156 RESPIRATION 



sisting 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 CO 2 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 proportion to 

 its percentage in the mixture, and this is called its "partial pres- 

 sure." Now, the extraction of O and CO 2 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 nec- 

 essary to keep it in solution, it follows that if the pressure be 

 diminished the gas will partly escape. If an atmosphere con- 

 taining, say, O at a certain partial pressure be brought in con- 

 tact 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 

 separated by an animal membrane. 



This is the condition which obtains in the pulmonary alveoli. 

 The partial pressure of O in the alveolar air is much greater 

 than the tension of O in the blood; consequently the current is 

 from the air to the blood. The tension of CO 2 in the venous 

 blood is much greater than the partial pressure of the CO 2 in the 

 alveolar air; consequently the current is from the blood to the air. 



But, here, as in the last analysis of almost all physiological 

 phenomena, it is found that, while these purely physical laws 

 are certainly concerned in the pulmonary interchange of gases, 



