Entropy generation and water conservation in the mammalian nephron.

During the transition from fresh waters to terrestrial habitats, significant adaptive changes occurred in kidney function of vertebrates to cope with varying osmotic challenges. We investigated the mechanisms driving water conservation in the mammalian nephron, focusing on the relative contributions of active ion transport and Starling forces. We constructed a thermodynamic model to estimate the entropy generation associated with different processes within the nephron, and analyzed their relative importance in urine formation. We demonstrate that active ionic reabsorption exerts a pressure above 15,000 torr, a value more than 500 times greater than Starling forces. The entropy generation of the reabsorption process is found to be 20-fold higher than that of renal blood perfusion. These findings imply that the evolutionary history of vertebrates, particularly terrestrial mammals, has shaped the renal architecture to prioritize water conservation by means of an entropically costly process. This approach to the nephron function provides insights into the physiological adaptations of terrestrial vertebrates to conserve water and sheds light on the intricate interplay between environmental conditions and evolutionary responses in renal physiology.