The kidney before countercurrent multiplication

THEORY: Evolutionarily, the loops of Henle predate countercurrent multiplication, and evolved for an ancestral “single effect” that is still in use within the kidney today to some extent. This ancestral mechanism was the simple deposition of concentrated filtrate, generating concentrated filtrate by the resistance of the thin limb causing water reabsorption in the proximal tubule. The work that powered the process was the pressure gradient generated by the heart as a pump, filtration and reabsorption were connected in series down the same pressure gradient, and still are.