1. C02 is generated in the tissues and diffuses freely into the venous plasma and then into the RBCs.
2. In the RBCs, C02 combines with H20 to form H2C03, a reaction that is catalyzed by carbonic anhydrase. H2C03 dissociates into H+ and HC03"
3. HC03" leaves the RBCs in exchange for CI" (chloride shift) and is transported to the lungs in the plasma. HCO:i is the major form in which C02 is transported to the lungs.
4. H+ is buffered inside the RBCs by deoxyhemoglobin. Because deoxy-hemoglobin is a better buffer for H+ than is oxyhemoglobin, it is advantageous that hemoglobin has been deoxygenated by the time blood reaches the venous end of the capillaries (i.e., the site where C02 is being added).
5. In the lungs, all of the above reactions occur in reverse. HCO:i enters the RBCs in exchange for CI". HC03" recombines with H+ to form H2C03, which decomposes into C02 and H20. Thus, C02, originally generated in the tissues, is expired.
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