Fig. 1.

Diagrammatic representation of proximal tubular Ca²⁺ reabsorption. Filtered Na⁺ is reabsorbed from the proximal tubule (A), creating the osmotic driving force for water reabsorption, which, in turn, drives paracellular Ca²⁺ reabsorption either by creating a concentration gradient for Ca²⁺ (water removal increases the luminal concentration of Ca²⁺; B) or by convection/solvent drag (C). NHE3, Na⁺/H⁺ exchanger isoform 3.