404 THE RESPIRATION 



OXYGEN CARBON DIOXIDE TOTAL GAS 



Venous blood 12 48 60 



Arterial blood 20 40 60 



The estimation of the amounts of the gases, although of little value 

 in connection with the physiology of gas exchange, is very important in 

 supplying information regarding the respiratory activities of the various 

 organs and tissues. Just as we determine the total respiratory exchange 

 of an animal by measuring the differences in 2 and C0 2 in inspired and 

 expired air, so may we determine the local respiratory exchange of the 

 tissues by analysis of the gasses in blood removed from the artery and 

 vein of the tissue. It should be clearly understood, however, that it is 

 not the percentage but the total amount of the gases that must be con- 

 sidered, and that it is therefore necessary to know the volumes of blood- 

 flow as well as the percentage of the gases. Something will be said later 

 of the results of such investigations (see page 408). 



At present we are concerned with the manner in which the gases are 

 carried in the blood. The 2 , as we have seen is carried by the hemo- 

 globin, some being also in a state of simple solution in the plasma. The 

 C0 2 , to which we must now pay attention and which it will be noted 

 is present even in arterial blood in considerably greater amount than 

 the 2 , is carried by various agencies, the relative importance of each 

 of which is not as yet clearly understood. A most important feature 

 of the mechanism is that there is a considerable degree of interdependence 

 between the carrying agencies for C0 2 and 2 , an increase in the one 

 causing a decrease of the other. By comparison of the dissociation curve 

 of blood for oxygen at varying pressures of C0 2 we have already studied 

 this relationship in so far as C0 2 influences the 2 -carrying power of 

 blood. By adopting the same method but in the reverse way we may 

 also investigate the influence of varying tensions of 2 on the C0 2 carry- 

 ing power. 



The C0 2 -Dissociation Curve of Blood. This is constructed by expos- 

 ing defibrinated blood in a flask at body temperature to atmospheres 

 containing known percentages of C0 2 and then removing samples and 

 analyzing them for C0 2 in Barcroft's or Van Slyke's apparatus. The 

 results are plotted as shown in Fig. 142 by placing the tensions (calculated 

 from the percentages) of C0 2 in mm. Hg on the abscissae and the vol- 

 umes per cent of C0 2 absorbed on the ordinates. When the atmosphere 

 with which the C0 2 is mixed is a neutral gas such as hydrogen, the upper 

 curve (B) is obtained; when the atmosphere is air, the lower curve (A). 

 Clearly, reduced blood can carry considerably more C0 2 at all pressures 

 of this gas, than oxygenated blood. Disregarding for the moment the 

 agency by which the C0 2 is carried, it is important to note that the 



