Changes

The length of the loop of henle is the determining factor with regard to how concentrated urine an animal can produce.  In species which produce very concentrated urine almost if not all of their nephrons are juxtamedullary in type and they tend to have very long loops of henle.  However animals such as beavers where dehydration is not really an issue have very short loops of henle and excrete dilute urine.  One surprise is however that the kangaroo rat which can concentrate urine to the equivalent of a 20% saline solution actually has shorter nephrons than some larger species with less ability to concentrate urine.  This has been attributed to it having a much higher metabolic rate and thus more active transport meaning greater gradients can occur on any given length of its loop of henle.

The length of the loop of henle is the determining factor with regard to how concentrated urine an animal can produce.  In species which produce very concentrated urine almost if not all of their nephrons are juxtamedullary in type and they tend to have very long loops of henle.  However animals such as beavers where dehydration is not really an issue have very short loops of henle and excrete dilute urine.  One surprise is however that the kangaroo rat which can concentrate urine to the equivalent of a 20% saline solution actually has shorter nephrons than some larger species with less ability to concentrate urine.  This has been attributed to it having a much higher metabolic rate and thus more active transport meaning greater gradients can occur on any given length of its loop of henle.