1/5th of the pressure generated by the heart is invested in the kidney, where it sustains an axial pressure gradient, as a result of the pyramidal structure of the filtration system. This pressure gradient from cortex to papilla causes filtrate distal of bowman's capsule to prefer to move down the gradient, into the renal vein and lymphatic vessels. Because of the anatomical arrangement of glomeruli draining into a pyramidal duct system, powered by the heart, the filtrate is "trapped" in the urine compartment. Water will more easily escape, via osmosis as 99% of all molecules in the body are water. The increasingly concentrated urine along the axial pressure gradient will equilibrate with the interstitium along that gradient, and, as a result, there is also an osmotic gradient, but it is an effect of the axial pressure gradient already providing the concentrating mechanism for the kidney. Vasopressin and aldosterone contracts smooth muscle surrounding the duct system, favouring water reabsorption. The pelvic wall "pump" (Schmidt-Nielsen, 1987) operates by an analogous mechanism, it increases the axial pressure gradient. Evolutionarily, the reason for the extreme multiplication of nephrons is simply to generate this pressure gradient. The cortex is de facto packed with mesonephros, metanephros is thousands of mesonephros packed side by side. The evolutionary benefit: volume, acting along axial pressure gradient, powered by the heart investing 1/5th of its work into it. This is convergent evolution with the solution to urine concentration in the archinephros, its thin segment, an architecture that forces reabsorption of water, reused at a macroscale in the collecting duct system of the metanephros. This axial pressure gradient explains why loss of filtration rate causes impairment of the ability to concentrate urine (Berliner, 1958), and, why newborns with low blood pressure fail to concentrate urine.

Synapses

Schmidt–Nielsen, B. (1987). The renal pelvis. Kidney International, 31(2), 621–628. https://doi.org/10.1038/ki.1987.43

Berliner, R. W., Levinsky, N. G., Davidson, D. G., & Eden, M. (1958). Dilution and concentration of the urine and the action of antidiuretic hormone. The American Journal of Medicine, 24(5), 730–744. https://doi.org/10.1016/0002-9343(58)90377-2