326 RESPIRATORY FUNCTION OF THE BLOOD 



mental finding of 401 c.c. Normal blood contains 14 grams per 

 cent, of haemoglobin. It follows that each corpnscle carries 



= 28 X 10" 1- grams of haemoglobin. In other words, 



25 X 1012 s 



each cubic centimetre of oxygen postulates the presence of 



28 X 25 X 109 



= 0-7 gram oi Hb. 



1012 



The iron content of Hb must therefore be = gram 



7 X 400 280 



per gram of Hb. Putting this in molecular terms, one molecular 

 proportion of iron (56 grams) enters into the composition of 

 15,680 grams of Hb. The molecular weight generally given for 

 Hb is 16,660. 



Nature of union between oxygen and haemoglobin. 



Evidence has been produced which shows that when haemo- 

 globin is fully saturated with oxygen there are 401 c.c. of oxygen 

 for every gram of iron. That is, each combining proportion 

 of iron (56 grams) co-exists with two combining proportions 

 (32 grams) of oxygen. From this, some deduce the presence of a 

 compound FeOg in haemoglobin. That may be. On the other 

 hand, the idea of a chemical combination between haemoglobin 

 and oxygen raises hosts of difficulties which w'ould not arise on 

 the hypothesis of adsorption of gas on a colloidal surface. How- 

 ever, no reliable direct experimental evidence bearing on this has 

 come to hand. If analogies could be taken as a proof instead 

 of analysis, the adsorption theory would hold the field undisputed. 



Conditions controlling the reaction between haemoglobin and oxygen. 



Whatever may be the nature of the union between oxygen and 

 haemoglobin, it is established that the reaction between them is a 

 reversible one, the equilibrium point being determined by the 

 tension of oxygen and the active mass of haemoglobin, wdien all 

 other conditions, such as temperature, hydrogen ion concentration, 

 etc., are kept constant. 



In Table XL VII., if the tensions (1) are taken as abscissae, and 

 the degree of saturation (4) as ordinates, a curve (Fig. 82, heavy 

 line) is obtained, called the dissociation curve of blood. It has been 

 shown by Hill that this curve can be approximately represented 

 by the equation 



_ [Hb] [O,]- 



[HbO,] 



