Structure and concentrating ability of the mammalian kidney: correlations with habitat.

Mammals with relatively long loops of Henle for their body size tend to have greater than average urinary concentrating ability, but the relationship between urine osmolality (Uosm) and absolute length of the loop of Henle (generally estimated as medullary thickness) is neither proportional nor direct. Uosm is independent of the thickness of the outer medulla (corresponding to the length of the medullary thick ascending limb), which scales similarly with body mass in animals from mesic, arid, and freshwater environments. After adjustment for the effect of body size, there is a significant relationship between the thickness of the inner medulla (corresponding to length of the thin ascending limb) and concentrating ability, but only in species from mesic environments; for these, the thickness of the inner medulla accounts for only 16% of the interspecific variability in Uosm. In marine mammals, both the cortex and medulla are surprisingly thin, yet these animals produce very concentrated urine for their size. A functional dependence of urinary concentrating ability on the length of the loop of Henle is a central tenet of countercurrent multiplier theory, but the correlation of maximum urine concentration with loop length is weak at best and largely reflects the influence of the thin ascending limb.