RESPIRATION BEYOND THE LUNGS 391 



OXYGEN CARBOX 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 0, and C0 2 in inspired and 

 expired air, so may we determine the degree of tissue respiratory ex- 

 change by analysis of the gases 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 393). 



At present we are concerned with the manner in which 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 , which it will be noted is present even in arterial blood in con- 

 siderably greater amount than the 2 , is partly combined with alkali to 

 form bicarbonates. The alkali available for this purpose varies from 

 time to time according to the amount of other acid substances present. 

 Since these are stronger acids than carbonic, any increase in their 

 amount (acidosis) causes displacement of some of the C0 2 , thus bring- 

 ing about, as we have seen, a relative increase in free C0 2 in the blood 

 and therefore raising the C H . 



What particularly interests us here is the agency by which the corn- 

 lined C0 2 is carried in the Uood. If blood is exposed to a full atmos- 

 phere of C0 2 , it will take up as much as 150 per cent of the gas that 

 is, between two and three times the amount ordinarily present in it. 

 It has therefore a great reserve capacity for C0 2 . A greater propor- 

 tion of the C0 2 is carried in the plasma than in the corpuscles; but if 

 plasma (or serum) is exposed in a vacuum, all of the C0 2 present in it 

 will not be evolved. When blood itself is similarly exposed, on the 

 other hand, all the C0 2 is given off. To liberate all of the C0 2 from 

 plasma in vacuo, some acid must be added, from which it has been in- 

 ferred that blood corpuscles act like weak acids. It is commonly stated 

 that hemoglobin or some constituent of blood is capable of freeing C0 2 

 from solutions of sodium carbonate, but the recent work of Buckmaster 32 

 shows that this is not the case. The decomposing power of blood is 

 caused by the development of acidity in the shed blood and any similar 

 power that the corpuscles may exhibit is due to a discharge from 



