CHAPTER XLV 

 RESPIRATION BEYOND THE LUNGS Cont'd 



THE MEANS BY WHICH THE BLOOD CARRIES THE GASES 



In the foregoing account of the physiology of the blood gases, empha- 

 sis is placed on the tension under which the gases exist rather than on 

 the total amount of each gas present in the blood. This has been done 

 because the exchange of gases between alveolar air and blood and be- 

 tween blood and tissues proceeds according to the laws of gas diffusion, 

 which are of course dependent upon differences in gas pressure or 

 tension. 



Something must now be said regarding the amount of the gases. This 

 may be measured either by physical or by chemical methods. In the 

 former, a measured quantity of blood is received into an evacuated glass 

 vessel, which is then attached to a mercury pump, by which the gases 

 are sucked out of the blood and transferred, by suitable manipulations 

 of stopcocks, to a graduated tube, in which they are then analyzed by 

 chemical means. The principle of the chemical method has already been 

 described in connection with the measurement of oxygen in hemoglobin 

 solutions (see page 393). A measured quantity of blood, kept free from 

 contact with the air, is transferred under some weak ammonia solution 

 to one of the blood-gas bottles of the blood-gas differential manometer, 

 and a few drops of a saturated solution of potassium ferricyanide are 

 placed in the pocket of the bottle. After the blood has been laked and 

 temperature conditions adjusted, the ferricyanide is mixed with the 

 blood solution, thus causing the 2 to be quantitatively displaced. From 

 the increased pressure produced in the manometer the amount of 2 can 

 readily be computed. To determine the C0 2 of the blood, the bottle is 

 now removed from the manometer and a few drops of a saturated solu- 

 tion of tartaric acid placed in the pocket. When this is mixed with the 

 deoxygenated blood mixture, after the usual adjustment for tempera- 

 ture, the pressure caused by the evolved C0 2 is recorded and the amount 

 present calculated. 



The results of the analysis are expressed as the number of cubic centi- 

 meters of gas present in 100 c.c. of blood the volume percentage, as it 

 is called. The following are approximate percentage values: 



403 



