Urea Excretion in Mammals

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Abstract
The currently accepted concepts for the renal excretion of urea in mammals are inadequate to explain a number of observations. Of particular importance are: (1) the renal tubular regulation of the urea clearance found in rodents, ruminants, dog and man; (2) urea clearances which approach or in certain cases exceed the glomerular filtration rate; (3) the exaltation and abatement of the urea clearance at rising and falling urine flows, respectively; (4) the concentration gradients of urea in the renal tissue, including the fact that the concentration in the tissue of the renal papilla is identical to the concentration in the simultaneously produced urine. A hypothesis suggested for the urea excretion mechanism is supported by physiological as well as anatomical findings. This hypothesis is based on the counter-current multiplier principle. It is suggested that urea is concentrated in the renal papilla, and consequently in the urine, through an active transport of urea over a relatively low gradient, which is amplified by the multiplier system of Henle''s loops and vasa recta. This hypothesis can adequately explain many physiological findings that have previously been difficult or impossible to interpret. Furthermore, a number of anatomical features that have not had any adequate functional interpretation become meaningful and give weighty support to the revised view on renal excretion of urea.

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