Anatomy & Physiology

Anatomy & Physiology Study Guide identical to growth hormone–inhibiting hormone (GH–IH), a hypothalamic regulatory hormone. Finally, F cells produce the hormone pancreatic polypeptide (PP). 17.8 Secondary Endocrine Organs As noted earlier, many organs of other body systems have secondary endocrine functions. Examples are the intestines (digestive system), the kidneys (urinary system), the heart (cardiovascular system), the thymus (lymphatic system), and the gonads—the testes in males and the ovaries in females (reproductive system). The Intestines The intestines, which process and absorb nutrients, release a variety of hormones that coordinate the activities of the digestive system. Although the pace of digestive activities can be affected by the autonomic nervous system, most digestive processes are hormonally controlled locally. The Kidneys The kidneys release the steroid hormone calcitriol, the peptide hormone erythropoietin, and the enzyme renin. The kidneys secrete a steroid hormone called Calcitriol in response to the presence of parathyroid hormone (PTH). The best-known function of calcitriol is the stimulation of calcium and phosphate ion absorption along the digestive tract. Erythropoietin (EPO) is a peptide hormone released by the kidneys as a response to decreased oxygen levels in kidney tissues. EPO stimulates the production of red blood cells by bone marrow. The increase in the number of red blood cells elevates blood volume. Renin is released by specialized kidney cells in response to sympathetic stimulation or a decline in renal blood flow. Once in the bloodstream, renin functions as an enzyme that starts an enzymatic cascade known as the renin–angiotensin system. The Heart The endocrine cells in the heart are cardiac muscle cells located in the atria walls (chambers that receive blood from the veins) and the ventricles (chambers that pump blood to the rest of the body). If blood volume becomes too great, these cells are stretched excessively, to the point at which they begin to secrete natriuretic peptides . Natriuretic peptides promote the loss of Na + and water at the kidneys and inhibit renin release and the secretion of ADH and aldosterone. They also suppress thirst and blood pressure elevation. The Thymus The thymus is located in the mediastinum, deep to the sternum. The thymus produces several hormones that are important to the development and maintenance of immune defenses. Thymosin is the name originally given to an extract from the thymus that promotes the development and maturation of lymphocytes, the white blood cells responsible for immunity. The Gonads The interstitial cells of the testes produce the male hormones known as androgens. The most important of these androgens is testosterone . During embryonic development, the production of testosterone affects the development of CNS structures, including hypothalamic nuclei, which will Achieve Page 214 of 368 ©2018