Countercurrent Multipliers in Avian Kidneys

The capacity to conserve urinary water by producing a concentrated urine is directly related to the number of Henle's loops in the kidneys of three terrestrial birds. This suggests that a Henle's loop countercurrent multiplier is responsible for urine concentration in these birds. Several features of the organization of the kidneys of these birds may account for the importance of multiplier number, as contrasted to multiplier length in mammals, in determining maximum urine concentration.