Interdependence of urea and electrolytes in production of a concentrated urine

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Abstract
In most mammals, ability to concentrate urea in the urine exceeds that for electrolytes; this was not found in beaver, pig, and the desert rodent Psammomys. When these animals were exposed to water deprivation, Pitressin administration, urea, and salt loading, the urine osmotic concentration was constant regardless of which solute dominated in the urine. When the urea concentration was high, total electrolyte concentration was low, and vice versa. In salt-loaded animals, urea and electrolyte concentrations in the papilla nearly equaled urea and total electrolyte concentrations in the urine. In urea-loaded animals, urea concentration in the papilla was markedly lower than in urine and electrolyte concentration was correspondingly higher, so that calculated osmolarity in the tissue still equaled the osmolarity in the urine. It is possible that the permeability of the collecting ducts to urea changes with the loading solute. This could explain the inverse relationship between the urea and electrolyte concentrations in the urine.

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