Kidneys

• Kidneys (Urinary System)

The kidneys are not primarily endocrine organs, but are involved in the production of hormones and affected by them.

Major hormones controlling kidney function include:

• ADH (antidiuretic hormone), secreted by the posterior pituitary (neurohypophysis) gland, stimulates the reabsorption of water, and allows the kidneys to concentrate urine.
• Aldosterone is a hormone secreted by the adrenal cortex which regulates electrolyte and water balance. It stimulates the reabsorption of sodium and water and the excretion of potassium. It also increases blood volume thereby increasing blood pressure.
• ANF (atrial natriuretic factor) is also known as ANP (atrial natriuretic peptide). It is secreted by the atria of the heart which then decreases the reabsorption of sodium. ANF also causes greater excretion of sodium and water by the kidney. It has the opposite effect from that of aldosterone and ADH.
• Angiotensin II, converted from angiotensin I, is a substance formed in the blood when renin is released by the kidney. It is a powerful vasocompressor and stimulator of aldosterone secretion.
• Calcitriol is a product converted in the kidneys after the release of D3 by the liver. It is important to calcium ion homeostasis. Its synthesis is dependent on the availability of O3.
• PTH (parathyroid hormone) is secreted by the parathyroid gland. It is involved with two electrolytes: the reabsorption of calcium and excretion of phosphate.
• REF (renal erythropoietic factor) or EPO (erythropoietin) is secreted by the kidney in the adult and by the liver in the fetus. It acts on the stem cells of the bone marrow to stimulate red blood cell production, and, along with renin, is involved in the regulation of blood pressure and volume.
• Renin is synthesized, stored, and secreted by the kidney. It is released in response to a decline in blood volume, blood pressure, or both, thereby, stabalizing blood pressure. Once in the bloodstream, it functions as an enzyme leading to the formation of angiotensin II, which has several functions, including the stimulation of aldosterone production by the adrenal cortex. It is the hormone that initiates a series of events called the renin-angiotensin-aldosterone system.