THE CHEMISTRY OF RESPIRATION 



1119 



alkaline reserve of the plasma increases while the chlorides diminish, and 

 the reverse change must take place when the carbonic acid tension in the 

 blood is diminished as on exposure of the blood to a vacuum. 



EXCHANGE OF GASES IN THE LUNGS 



A fluid gives off gas to or takes up gas from any other medium with 

 which it is in contact, according to the relative pressures of the gas. The 

 question arises whether the physical conditions in the lungs are such as to 

 account for the absorption of oxygen and the evolution of carbon dioxide 

 by the blood in its passage through these organs. In order to answer this 

 question we must know the partial pressures or tensions of oxygen and of 

 carbon dioxide in the alveolar 

 air, in the venous blood coming 

 to the lungs, and in the arterial 

 blood leaving the lungs. In the 

 alveoli the ^pressures are given 

 by the analysis of alveolar air. 

 The determination of the gaseous 

 tensions in the blood presents 

 however considerable difficulty. 

 It is necessary to bring the blood 

 in contact with gaseous mixtures 

 containing various proportions 

 of the gas whose tension in the 

 blood it is desired to measure. 

 By making various experiments 

 a gaseous mixture will be found 

 with which the blood is in 

 equilibrium. If we know before- 

 hand the amount of gas in this 

 mixture, we know its tension and therefore the tension of the gas in the 

 liquid. 



Pfliiger's aero tonometer (Fig. 514) consists of two glass tubes, R and R, contained in 

 a vessel filled with water at the temperature of the body. The upper ends of the tubes 

 are connected by the tube a with the artery or vein in which it is desired to estimate 

 the tension of the blood gases. If, for instance, we wish to determine the tension of 

 CO 2 in venous blood, wnere we expect the tension to amount to about 4 per cent, of an 

 atmosphere, one tube R is filled with a gaseous mixture containing 3 per cent. C0 2 , and 

 the other tube R with a mixture containing 5 per cent. CO 2 . a is now connected with 

 the distal end of the jugular vein or with the central end of the carotid artery, and 

 blood is allowed to flow hi a thin stream down the walls of the tubes R and R, thus 

 presenting a large surface to the contained gases. The blood collects in the lower 

 narrower portions of the tubes, and runs out into the vessels b, 6, whence after defibrina- 

 tion it is returned at intervals into the veins of the animal. 



In all such instruments the main difficulty is in obtaining a sufficient 

 surface of the blood exposed to the gaseous mixture. The interchange of 



FIG. 514. Pfliiger's aerotonometer. 



