TRANSPORT OF OXYGEN 99 



of the metal. In other words, haemoglobin is a kind of boat 

 in which iron is enabled to float in the blood. 



When oxyhsemoglobin is treated with potassium ferri- 

 cyanide its oxygen is quantitatively evolved. The nature 

 of the reaction is very compHcated, for the oxyhsemoglobin 

 is not reduced but is converted into a substance known as 

 methsemoglobin, which contains just as much oxygen as 

 oxyhsemoglobin. The oxygen, however, is in more perma- 

 nent combination. Notwithstanding its complex nature, 

 the process provides what is now the standard method for 

 estimating the amount of oxygen originally present. 



The combination of haemoglobin with oxygen is a 

 reversible reaction, the direction in which the reaction 

 proceeds being determined by the pressure of oxygen to 

 which the haemoglobin is exposed. 



The relation between the degree of combination and the 

 oxygen pressure can be estimated by exposing a solution 

 of pure haemoglobin to different pressures of oxygen and 

 estimating by the ferricyanide method the amount of the 

 gas which has entered into combination. The result is 

 expressed in the accompanying curve (Fig. 21 H), which is 

 seen to be a rectangular hyperbola. It corresponds to 

 the curve which is obtained theoretically from the equation 



Hb + 0., Z HbO^. 



The respiratory function of haemoglobin therefore Hes in 

 its capacity for combining with oxygen when the pressure 

 of oxygen is high, as in the lungs, and of parting with the 

 gas when the pressure is low, as in the tissues. 



If, however, the dissociation curve of haemoglobin in the 

 blood resembled the curve for pure haemoglobin, this sub- 

 stance would but inefficiently fulfil its function. Reference 

 to Fig. 20 will show that even at as low an oxygen pressure 

 as 10 mm. Hg the blood would still be 55 per cent, saturated ; 

 in other words, the affinity of haemoglobin for oxygen 

 would be too great for the transference of an adequate 

 amount of blood to the tissues. In the body, however, 



