kidney

one of two GLANDS /organs that remove nitrogenous wastes, mainly UREA , from the BLOOD and also adjust the concentrations of various salts. The kidney is roughly 10 cm long, 6 cm wide and 4 cm thick, and is positioned at the back of the abdomen, below the diaphragm. BLOOD is supplied to the kidney by the renal ARTERY and leaves via the renal VEIN . Each kidney is held in place by fat and CONNECTIVE TISSUE , and comprises an inner MEDULLA and outer CORTEX . The kidneys produce and eliminate URINE by a complex process of filtration and reabsorption. The 'active' parts are the nephrons, which filter blood under pressure, reabsorbing water and other substances. A nephron comprises a renal TUBULE and BLOOD VESSELS . The tubule expands into a cup shape ( BOWMAN'S CAPSULE ) that contains a knot of capillaries (see CAPILLARY ), called the glomerulus, and this brings the water, UREA , salts, etc, Filtrate passes from the glomerulus through three areas of the tubule (proximal convoluted tubule; the loop of Henle, distal convoluted tubule, which together form a shape resembling a hairpin), leaving as urine. The kidneys contain roughly two million nephrons and receive between one and two thousands litres of blood each day, processing 150 to 200 litres of filtrate, resulting in roughly 1.5 litres of urine.