2 5 8 



ESSENTIALS OF PHYSIOLOGY. 



oxyhtemoglobin. This difference is chiefly due to the presence of salts, 

 particularly potassium salts, in the blood, which render the combination 

 between hemoglobin and oxygen more unstable ; when haemoglobin 

 is dissolved in water containing the same salts as those normally 

 present in blood, its dissociation curve is identical with that of blood. 



The other factors, which alter the form of the dissociation curve of 

 blood, are (1) the^temperature of the blood, and (2) the presence of 

 carbonic or other acids. The higher the temperature of the blood 

 and the greaterjthe percentage of carbonic or other acids, the more 



S 60 



100 



90 



80 



I 70 



X 



o 



s 



I" 



1 40 



I 30 



20 



10 



i\ 



10 



90 IOO 



20 30 40 SO 60 70 80 



Oxygen pressure in mm. of mercury. 



FIG. 99. Dissociation curves (A) of haemoglobin dissolved in water at 

 37 C., and (B) of blood at 37 C. 



readily does the oxyhremoglobin dissociate. The effect of carbonic 

 acid and of lactic acid is shown in fig. 100. These factors not only 

 modify the extent to which oxyhsemoglobin dissociates at the same 

 partial pressure of oxygen, but also the rate at which it loses oxygen 

 and becomes reduced ; when an indifferent gas such as nitrogen 

 is bubbled through a solution of blood, the blood becomes reduced 

 much more rapidly if it contains an excess of carbonic acid or a small 

 amount of lactic acid. The influence of carbonic or other acids on the 

 readiness with which oxjheemoglobin dissociates, when the pressure of 

 oxygen to which it is exposed is low, is of great physiological importance. 

 As the blood passes through the capillaries, it not only gives off 

 oxygen, but also receives from the tissues carbonic acid and frequently 



