THE CHEMISTRY OF RESPIRATION 1195 



of man, though an approximate valuation of these tensions can be 

 obtained by knowing the degree to which the arterial and venous 

 blood respectively are saturated with oxygen or carbon dioxide. An 

 indirect method may be employed to measure the gaseous tensions 

 in the venous blood coming to the lungs. It is possible, as Loewy 

 has shown, to block the right bronchus in man by introducing a catheter 

 through the larynx and trachea, so that the renewal of air in the right 

 half of the lung is entirely stopped for some time. A sample of air 



to 



19 



18 

 17 

 16 

 15 

 1*t 

 13 

 12 

 11 

 V) 



ISO 



2095 



in inspired 

 air 



50 2 10 30 



so 3 to 



FIG. 502. Tensions of gases in alveolar air and in arterial blood. 



A, during artificial increase of oxygen tension in alveoli ; B, during artificial 



increase of CO 2 tension in alveoli. 



in the blocked lung can be taken at any time by means of the catheter. 

 The interchange of gases between alveolar air and blood will go on 

 until the tension of gases in the air is the same as that coming to the 

 blocked portion of lung. By this means the tension of the oxygen in 

 the venous blood was found to be 5'3 per cent. = 37 mm. Hg, and 

 that of the carbon dioxide 6 per cent. = 46 mm. Hg. 



The tensions in the alveolar air of man may be taken as f oUows : 



Oxygen 

 Carbon dioxide 



107 mm. Hg. 

 40 



As the venous blood enters the lungs there is thus a difference of 

 pressure of 107 - 37 = 70 mm. Hg, which will tend to cause a flow 

 of oxygen from alveolar air to blood and a difference of 46 - 40 = 

 6 mm. Hg, tending to cause a flow of carbon dioxide from blood to 

 alveolar air. Is this difference sufficient to account for the amount 



