Function of the excretory system of the crocodile (Croc~d~?h acutus)’

SCHMIDT-NIELSEN, BODIL, AND ERIK SKADHAUGE, Function of the excretory system of the crocodile (Crocodylus acutus) Am. ,T. Physiol. 212(5): 973-980. I 967 .-Glomerular filtration rate (GFR), tubular and cloaca1 functions were studied, during normal conditions, water loading, salt loading, and isosmotic expansion of the extracellular fluid volume. Na+, K+, Cl-, NH& and osmolality were determined in ureteral and cloaca1 urine. GFR varied little compared to that of other amphibious reptiles. It increased about 20 y0 after a water load corresponding to I 07~ body wt, and showed no increase after an isosmotic load. It decreased only 40 y0 after dehydration ( I o y. of body wt). The tubules have little or no capacity for regulating the osmolality or electrolyte composition of the urine. Ureteral urine osmolality remained around 807~ of the plasma osmolality under all conditions. The electrolyte concentration of the ureteral urine varied only slightly with the variations in salt load. Cloaca1 absorption appeared to be more regulated. Under normal conditions Na+ and Clwere almost completely resorbed in the cloaca leaving NHA+ and probably HCO3 behind. After salt load Na+ and Clresorption in the cloaca was less complete. The kidney function of the crocodile resembles that of terrestrial reptiles more than that of other amphibious reptiles. The lack of regulation of glomerular filtration rate and the tubular osmolality may be characteristic for uricotelic reptiles. It may also reflect the ancestry, since the crocodile is a descendant of the terrestrial ruling reptiles.