What do the kidneys do, exactly?

Most people have two of these bean-shaped organs (nearly one in 750 people are born with only one kidney). They're about the size of a fist and are located halfway up the back on both sides of the spine. Often, the left kidney is positioned up to an inch higher than the right kidney.

The kidneys contain millions of nephrons, tiny units made up of blood vessels (glomeruli) and fluid-collecting tubes (tubules), sifting approximately 200 quarts of blood a day. Over a 24-hour period, they filter the entire volume of blood 30 to 40 times, extracting two quarts of waste products and extra water for removal in the form of urine.

In addition to acting as filters, the kidneys keep track of various substances in the blood and, if necessary, release hormones to help replenish missing stock. They produce erythropoietin (EPO) to stimulate red blood cell production, renin to help control blood pressure, and the active form of vitamin D (calcitriol) to maintain calcium levels for healthy bones.

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