CHAPTER IV 



The Blood as a Carrier of Oxygen. 



The evidence has already been referred to that nearly all the 

 available oxygen in the blood is present in the form of a chemical 

 compound with the haemoglobin of the red corpuscles, and that 

 this compound has the remarkable property of dissociating with 

 fall in the partial pressure of oxygen, at the same time changing 

 its color from bright scarlet to a dark purple. It dissociates com- 

 pletely when the oxygen pressure is reduced to zero, and the 

 readiness with which the dissociation occurs is dependent on 

 temperature and other conditions which will be discussed below. 

 It is contained in the corpuscles to the extent of about 30 per cent 

 of their weight, and on liberation from them it can be crystallized 

 out with comparative ease by the help of cold and of substances 

 which diminish its solubility. There is considerable variation in 

 the form of the crystals obtained from the blood of different 

 animals. 



To what extent, and in what directions, the elementary composi- 

 tion of haemoglobin varies is not yet definitely known; but the 

 haemoglobin of birds has been found to contain phosphorus, while 

 none is present in the haemoglobin of mammals. Iron is always 

 present. A given amount of blood, whether or not the corpuscles 

 have been dissolved and the haemoglobin liberated and diluted, 

 takes up, on saturation with air at room temperature, a perfectly 

 fixed and definite amount of oxygen in chemical combination. No 

 further measurable quantity is taken up, except in simple physical 

 solution, on saturation with oxygen. An exactly equal volume of 

 carbon monoxide or nitric oxide is taken up in combination in 

 presence of either of these gases. There is no shadow of doubt that 

 the combination is a chemical one, though some extraordinary 

 attempts, based on ignorance of well-ascertained facts, have re- 

 cently been made to explain the combinations of oxygen and CO2 

 in blood as due to adsorption. 



Haemoglobin not only enters into dissociable chemical combina- 

 tions with oxygen, carbon monoxide and nitric oxide, but also in 

 presence of various oxidizing agents, such as ferricyanides or 

 chlorates, or very weak acids, etc., when oxygen is also present, 

 passes into a modification called by Hoppe Seyler methaemoglobin. 



