Uptake and release in capillaries of carbon dioxide

Carbon dioxide diffuses from the site of production in metabolizing cells, through the interstitial fluid, across the wall of the systemic capillary and into the blood plasma. It readily diffuses across the membrane of the erythrocyte. When carbon dioxide enters the erythrocyte, bicarbonate and hydrogen ions are formed. The rise in intracellular concentration of bicarbonate leads to the chloride shift, described in Chapter 4. The hydration of carbon dioxide in the erythrocyte occurs very quickly, due to the presence of the enzyme carbonic anhydrase, which catalyses the reaction  

The speed of reaction within the erythrocytes is about 13 000 times faster than in the plasma. This accounts both for the rapid uptake of carbon dioxide by the blood during the short transit time (about one sec.) of blood through the systemic capillary and for the rapid release of carbon dioxide from the blood to the alveolar gas during the transit of the blood through the pulmonary capillaries. Without carbonic anhydrase, carbon dioxide partial pressures in the venous blood, and hence in the tissues would be much higher. Indeed when 

COMPARISON OF THE OXYGEN AND CARBON DIOXIDE DISSOCIATION CURVES OF BLOOD 

Since carbon dioxide reaets with water to yield bicarbonate which is readily transported by the blood, no special carrier molecule, analogous to haemo- 

OXYGEN CARRIAGE, RESPIRATION AND ACID-BASE PHYSIOLOGY 

---