370 RESPIRATION [CH. XXVI. 



Under these conditions, viz., in the presence of potassium salts, at 

 37 C., and in the presence of a carbonic acid pressure of 40 mm. Hg, 

 the dissociation curve is that shown in the next diagram (fig. 279). 



The two coloured figures (295 and 297) should be carefully 

 compared; they present to the eye graphically the superiority of 

 hsemoglobin as an oxygen carrier when it is present in the actual 

 blood,over that which it possesses in a pure solution. In the second 



FIG. 297. Dissociation curve of hsemoglobin in the actual blood. Purple, reduced 

 haemoglobin ; red, oxyhsemoglobin. 



curve, that of the blood itself, it will be seen that at an oxygen 

 pressure of over 60 mm. of mercury (the pressure in the lung alveoli 

 is about 100) the blood will nearly saturate itself with oxygen, and 

 that at pressures below 50 the blood loses its oxygen rapidly, whilst 

 at 10 mm. pressure it is nearly completely reduced. As the rate at 

 wh'ich oxygen can diffuse out of the capillaries into the surrounding 

 tissues depends upon the pressure it exerts in the plasma, it is 

 important that the haemoglobin of the blood should be capable of a 



