Mechanisms of CSF formation: Net ion transport and electrochemical gradients A. CSF secretion by CP is by net transport of Na+, K+, Cl-, HCO3- and water, from plasma to ventricles. Reabsorptive ion fluxes CSF to blood occur simultaneously with active secretion but overall, there is net movement of ions and water into ventricles. CSF osmolality resembles plasma. B. Na+ moves down a concentration gradient via secondary active transport (e.g., Na+-H+ exchange) in the basolateral membrane (Fig. 5) [38, 43]. K+, Cl- and HCO3- diffuse down their electrochemical gradients  via ion channels in apical membrane  (Fig. 5). Arrow for Na+ symbolizes a steep inward concentration and electrochemical gradient . For K+, Cl- and HCO3-, the respective arrows depict outwardly-directed electrochemical gradients promoting ion diffusion via channels into CSF . Choroidal cell concentrations (mM) for Na+, K+, Cl- and HCO3-, respectively, are about 48, 145, 65, and 9.5 for rat . The potential difference across the CPe membranes is 45–50 mV, CSF being about 5 mV positive to plasma at pH 7.4.