396 PHYSIOLOGY CHAP. 



Since the tension of the carbonic acid constantly increases with 

 the passage of the blood into the aortic capillaries, this must 

 facilitate the dissociation of oxygen, and increase its concentration 

 in the blood plasma, so that it can be easily absorbed by the 

 endothelia of the capillaries and the tissue cells. The increase in 

 tension of the carbonic acid partly compensates for the diminished 

 concentration of oxygen in the plasma due to its consumption by 

 the tissues. This regulation is of especial importance in asphyxia, 

 when the oxygen of the blood is much attenuated. 



On the other hand the high tension of carbonic acid in the 

 pulmonary capillaries does not in any way dimmish the absorption 



30 40 50 60 70 80 90 100 110 120 130 140 150 



FIG. 172. Curve showing influence of varying quantity of CO-> on absorption of oxygen at different 

 pressures, from defibrinated dog's blood at 38 C. (Bo'hr, Hasselbach, and Krogh) The 

 percentage amount of oxygen absorbed is marked on the axes of the ordinates the different 

 pressures of oxygen in mm. Hg, on the axes of the abscissa, while the different pressures of 

 COo acting simultaneously in the pressure of biood, are indicated above the respective curves. 



of oxygen, for which, as has been shown, the influence of carbonic 

 acid becomes negligible in the presence of high oxygen tensions. 



This fact suffices to explain on a simple process of diffusion 

 the gas exchanges between the blood and the tissues, by which 

 the latter continually absorb oxygen and give off carbonic acid, 

 converting arterial into venous blood. The data in regard to 

 external respiration, do not, however, preclude the probability that 

 the physical laws of diffusion may, in the case of internal 

 respiration also, be modified by the activity of the cells which 

 build up the walls of the capillaries. 



X. We have seen that Lavoisier conceived of respiratory 

 chemistry as a slow double combustion of carbon and hydrogen 

 by which are formed carbonic acid and water. He was the first to 

 show that the amount of 2 absorbed exceeds that of the C0 9 

 exhaled, from which fact he deduced the formation of water. 



