According to The Merck Manuals Online Medical Library, the primary function of the kidneys is to maintain the proper balance of water and minerals (electrolytes) in the body. The kidneys play an additional important role in helping to regulate activities of the endocrine system, secreting two hormones and an enzyme that play important roles in regulating red blood cells, healthy bones and blood pressure.
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Secretion of Erythropoietin (EPO)
The kidneys secrete a vital hormone called erythropoietin (EPO) that is responsible for stimulating the production of red blood cells in bone marrow. The bone marrow then produces and releases just the right amount of red blood cells into the bloodstream to carry needed oxygen to vital organs. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), if the kidneys become diseased and fail to produce enough EPO, the marrow makes insufficient blood cells, resulting in anemia. When oxygen-carrying red blood cell count is too low, the body’s tissues and organs can be deprived of the oxygen they need to function properly. This can lead people with anemia to become pale, weak and easily fatigued, and it could lead to serious heart problems.
Secretion of Calcitriol
The kidneys produce an active form of vitamin D called calcitriol that is vital to the growth and maintenance of healthy bones. The vitamin D we consume through food or that is produced in the skin through direct exposure to sunlight is inactive. The kidneys convert that vitamin D into calcitriol which then acts like a hormone to stimulate absorption of dietary calcium by the blood and bones. According to the NIDDK, those whose kidneys cease to function stop making calcitriol. When this occurs, the body is unable to absorb calcium from food and instead starts to remove it from the bones.
Secretion of Renin
The kidneys play an important role in helping to regulate blood pressure. One of the ways they accomplish this is by regulating the excretion of excess sodium from the body; they also control blood pressure through their secretion of the enzyme renin. When blood pressure gets too low, the kidneys secrete renin into the bloodstream, activating a series of reactions in the blood. According to The Merck Manuals, renin indirectly stimulates another hormone called angiotensin II to cause arteriole walls to constrict, thus increasing blood pressure. Failing kidneys lose their ability to monitor blood pressure and release renin accordingly; consequently, many people with kidney failure end up with high blood pressure.