FIGURE 3.

The transportation of free water and ions of the cornea under physiological state. The epithelial cells and the tight junction prevent free water of the tear film from entering the cornea; the endothelial cells create an osmolarity gradient towards the anterior chamber, which allows the free water from the stroma to be “pumped” into the anterior chamber. The osmolarity gradient is facilitated by a group of enzymes and ion channels (left panel). Firstly, the carbonic anhydrase (CA) 12 expressed in the stroma synthesizes ${\text{HCO}}_3^{‐}$, which is then passively transported into the endothelial cells by the $\text{Na}/{\text{HCO}}_3^{‐}$ co‐transporter (NBC1) down the chemical‐potential gradients created by a 3Na⁺/2K⁺‐ATPase. In the cytoplasm of endothelial cells, the ${\text{HCO}}_3^{‐}$ breaks down into CO₂ and H₂O by CAII, then the CO₂ diffuses back to the stroma or anterior chamber. The remaining ${\text{HCO}}_3^{‐}$ is transported to the anterior chamber via a ${\text{HCO}}_3^{‐}/{\text{Cl}}^{‐}$ exchanger. This ${\text{HCO}}_3^{‐}$ transportation lowers the osmolarity at the basolateral/stromal interface relative to that in the aqueous humor, allowing the free water to flush into the anterior chamber, possibly via the water channel aquaporin (AQP) 1