K KSIMRATION BEYOND THE LUNGS 



385 



(D in Fig. 138). The results are very different. At a partial pressure 

 of 2 of about 60 mm. Hg that is, a lower pressure than exists in the 

 lung alveoli (100 mm.) the blood becomes nearly saturated with O 2 , 

 whereas at pressures below 50 mm. it readily loses 2 , so that at 10 mm! 

 there is nearly complete reduction. 



The question is: What are the environmental conditions under which 

 the hemoglobin in the blood so alters its combining power for 2 as to 



Fig. 139. Dissociation curves of hemoglobin. 



Ordinates Percentage saturation of hemoglobin. 

 Abscissa Tension of oxygen in mm. of mercury. 



I. Dissociation curve of hemoglobin dissolved in water. 



II. Dissociation curve of hemoglobin dissolved in 7% NaCl. 



III. Dissociation curve of hemoglobin dissolved in 9% KC1 

 Temperature 37-38 C. (From Joseph Barcroft.) 



produce such a difference in the dissociation curve? By experimenting 

 with hemoglobin solutions, three such factors have been found to come 

 into play: (1) the presence of inorganic salts, (2) the hydrogen-ion con- 

 centration (C0 2 tension) of the solution, and (3) the temperature. If 

 hemoglobin is dissolved in water containing the various salts of plasma 

 in the same proportion as in blood (artificial plasma), the dissociation 

 curve will be found to change so as to resemble that of blood (Fig. 139). 



