The kidneys are primarily responsible for the maintenance of the internal environment of the human body. They share the following structural features:
* They are paired retroperitoneal organs.
* They weigh 110-170geachin the adult male, are 10-12 cm long, 5-7.5 cm wide and 2.5-3 cm thick.
* They receive 20%-25% of the cardiac output, which corresponds to 1000-1200 ml/minute, but only account for about 10% of the oxygen consumption of the body.
* The kidneys thereby receive the highest blood flow per gram of organ weight in the human body, while accounting for only 0.4% of the body weight. The entire plasma volume is cycled through the glomerular system 20 times per hour.
* Their internal structure consists of an outer cortex and an inner medulla. The medulla consists of ten pyramids, with their bases near the cortex and apices (papillae) which project into the calyceal sinuses. The pyramids are separated by columnar extensions of the cortex. Ninety per cent of the renal blood supply goes to the cortex.
* The cortex comprises glomeruli and proximal convoluted tubules, while the medulla comprises the loops of Henle, the distal convoluted tubules and the collecting ducts.
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