524 THE RESPIRATORY EXCHANGE 



In the case of liquids containing substances which form loose 

 chemical combinations with gases, the relation of pressure to the 

 gas absorbed is frequently expressed by a curve in which the 

 abscissa represents the pressures and the ordinates the amounts 

 of gas taken up by the fluid. Under the ordinary pressure of 

 oxygen in the atmosphere haemoglobin readily combines with 

 oxygen, but, if the external pressure be lowered sufficiently, oxygen 

 is given off, the oxy-hsemoglobin undergoing dissociation. The 

 force with which the oxygen separates from the haemoglobin under 

 these circumstances is called the pressure or tension of dissociation, 

 and is represented graphically by such a curve as that just 

 described. 



The blood, considered as a tissue, can be divided into two 

 portions, the fluid matrix, the plasma, and the cellular elements, 

 the corpuscles ; the distribution of its gases will, therefore, be con- 

 sidered in relation to the whole blood, the plasma and corpuscles 

 respectively. It will be shown later that the investigation of the 

 combination of gases with pure solutions of haemoglobin is not 

 so important a method as it was once thought to be ; indeed, 

 many of Hiifner's results with solutions of haemoglobin have been 

 shown to be wrong ; and even the correct ones are not directly 

 applicable to the haemoglobin as it exists in the red corpuscles. 

 It is difficult to obtain pure haemoglobin, and it is probable that 

 the properties of the crystals are not exactly the same as those 

 of the haemoglobin in the red corpuscles in which it exists in close 

 relationship with proteids and salts. Moreover, different results 

 are obtained for the dissociation of oxy-haemoglobin, according to 

 the method used for the preparation of the crystals and the per- 

 centage of haemoglobin in solution. 



The Oxygen in Blood. When blood is shaken with air, it 

 takes up a much larger volume of oxygen than the amount which 

 can be held in solution by an equal volume of water, and animals, 

 as shown by Regnault and Reiset's experiments, absorb the same 

 amount of oxygen whether they breathe oxygen or air. The last 

 fact led Liebig to the conclusion that the gases of the blood are 

 present in a state of loose chemical combination with some of 

 the constituents of the blood, and Fernet found that, as regards 

 oxygen, the constituent concerned was the red corpuscle. Further 

 proofs of the chemical combination of oxygen were supplied by 

 Bernard and Hoppe-Seyler, who discovered that the oxygen of 



