Reptiles evolved during a dry climate, and their nephrons are designed to conserve water. The main waste product of both reptiles and birds is uric acid. The glomeruli are small, so less water is filtered from the blood, while still removing most of the uric acid. The peritubular capillaries then reabsorb water from the filtrate. When the uric acid has been flushed into the cloaca, most remaining water is absorbed there, and the uric acid is eliminated as a white paste. Note also that the renal portal vein supplies most of the blood to the peritubular capillaries.