Figure 1. Bicarbonate reabsorption mechanisms in the proximal tubular cell.

On the luminal side, the Na⁺-H⁺ exchanger, NHE3, carries out the majority (~80%) of hydrogen ion secretion, while the proton pump H⁺-ATPase is responsible for ~20% of proton transport. Note that CO₂ entry via an AQP1 channel is controversial and not fully proven.

AQP1, aquaporin 1; CA, carbonic anhydrase; NBCe1-A, sodium bicarbonate cotransporter 1 variant A; NHE3, Na⁺/H⁺ exchanger isoform 3. Modified from Singh et al [4] with permission.