374 



RESPIRATION 



[CH. XXVI. 



relationships of the carbon dioxide; the alveolar tension of this gas 

 (dotted line) is always lower than that of the arterial blood (continu- 

 ous black line). It will further be noticed that the pressure 

 differences are less in the case of carbonic acid than in that of 

 oxygen ; this coincides with the ease with which carbonic acid passes 

 out through the membrane which separates the blood from the air. 



10 20 30 40 50 60 70 80 90 minutes. 



FIG. 299. Diagram to represent the relationship between the tensions of oxygen and carbon dioxide 

 in blood and alveolar air in rabbit (after Krogh). 



Some authorities consider that in cases of definite oxygen want, 

 such as during violent muscular exercise, or on the tops of high 

 mountains, the lining epithelium of the pulmonary alveoli can, by a 

 process of active secretion, transfer oxygen from the alveolar air to 

 the blood. In the case of exercise the observations made by different 

 workers are at variance, whilst at high altitudes they are so few as 

 to make further work desirable before physiologists generally can 

 accept the possibility of the secretion of oxygen by the lung. That 



