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Renal ThresholdIn physiology, the renal threshold is that concentration of a blood substance above which the kidneys begin to pass it through into the urine. Kidneys function by selectively passing dissolved blood substances into the urine, thus removing them from the body. Renal thresholds vary by substance, by species and, within species, by physiological condition. Thus hibernating animals have different renal thresholds for various substances than do non-hibernating ones, and some animals have very different thresholds for, say, urea. Urea is removed at very low concentrations, while glucose must reach much higher levels before it is passed through. Functionally, this removes a low potency poison (urea in this case) while retaining the major form of fuel for cellular processes (glucose). In the case of glucose, the most common circumstance in which the renal threshold is ever exceeded is diabetes. The renal thresholds for various substances can be altered by many drugs and also change in characteristic ways during illnesses, as for example infections of various types. Taken together, the collection of renal thresholds a kidney exhibits essentially defines much of its function in renal physiology. There are various tests of kidney function, some of which are measures of renal thresholds for various substances.
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