540 



VERTEBRATE LIFE AND ORGANIZATION 



reaction occurs much more rapidly in the erythrocytes than in the 

 plasma because they contain the enzyme carbonic anhydrase, which 

 speeds up the reaction 1500 times. Carbonic acid in turn dissociates into 

 hydrogen and bicarbonate ions, and many of the bicarbonate ions diffuse 

 out ot the erythrocytes into the plasma. A great deal of carbon dioxide, 

 then, is carried as bicarbonate ions. The hydrogen ions combine with 

 oxyhemoglobin and this facilitates the dissociation of oxyhemoglobin and 

 the release of oxygen to the tissues. Oxyhemoglobin is a stronger acid than 

 hemoglobin and the conversion of oxyhemoglobin (HbOo) to hemoglobin 

 (Hb) would tend to raise the pH within the red cell (make it more 

 alkaline). The formation and dissociation of carbonic acid would tend 

 to lower the pH within the red cell (make it more acid). These two 

 opposing phenomena tend to balance each other and the pH of the 

 erythrocyte is maintained essentially unchanged. The potassium ions 

 (K + ) previously neutralized by the oxyhemoglobin are now neutralized 

 by the bicarbonate ions: 



CO2 



Tissues 



Plasma 



carbonic 



anhydrase 

 H+ + KHb02 



-> H2CO3 



^H+ + HCO3 



HCOy + K+ 



->K+ + HHb + O2 

 > KHCO3 



Red 



Cell 



In the lung capillaries, where the tension of carbon dioxide is lower 

 than in the venous blood, the reactions described above move in the 

 opposite direction and carbon dioxide diffuses out of the blood. Oxygen 

 entering the blood from the lungs combines with hemoglobin and facili- 

 tates its giving up hydrogen ions and hence the release of carbon dioxide 

 by the blood. Tfiis set of reactions may be expressed as follows: 



Lungs 



Plasma 



KHCO3 



>02 + HHb + K + 



■^ K+ + Hcor 



H+ + HCOJ 



-^H2C03- 



^KHbOa + H + 



carbonic 



anhydrase 



> H2O + CO2 



Red 



Cell 



These reactions enable the blood to carry a great deal more oxygen 

 and carbon dioxide than it could in simple physical solution, they 



