236 A MANUAL OF PHYSIOLOGY 



comes off in proportion to the reduction of the partial 

 pressure of the oxygen in the pump, and is simply in solution. 

 When blood is being pumped out, very little oxygen 

 comes off till the pressure has been reduced to about half an 

 atmosphere. At about a third of an atmosphere, if the 

 blood is nearly at body temperature, the oxygen begins to 

 escape a little more freely ; and when the pressure has fallen 



FIG. 84. CURVE OF DISSOCIATION OF OXYH^EMOGLOBIN AT 35 C. (AFTER 



HiJFNER'S RESULTS.) 



Along the horizontal axis are plotted the partial pressures (numbers below the 

 curve) of oxygen in air, to which a solution of haemoglobin was exposed. The corre- 

 sponding percentages of oxygen are given above the curve. Along the vertical axis is 

 plotted the percentage saturation of the haemoglobin with oxygen. Thus, on exposure 

 to an atmosphere in which oxygen existed to the extent of i per cent., corresponding 

 to a partial pressure of 7 '6 millimetres of mercury, the haemoglobin took up about 

 75 per cent, of the amount of oxygen required to saturate it. When the oxygen was 

 present in the atmosphere to the amount of about 10 per cent., corresponding to a 

 partial pressure of 76 millimetres of mercury, the quantity taken up by the haemo- 

 globin was about 96 per cent, of that required for saturation. 



to about one-sixth of an atmosphere (corresponding to a 

 partial pressure of oxygen of 25-30 mm. of mercury), it is 

 disengaged with a burst. This shows that it is not simply 

 absorbed, but is united by chemical bonds to some con- 

 stituent of the blood. The same thing is seen when de- 

 fibrinated blood is saturated at body temperature with 

 oxygen at different pressures. The quantity taken up 



