THE RESPIRATORY SYSTEM. 



257 



by Barcroft's blood-gas apparatus. If, for example, the partial pressure 

 of oxygen in the mixture of gases placed in the aerotonometer is 

 40 mm. Hg (corresponding with about 5 per cent, of oxygen), and 



Percentage saturation 

 with oxygen 



100 



Oxygen pressure 

 mm. 



FIG. 97. Dissociation curve of hremoglobin dissolved in water at 37 C. 

 (From Barcroft, Respiratory Function of the Blood.} 



obin is red, and reduced haemoglobin is purple. 



the percentage of oxyhaemoglobin in the blood, when in equilibrium 

 with this mixture, is 70 per cent., that point can be marked on the 

 chart. By making a series of such observations, and using varying 

 percentages ofx^xygen in the 

 gaseous mixture, & series of 

 points is determined and plotted 

 The line joining these 



out. 



FIG. 98. Barcroft's tonometer. 



/ * x 



points is the dissociation curve. 



The form of the curve can be greatly altered by varying circum- 

 stances, and the dissociation curve of blood differs considerably from 

 that of a somtion of^pHfe haemoglobin, as may be seen in fig. 99. A 

 comparison of the two curves shows that oxyhaemoglobin dissociates 

 more readily in blood than when simply dissolved in water. When 

 the partial pressure of oxygen is 20 mm. Hg, blood contains only 

 30 per cent, of its haemoglobin in the form of oxyhaemoglobin, whereas 

 in a watery solution of pure haemoglobin 72 per "cent, exists as 



