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Human Body
tively reabsorbed into the circulation
Selection
Finally the process of selection between the unwanted and wanted substances takes place. The unwanted urea and excess materials pass into the collecting duct where they are then known as urine. A certain amount of water is essential to keep all these unwanted substances dissolved and expelled as urine.
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Besides water, glucose, vitamins, proteins, amino acids and various inorganic salts are sent back for the body's use. Thus, the fluid that finally trickles down the ureters has a quite different composition from the initial filterate and the blood from which it was formed. About 1200 ml. of blood (about one-quarter of the total cardiac output) passes through the kidneys every minute. Out of this, only 1 ml/min. flows into the bladder. Drinking large amounts of liquids results in dilution of plasma and the filtration rate is increased. Dehydration, on the other hand, decreases this rate.
At night, during sleep, under the influence of ADH, the reabsorption of water is greatly increased and smaller amounts of more concentrated urine are produced. As a result, the bladder is usually able to cope with a night' s urine production allowing the person to sleep undisturbed. Occasionally, the dissolved salts in the urine may solidify to form kidney stones-either small gritty particles or large stones, which require surgical removal.
Regulation: The organs of urinary system are linked in an effective feedback system with the hypothalamus and pituitary gland. The amount of water excreted by the system is controlled mainly by the hormone ADH (anti-diuretic hormone) produced by the hypothalamus and secreted by the posterior pituitary. In the absence of ADH, the urine output can rise to as high as 15 to 18 ml. per minute; under the influence of the hormone, the urine output may be reduced to 0.35 ml/min. Thus the hypothalamuspituitary-kidney link maintains a precise balance of the tissue fluids by conserving water for the body and returning the concentration of the fluids to the optimum values.
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