F. Carbon Dioxide

Carbon dioxide is more soluble in water than oxygen and is reactive with water, forming carbonic acid (H₂CO₃). Carbonic acid quickly dissociates to form H⁺ (a proton or hydronium ion) and HCO^{₃ -} (bicarbonate ion). Carbon dioxide that reacts with water to form carbonic acid no longer contributes to the partial pressure (P_CO2). For both of these reasons, there is more inorganic carbon (CO₂ and H₂CO₃) in water at a given partial pressure than oxygen. Also, there is much less CO₂ in the atmosphere than O₂ (0.035% versus 21%), so there is usually a tiny fraction of the pressure (0.25 mm Hg at air saturation) of CO₂ in water compared to O₂ (150 mm Hg)_.

Aerobic metabolism produces a molecule of CO₂ for each molecule of O₂ . So the same amount of CO₂ as O₂ must be carried by the blood and diffuse across the gill. Since CO₂ is very soluble and, more importantly, reactive, the blood easily carries all that is produced by metabolism at low partial pressure (about 2 mm Hg) without the necessity of a specific carrier molecule (hemoglobin) that oxygen requires. That is not to say that red blood cells (RBCs) and Hb do not play a role in CO₂ transport. RBCs and gill epithelial cells contain an enzyme, carbonic anhydrase (CA), that catalyzes the reaction (both ways) between water and CO₂. The CA in the red blood cells of venous blood converts the vast majority (95%) of the CO₂ into bicarbonate. Of the remaining CO₂, a small amount is in solution and some is bound to the Hb. When the the blood reaches the gill, the Haldane effect causes much of the bicarbonate to shift back to CO₂ increasing the partial pressure. The partial pressure gradient between blood and air at the gill for CO₂ is still relatively small compared to that for oxygen, however. In addition to CA and the Haldane effect, there is an enzyme system that is powered by ATP that actively pumps the bicarbonate ion out of the cell. This is independent of both partial pressure and osmotic gradients. The export of bicarbonate (HCO₃^-) is coupled with an import of chloride (Cl^-). Once the bicarbonate ion is pumped out into the water, much of it will split back to CO₂ and H₂O, thus potentially lowering the already small pressure gradient for carbon dioxide between the water and the blood. However, since there is no carbonic anhydrase in the water the reaction back to CO₂ is relatively slow and by the time much of the carbon dioxide has formed, the water has passed out of the gills. So, practically, the bicarbonate pump has little effect on the carbon dioxide gradient.

In Summary, a fish must move as much CO₂ out of the blood at the gill as it moves O₂ into the blood. The problem is that the high solubility and reactivity of CO₂ makes for a much low partial pressure gradient than exists for O₂ meaning diffusion alone will not move the CO₂ as rapidly as O₂. This is addressed by the enzyme CA and the Haldane effect that temporarily boost the partial of CO₂ as the blood enters the gill and the bicarbonate pump that acts independently of partial pressure.