Our kidneys perform four functions, of which the most obvious is the ejection of waste materials containing nitrogen from the body. They also keep the acid-base balance of the blood constant, and regulate the volume of circulating blood and the fluid content of the body as a whole. Finally they regulate pressure relationships between the blood and the tissues.
The kidneys, bean-shaped and about four inches long, lie on the back wall of the abdomen just above the waist, one on each side of the spinal column.
Blood enters the kidney by the renal artery. Some of its plasma is filtered off and, after other processes, ends as urine. The rest leaves the kidney by the renal vein to return to the heart. The urine collects in the central activity (pelvis) of the kidney passes down to the bladder via a tube (ureter) and is finally expelled.
A human being can survive without kidneys for only two or three weeks. Nearly one-fifth of the blood pumped out of the heart goes through the kidneys. The kidneys thus regulate the body fluid - a fluid which Claude Bernard (1813-1878), the great French physiologist, called our "internal environment".