684 



PHYSIOLOGY OF RESPIRATION. 



dissociation curve of the blood in man it is desirable, therefore, 

 to obtain the figures expressing the percentage saturation of the 

 hemoglobin in atmospheres containing carbon dioxid in amounts 

 equal to that found in the alveolar air of the individual examined. 

 In Fig. 275 a a dissociation curve for human blood is given in 

 which this factor was considered. The curve shows also, in the 

 thickness of the line, the individual variations that may be ex- 

 pected in the bloods of normal persons. For example, at an 

 oxygen pressure of 40 mms. the percentage saturation of the hemo- 

 globin for oxygen varies between 70 and 78 or 79. For a blood 



10 20 30 



60 60 70 80 90 100 



Fig. 275 a. The dissociation curve for human blood. The thickness of the curve indicates 

 the limits within which the curve may fall for normal persons. The figures along the abscissa 

 indicate the pressure of oxygen in mms. of mercury; those along the ordinate indicate the per- 

 centage saturation of the hemoglobin with oxygen. (From Barcroft, "The Respiratory Func- 

 tion of the Blood." By permission of University Press, Cambridge, England, Publishers.) 



that falls within the limits indicated by the curve Barcroft* pro- 

 poses the designation mesectic. Bloods which at any given pres- 

 sure, below saturation, take up a larger percentage of oxygen 

 (higher curve) may be called pleonedic, while those which under 

 the same conditions are less saturated than normal (lower curve) 

 are mdonectic. 



Condition of the Carbon Dioxid in the Blood. The condition 

 in which the carbon dioxid is held in the blood is not entirely 

 understood. It has long been recognized that a certain small 

 percentage is held in simple physical solution in the plasma and 

 in the corpuscles, and that a certain additional amount, much 



* Barcroft, "The Respiratory Function of the Blood," 1914. 



