156 RESPIRATION 



which is dissociated on the addition of an acid, or corpuscles, 

 or hemoglobin, etc. It may be that as the blood passes 

 through the lungs there is set free, in the formation of oxy- 

 hemoglobin, an acid which immediately unites with the bases 

 holding the CO in combination the liberation of the latter 

 being the consequence. 



The O being thus in the air vesicles, and the CO thus 

 free, or set free, in the blood, with the very thin animal mem- 

 brane consisting of the vesicular and capillary walls between 

 them, it remains to be seen what forces are concerned in the 

 interchange of these gases. It has been noted that only one- 

 fourth of the O entering the lungs in the air is taken up by 

 the blood ; so it is to be remembered that not all the CCte 

 entering the lungs in the venous blood is taken up by the air. 



Interchange of Oxygen and Carbon Dioxide in the Lungs. 

 The condition of "partial pressure" of gases in mixture 

 has been mentioned. Each gas exerts a pressure in propor- 

 tion to its percentage in the mixture, and this is called its 

 "partial pressure." Now, the extraction of O and COs from 

 the blood by placing it in a vacuum shows that both these 

 gases exist in the blood under a certain degree of tension. 



The tension of a gas in solution being only the pressure 

 necessary to keep it in solution, it follows that if the pres- 

 sure be diminished the gas will partly escape. If an atmos- 

 phere containing, say, O at a certain partial pressure be 

 Drought in contact with a fluid containing O at a certain 

 tension, unless the partial pressure of the O in -the air be 

 equal to its tension in the fluid there will be an escape of the 

 gas from the point of higher to the point of lower pressure 

 or tension. If the partial pressure of the gas be less in the 

 atmosphere than its tension in the fluid, the current will be 

 from the latter to the former and vice versa. This will be 

 the case whether the media are in actual contact or separ- 

 ated by an animal membrane. 



This is the condition which obtains in the pulmonary alve- 

 oli. The partial pressure of O in the alveolar air is much 



