Respiration - 365 



°/o Hbos 

 HbO z 



100 



90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 



10 20 50 40 50 60 70 



90 100 110 120 150 140 150 



£ . pressure 



Fig. 19-8. The "oxygen dissociation curves" of hemoglobin are shown at 

 four different carbon dioxide pressures, indicated above each curve. Carbon 

 dioxide has a marked influence on the amount of oxygen with which hemo- 

 globin will combine at any particular oxygen pressure, and this action of 

 carbon dioxide is very important in the normal functioning of hemoglobin. 

 The normal carbon dioxide pressure in arterial blood is slightly over 40 milli- 

 meters, so that the heavy curve shows approximately the physiological condi- 

 tion. (Drawn by E. M. From Gerard, The Body Functions. Permission of John 

 Wiley and Sons, Inc.) 



plasma. Moreover, there is scarcely any shift 

 in the pH of the blood, despite the fact that 

 C0 2 , dissolved in aqueous media, gives rise 

 to carbonic acid (H 2 C0 3 ). The normal range, 

 pH 7.3 to 7.5, outside of which the cells of 

 the blood fail to maintain normal structure 

 and activity, is not disturbed by the influx 

 of C0 2 into whole blood; whereas plain 

 plasma, saturated with C0 2 , displays a dis- 

 tinctly acidic reaction (less than pH 6). 



The hemoglobin of the red cells plays an 

 important role in the transport not only of 

 2 but also of C0 2 . To be sure, only a small 

 fraction (perhaps 10 percent) of the C0 2 

 actually unites with hemoglobin, forming a 

 loose combination called carbamino hemo- 

 globin, which liberates C0 2 when the blood 

 reaches the lung. But changes in the dissocia- 

 tion tendencies of hemoglobin when it is 

 oxygenated to oxyhemoglobin are intimately 

 associated with the binding and freeing of 

 carbon dioxide and with the stabilization of 

 blood pH during these transactions. Both 

 forms of hemoglobin, at the pH of blood, are 

 in the acid range of their dissociation. Thus 



we may write H-Hb <=•: H+ + Hb~ and 

 H-HbOo ?=> H+ + HbO^. Moreover it is im- 

 portant to realize that H-Hb0 2 displays a 

 stronger tendency to dissociate, yielding a 

 greater concentration of H+ ion, than does 

 H-Hb. 



The relationship between the equilibrium, 

 H-Hb + 2 ?=±H-Hb0 2 , and the binding 

 and freeing of C0 2 is very complex. There- 

 fore it can be presented in boldest outline 

 only, with particular reference to what goes 

 on in the corpuscles. 



Aside from the small amount transported 

 as carbamino hemoglobin, C0 2 is carried in 

 the blood in the form of bicarbonates, partly 

 in the corpuscles as KHC0 3 and partly in 

 the plasma as NaHC0 3 . In the lung capil- 

 laries, H-Hb is converted to H-Hb0 2 and 

 this conversion yields extra hydrogen ion 

 (H+) needed for the liberation of C0 2 from 

 the bicarbonates. Simultaneously, however, 

 H-Hb0 2 is converted to KHb0 2 , a rela- 

 tively stable form of oxyhemoglobin, which 

 serves as the actual oxygen transport agency. 

 Thus we may write: H • Hb0 2 + K ■ HC0 3 «+ 



