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A TEXTBOOK OF PHYSIOLOGY 



The subject inspires through the mouthpiece M, and expires 

 through the meter (Fig. 144). Water is allowed to run from J? into 

 C, which contains pure CO, and displaces this gas at such a rate as to 

 give a known concentration of CO in the volume of air, indicated by 

 the meter, which is being breathed. The breathing is continued long 

 enough for the blood to come into equilibrium with the partial pres- 

 sure of CO and O 2 which is being breathed (the partial pressure of 

 O is that of the air). The finger is then pricked, and samples of blood 

 taken, and the saturation determined by the carmine method given 



0,05 



O f 05 bk% Co2 ia, a spired air 



220 30 



10 



20 30 t 



FIG. 145. 



so 



10 gO 30 



Dotted line =pressure in the air at bifurcation of trachea. 

 Solid line = pressure in the blood. 



In the upper part of the figure are shown the pressures of oxygen, in the lower part the 

 pressures of carbon dioxide. Ordinate pressure of oxygen expressed in per- 

 centages of an atmosphere. Abscissa = time. (Krogh.) 



above. A sample is shaken with the air (containing the known per- 

 centage of CO), which was breathed. If this sample is found to be 

 more saturated with CO than that drawn from the finger during the 

 breathing, the conclusion is drawn that the partial pressure of oxygen 

 in the blood in the pulmonary veins is higher than that in the air. 



The colorimetric method is liable to error ; moreover, a part of the 

 carbon monoxide actually absorbed may leave the blood and com- 

 bine with haemoglobin hi the muscles. There is no evidence in favour 

 of any of the carbon monoxide being destroyed in the body, error 

 does not seem to arise from that cause. 



