FIGURE 9.

The classical model of cerebral spinal fluid (CSF) production. 1: In this model of CSF secretion, the Na⁺-K⁺-ATPase is the primary driver of CSF production, and secretes Na⁺ directly into CSF. 2: The activity of the Na⁺-K^+-ATPase lowers intracellular Na⁺, which in turn promotes Na⁺ influx across the basolateral membrane, which depends on Na⁺/${\text{HCO}}_3^{-}$/Cl⁻ cotransporters to produce an increase in cytosolic anions (Cl^- and ${\text{HCO}}_3^{-}$) concentrations. 3: In turn, the high density of anion channels at the apical membrane facilitates efflux of Cl^- and ${\text{HCO}}_3^{-}$. 4: Water is drawn into CSF by the increase in osmolarity supported by aquaporin 1 (AQP1) water channels. AQP1 at the basolateral membrane likely also contributes to water influx to the cell. NCBE, Na⁺-driven Cl⁻/${\text{HCO}}_3^{-}$ exchangers.