STRUCTURE AND CONCENTRATING ABILITY OF THE MAMMALIAN KIDNEY - CORRELATIONS WITH HABITAT

Mammals with relatively long loops of Henle for their body size tend t o have greater than average urinary concentrating ability, but the rel ationship between urine osmolality (U-osm) and absolute length of the loop of Henle (generally estimated as medullary thickness)is neither p roportional nor direct. U-osm is independent of the thickness of the o uter medulla (corresponding to the length of the medullary thick ascen ding limb), which scales similarly with body mass in animals from mesi c, arid, and freshwater environments. After adjustment for the effect of body size, there is a significant relationship between the thicknes s of the inner medulla (corresponding to length of the thin ascending limb) and concentrating ability, but only in species from mesic enviro nments; for these, the thickness of the inner medulla accounts for onl y 16% of the interspecific variability in U-osm. In marine mammals, bo th the cortex and medulla are surprisingly thin, yet these animals pro duce very concentrated urine for their size. A functional dependence o f urinary concentrating ability on the length of the loop of Henle is a central tenet of countercurrent multiplier theory, but the correlati on of maximum urine concentration with loop length is weak at best and largely reflects the influence of the thin ascending limb.