STRUCTURAL UNITS OF THE KIDNEY
There are about one million working units called nephrons in each kidney. Each nephron consists of a tuft of capillaries known as the glomerulus attached to a long winding tubule that empties into collecting tubules.
The glomerulus filters the blood that circulates through it. Water together with glucose, amino acids, urea, sodium chloride, and other small molecules filter into the tubules. Large molecules such as the blood proteins are held back in the circulation. Each glomerulus filters only a tiny drop of fluid in a day, but the volume of plasma filtered by the two million glomeruli amounts to about 125 ml per minute or 180 liters in 24 hours. The amounts of glucose, sodium chloride, and other substances filtered are equally large; for example, the sodium chloride filtered is over kilogram which is roughly 100 times the daily intake of salt!
The winding tubules bring about selective reabsorption so that the normal concentration of substances in the blood is maintained at all times. Normally, the urine volume ranges from 1000 to 2000 ml, which means that over 99 per cent of the filtered water has been returned to the circulation. Likewise, all of the glucose and vitamin C, and almost all of the amino acids, sodium and other substances have been returned to the blood. For example, if you eat foods containing more salt than your body needs, the renal excretion of water and sodium will be increased. On the other hand, if you greatly reduce your salt intake or if the body sodium is depleted, the excretion in the urine will be very small.
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