THE ENDOCRINE FUNCTIONOF THE KIDNEYS
The kidneys secrete three hormones—calcitriol, erythropoietin, and renin. Calcitriol is important incalcium ion homeostasis; the other two hormones are involved in the regulation of red blood cell for-mation, blood pressure, and blood volume.
Calcitriol is a steroidal hormone secreted in response to parathyroid hormone. Calcitriol is formed from a precursor vitamin D3 or cholecalciferol, which is ab-sorbed from the diet or formed in the skin. In the skin, cholecalciferol is formed from 7-dehydrocholesterol by the action of sunlight. Cholecalciferol is absorbed from skin and the diet and is transported to the liver by the blood where it is converted into an intermedi-ate product and released into the circulation. This in-termediate product is converted by the kidneys into calcitriol. Vitamin D indicates this entire group of steroids—calcitriol, cholecalciferol, and the intermedi-ate product (Integumentary Systemfor more details).
Vitamin D production is modified mainly by changing levels of calcium and phosphates in the plasma.
Erythropoietin is the hormone responsible for main-taining normal hemoglobin levels. When a person bleeds heavily or if the oxygen levels in the blood are reduced, the hemoglobin synthesis and production of red blood cells by the bone marrow is enhanced. Con-versely, if the hemoglobin level is increased by blood transfusion, the body adapts by reducing the produc-tion of hemoglobin. These responses are a result of the hormone erythropoietin.
The action of renin is to convert the peptide an-giotensinogen present in the plasma into angiotensin I. Angiotensinogen is manufactured in the liver and secreted into the plasma. Angiotensin I is converted into a more active form—angiotensin II—by enzymes located in the endothelium of blood cells. This con-version mainly occurs in the lungs as the blood passes through.
Angiotensinogen → Angiotensin I → Angiotensin II → vasoconstriction
Angiotensin II is a potent hormone that produces constriction of arterioles, with a rapid increase in blood pressure. In addition, it stimulates the produc-tion of aldosterone from the adrenal cortex, stimu-lates the brain to directly increase blood pressure, and increases water intake—all measures toward maintaining blood pressure and volume.
Renin is produced by cells located in the afferent arterioles of the kidney. Secretion by these cells is triggered by falling pressure in the arterioles (as in hypotension and hemorrhage). Rising levels of an-giotensin II have a negative feedback effect on these secretory cells.