342 RESPIRATION 



studies on the respiration, that carbon dioxide is formed in the lung's; and 

 recently Bohr and Henrique have published experiments which purport to show 

 that a considerable part (two to sixty-six per cent) of the carbon dioxide given 

 off is formed there. If these results should be confirmed, it would be necessary 

 to suppose that in the combustions going on in the body in addition to carbon 

 dioxide, a number of intermediary products of decomposition are formed, given 

 off to the blood, and there further oxidized (cf. page 339) ; also, possibly, that 

 some final oxidation takes place in the lungs. 



2. EXCHANGE OF GASES BETWEEN BLOOD AND LYMPH 



During its passage through the capillaries the blood gives off oxygen to 

 the tissues and receives carbon dioxide from them. We know very little at 

 present about the manner of this exchange in the tissues. But, since the 

 tension of oxygen in the tissues is extremely small, while according "to Stras- 

 burg the tension of carbon dioxide there exceeds that of the venous blood 

 (C0 2 tension in venous blood 42 mm. Hg., in the intestine 59, in the bile 51, 

 in acid urine 67), the exchange might be looked upon as a simple matter of 

 equalizing the tension. In view of the facts with which we have just become 

 acquainted under respiratory exchange in the lungs, and since the consumption 

 of oxygen (cf. page 27) does not depend upon the oxygen tension but upon 

 the activity of the tissues, it is possible that the vital activity of the vascular 

 wall should exercise some influence but we have no positive information on 

 this at present. 



3. CHANGES PRODUCED IN THE RESPIRED AIR 



The excretory products eliminated in the breath are carbon dioxide, water 

 vapor and possibly some other gaseous substances as yet imperfectly known. 



Inspired air contains in round numbers twenty-one vols. per cent oxygen 

 and seventy-nine vols. per cent nitrogen, if we disregard argon, etc. To these 

 are to be added some carbon dioxide, which amounts to only 0.03 per cent in 

 atmospheric air, but sometimes to considerably more in room air, and water 

 vapor, the quantity of which varies within wide limits. 



The expired air is saturated with water vapor, which for the most part 

 has its source in the respiratory passages (cf. page 323). To what extent this 

 water vapor represents a product of metabolism cannot yet be decided. 



In different animals and in different individuals, as well as in the same 

 individual under different circumstances, the amount of carbon dioxide in the 

 expired air exhibits wide variations according to the depth and frequency of 

 the respiratory movements, etc. The figure generally given for the normal 

 percentage of C0 2 in the expired air of man is 4.1 vols. per cent (Vierordt). 

 With quicker and deeper respirations the lungs are better ventilated and the 

 amount of C0 2 sinks to about 2.5-2.78 vols. per cent. Along with this the 

 quantity of C0 2 given off per minute becomes greater, which in itself how- 

 ever serves only as an expression of the improved ventilation and signifies 

 nothing concerning the way in which the formation and elimination of C0 2 



