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kidney
(kidīne)
either of the two bean-shaped organs in the lumbar region that filter the blood, excreting the end-products of body metabolism
in the form of urine, and regulating the concentrations of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular fluid. This small organ contains over a million microscopic filtering units, the nephrons. Blood arrives at the kidney by way of the renal artery, and is distributed through arterioles into millions of capillaries
that lead into the nephrons. Fluids and dissolved salts in the blood pass through the walls of the capillaries and are collected
within the malpighian capsule in the center of each nephron. Within the capsule is a tuft of capillaries called the glomerulus that acts as a semipermeable membrane permitting a protein-free ultrafiltrate of plasma to pass through. This filtrate is
forced into the renal tubules, hairpin-shaped collecting channels in the nephrons. Capillaries in the walls of the tubules reabsorb the water and the salts
required by the body and deliver them to a system of small kidney veins which, in turn, carry them into the renal vein and
return them to the general circulation. Excess water and other waste materials remain in the tubules as urine.
Copyright 2007. An Elsevier publication. All rights reserved.
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