T RANSPORT OF G ASES · Part 2

CO . pO is a major factor which could affect this binding. When pCO is high and pO is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO is low and pO is high as in the alveoli, dissociation Partial pressure of oxygen (mm Hg) Percentage saturation of haemoglobin with oxygen Figure . Oxygen dissociation curve of CO from carbamino-haemoglobin takes place, i.e., CO which is bound to haemoglobin from the tissues is delivered at the alveoli. RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions. CO H O H CO Carbonic anhydrase Carbonic anhydra + → ← sse HCO H → ← + − + At the tissue site where partial pressure of CO is high due to catabolism, CO diffuses into blood (RBCs and plasma) and forms HCO – and H +, . At the alveolar site where pCO is low, the reaction proceeds in the opposite direction leading to the formation of CO and H O. Thus, CO trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO (Figure . ). Every ml of deoxygenated blood delivers approximately ml of CO to the alveoli.