THE CHEMISTRY OF RESPIRATION 1059 



yield as muck as twenty volumes per cent, of oxygen to a Torricellian 

 vacuum, the oxygen cannot be in simple solution, but must be in some form 

 of combination with some of the constituents of the blood. Of this oxygen 

 practically the whole is contained in the red blood-corpuscles in combina- 

 tion with haemoglobin, the plasma containing no more than could be 

 accounted for by simple solution. 



One gramme of crystallised haemoglobin can absorb 1-34 c.c. of oxygen. 

 If a solution of oxyhaemoglobin be subjected in an air-pump to gradually 

 diminishing pressure at the temperature of the body, very little oxygen 

 is given off until the partial pressure of the oxygen is diminished to about 

 30 mm. Hg. (Fig. 496). At this point a large evolution of gas begins, 

 and continues at falling pressure until at mm. pressure all the oxy- 

 haemoglobin is dissociated and converted into haemoglobin. The same 

 observation may be made in a reverse direction. If a solution of reduced 

 haemoglobin be exposed to gradually increasing pressures of oxygen, it will 

 be found that the greatest absorption takes place between and 30 mm. Hg. 

 After this point the oxygen is more slowly absorbed up to the point of 

 complete saturation. 



Since there is no direct proportion between the partial pressure of the 

 oxygen and the amount absorbed, it is evident that the oxygen combines 

 with haemoglobin to form an unstable chemical compound, and that this is 

 not a mere question of solution. This is further proved by the fact that 

 we can displace the oxygen (0 2 ) from the oxyhaemoglobin by equivalent 

 amounts of CO or NO. Haemoglobin is also supposed to form an unstable 

 combination with carbon dioxide, since it takes up much more of this gas 

 than the corresponding bulk of water or salt solution would do. Although 

 carbon dioxide combines with haemoglobin, it does not displace oxygen from 

 the oxyhaemoglobin molecule. Thus we may have haemoglobin saturated 

 at the same time with oxygen and with carbon dioxide. The presence of 

 carbon dioxide does, however, alter the case with which oxyhaemoglobin 

 dissociates. 



The relation between the partial pressure of oxygen and the amount 

 of oxyhaemoglobin formed under varying conditions can be investigated in 

 the following way (Barcroft) : 



A large glass globe with a stop-cock at one or both ends (Fig. 495) is filled with a 

 gaseous mixture of known composition containing oxygen. Into it are introduced 2 or 3 c.c. 

 of blood or of haemoglobin solution. It is then tightly stoppered and immersed in Ta 



