THE CHEMISTRY OF RESPIRATION 



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and 517. In Fig. 517 A and B (Krogh) the composition of the alveolar air 

 artificially altered by increasing the percentage of carbon dioxide and of 

 oxygen respectively. It will be seen in each case that there was a corresponding 

 alteration of the tension in the arterial blood, the tension of carbon dioxide 

 being higher and that of oxygen lower in the blood than in the air through- 

 out the experiment. We have no direct determinations of the tensions of the 

 gases in the blood of man, though an approximate valuation of these tensions 



FIG. 515. A, Krogh's microtonometer. B, upper part of microtonometer showing 

 capillary tube into which the bubble is returned for measurement and analysis. 



can be obtained by knowing the degree to which tjie arterial and venous 

 blood respectively is 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 cathether 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 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. 

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