GASES OF THE BLOOD 519 



mination of the volume of oxygen or carbon monoxide, which the 

 haemoglobin of blood can absorb, can be made without the aid 

 of a gas-pump ; the method is based upon the reaction of ferri- 

 cyanide with solutions of oxy-haemoglobin or carbon monoxide 

 haemoglobin, the combined gas is set free and methsemoglobin 

 is formed. 



The most important analyses of the gases of blood must now 

 be examined ; in the first place, those of the venous blood which 

 is passing to the lungs. The venous blood is liable to marked 

 differences in its gaseous contents, according to the nature and 

 condition of the organs of the body from which it is received ; 

 it is therefore only possible to obtain results which are directly 

 applicable to the question of the exchange of gases in the lungs, 

 when the analyses are performed upon samples of blood removed 

 from the right ventricle of the heart. Such samples can be 

 obtained by passing a catheter from the right external jugular 

 vein through the right auricle and into the right ventricle. 



The following table (see p. 520) gives the results of analyses of 

 simultaneous samples of venous blood from the right ventricle 

 and arterial blood. The figures represent the volumes of gas 

 in 100 c.c. of blood, and those in brackets the maximal and 

 minimal values of several analyses. 



An examination of the table shows that there are considerable 

 variations in the percentage volumes of gas ; thus in the case of 

 dogs the oxygen of the venous blood varies from 5*5 to 17*3, the 

 carbon dioxide from 24*96 to 61*08, and the oxygen of the arterial 

 blood from 15*0 to 25*6, the carbon dioxide from 30*65 to 47-33. 

 The range is greater in the venous than in the arterial blood ; 

 this result would be expected, for the composition of the mixed 

 venous blood of the right side of the heart is liable to variations 

 according to the activity of the muscles and glands of the body 

 and the different distribution of blood in the various organs. 

 Other causes of the variations are to be sought in individual differ- 

 ences and in errors of experiment. It is impossible to obtain two 

 animals in exactly the same condition, and it is very difficult, 

 except with the most perfect gas-pumps, to avoid some dilution 

 of the blood-gases with air. This error shows itself in the figures 

 for the amount of nitrogen contained in the blood, and is corrected 

 by many observers in the following way. As the basis of correc- 

 tion, it is assumed that blood holds in solution a similar volume 



