The kidneys also work in conjunction with the heart, in this case, to cleanse the blood the heart pumps to the rest of the body. The kidneys filter out metabolic impurities and waste products from the blood, waste products such as urea.

Each kidney is made up of nephrons and tubules, which at one end have a concentration of intertwined capillaries (the glomerulus), and at the other, an opening into the interior cavity of the kidney itself. Each tubule consists of various sections along its length which carry out various filtration functions, straining out waste and returning much of the water back to the body. At its end stage, the "finished" urine is emptied into the kidney's central cavity, and from there to the bladder. Other than when we are signaled by a full bladder, we are never aware of this complex process. Until, as with the lungs, something goes wrong.

This something can be anything from injury, to disease, to cellular impairment brought on by the decay of age. Boxers who have suffered "kidney punches"; are sometimes made aware of kidney injury by blood in the urine. Other problems with the kidneys may be determined by means of various tests. Where kidney failure is very pronounced, a patient may have to go on dialysis.

Just as the FEV1 test of the lungs was a reliable indicator of life expectancy, so too the kidneys may be tested to get a prediction about life expectancy from their perspective. With the kidneys, as with the lungs, much of the data on HGH therapy comes from studies done on people who were already experiencing kidney problems.