kidney

In vertebrates, one of a pair of organs responsible for fluid regulation, excretion of waste products, and maintaining the ionic composition of the blood – in other words the regulation of the concentrations of some chemicals in the blood. The kidneys are situated on the rear wall of the abdomen. Each one consists of a number of long tubules (see nephron) – the outer parts filter the aqueous components of blood, and the inner parts selectively reabsorb vital salts, leaving waste products in the remaining fluid (urine), which is passed through the ureter to the bladder.

The kidneys tasks can be broken down into a number of stages: filtering of the blood; re-absorbance of all sugar; re-absorbance of the dissolved ions needed by the body; re-absorbance of as much water as the body needs; and release of urea, excess ions, and excess water as urine.

The action of the kidneys is vital and so kidney failure is serious. However, if one kidney fails, the other enlarges to take over its function. A patient with two defective kidneys may still continue near-normal life with the aid of dialysis using a kidney machine or continuous ambulatory peritoneal dialysis (CAPD), or by a kidney transplant. Other diseases of the kidney can include the formation of kidney stones. These hard stones can build up as a result of high levels of blood calcium or high levels of uric acid, and can cause intense pain as they travel down the ureter, as well as causing bleeding in the tissues of the urinary tract.