1. Dissolved in plasma: A small percentage of CO2 is carried directly dissolved in the plasma. This accounts for about 5-10% of the total CO2 transported.
2. Bound to hemoglobin: About 20% of CO2 is bound to hemoglobin, forming carbaminohemoglobin. This occurs at the amino acid residues of the globin protein, not at the heme group where oxygen binds. Unlike oxygen, CO2 can bind to hemoglobin simultaneously with oxygen.
3. As bicarbonate ions (HCO3-): This is the most significant way CO2 is carried in the blood, accounting for about 70%.
Here's how this process works:
* CO2 enters red blood cells (RBCs): CO2 diffuses from the tissues into the RBCs.
* CO2 reacts with water (H2O) to form carbonic acid (H2CO3): This reaction is catalyzed by the enzyme carbonic anhydrase, which is present in high concentrations in RBCs.
* Carbonic acid dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+): The H+ ions bind to hemoglobin, which helps buffer the blood and prevent pH changes.
* Bicarbonate ions (HCO3-) diffuse out of the RBCs into the plasma: This creates a concentration gradient that drives the further conversion of CO2 into bicarbonate ions.
* Chloride shift: To maintain electrical neutrality, chloride ions (Cl-) move into the RBCs from the plasma.
At the lungs:
* Bicarbonate ions (HCO3-) diffuse back into the RBCs: This is driven by the lower concentration of bicarbonate ions in the RBCs compared to the plasma.
* HCO3- reacts with H+ to form carbonic acid (H2CO3): This reaction is again catalyzed by carbonic anhydrase.
* Carbonic acid (H2CO3) breaks down into CO2 and water (H2O): The CO2 diffuses out of the RBCs and into the alveoli, where it is exhaled.
This complex process ensures efficient transport of CO2 from the tissues to the lungs, contributing to the regulation of blood pH and maintaining the oxygen-carrying capacity of hemoglobin.
