13% apical CO₂ increases net solute and fluid absorption across the RPE. The transporters and channels depicted in this model are adopted from earlier studies of frog, bovine, human, and cultured human RPE. Apical membrane proteins: Na/K ATPase, Na/K/2Cl cotransporter (NKCC1), Na/H exchanger (NHE), and Na/2HCO₃ cotransporter (NBC1). Basolateral membrane proteins: Ca²⁺-activated Cl channels, cAMP-sensitive CFTR, Cl/HCO₃ exchanger (AE2), and Na/nHCO₃ cotransporter (NBC). Increasing apical CO₂ increases Na (Cl + HCO₃) and fluid absorption from the SRS to the choroid.