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Definition: In vertebrates, one of a pair of organs responsible for regulating blood composition and excretion. The kidneys are at the back of the abdomen, either side of the backbone. Each is c.10cm (4in) long and 6.5cm (2.5in) wide. The human kidney consists of an outer cortex and an inner medulla with about one million tubules (nephrons). Nephrons contain numerous capillaries, which filter the blood entering from the renal artery. Some substances, including water, are reabsorbed into the blood. Urine remains, which is passed to the ureter and on to the bladder. One healthy kidney is needed to maintain life. A serious kidney disease may require a kidney transplant or dialysis using a machine.
Source: Philip's Encyclopedia 2008. London: Philip's, 2008. Credo Reference. Web. 10 August 2012.