanism, regulates functions of various peripheral endocrine glands (thyroid gland, adrenal cortex, and sex glands).
Negative feedback is a fundamental feature of the endocrine system function. The hypo-thalamus produces releasing hormone (releasing factor, liberin) that stimulates the function of the anterior pituitary target trophic cells, which, in their turn, produce respective tropic hormone. This hormone of the anterior pituitary is secreted into the blood and reaches a target peripheral gland, thus stimulating its proliferation and secretory activity. Peripheral gland hormones are secreted into the systemic bloodstream, reach target tissues and exert their effects. Simultaneously, reaching the hypothalamus and pituitary gland, they stimulate local specific receptors, and inhibit the production of respective releasing hormones of the hypothalamus, and tropic hormones of the anterior pituitary gland. Therefore, the higher the blood hormone concentration, the lesser its production by a respective gland (and vice versa).