Most of bicarbonate is present in the plasma. RBC's aren't too permeable to bicarbonate. CO 2 equilibrates across the membrane. D. Transfer of CO 2 to and from blood. Most of CO 2 goes through RBC's.
Rapid conversion leads to formation of a gradient.
See figure 13-11. Passive transport of bicarbonate depends on direction of gradient. In addition to facilitating CO 2 release, this outcome causes buffering of pH. 3. Carbonic anyhdrase at the lung converts plasma bicarbonate to CO 2 rapidly. This helps pull bicarbonate out of RBC and then CO 2 out of plasma down concentration gradients.