THE CHEMISTRY OF RESPIRATION 



501 



and p for the height of the barometer. P may be taken as 10,000 mm., so that the 



y 

 expression may be made to serve as the constant (c) of the apparatus. Then 



x = y X c. 



Having determined the oxygen content of this sample of blood, its carbon 

 dioxid content may be ascertained by the same procedure with the aid of tartaric 

 acid. If it is desired to compare the gas content of two different samples of blood, 

 they are placed in these two adjoining receptacles, 1 c.c. of each under 1.5. c.c. 

 of weak ammonia. They are then immersed in the water bath until the level of 

 the oil remains constant. The blood is then laked in the usual way. If the same 

 quantity of oxyhemoglobin is present in these samples of blood, the level of the oil 

 in the two tubes remains the same ; while if unequal amounts are present, the more 

 decidedly venous blood will absorb more oxygen from its bottle than the other. 

 Consequently, the level of the oil must rise on this side, the difference in the 



FIG. 257. BARCROFT'S BLOOD-GAS APPARATUS. 



levels indicating the amount of oxygen taken up, and hence, also the content in 

 hemoglobin. 



The quantities of oxygen and carbon dioxid vary greatly in different 

 samples of arterial and venous blood. Much depends upon the char- 

 acter of the blood-vessel, or rather, upon the intensity of the metabolism 

 of the tissue supplied by it. Still greater differences are encountered 

 if the blood of different animals is examined. Obviously, these 

 variations pursue a course parallel to the hemoglobin content, as well 

 as to the affinity which this body displays toward oxygen. It is the 

 general opinion that the percentage of oxygen is greater in carnivora 

 than in herbivora and birds, while the percentage of carbon dioxid 

 is smaller. The experiments of Pfliiger and others have furnished 

 such values as are included in the following table: 



