THE CHEMISTRY OF RESPIRATION 



1181 



From this Table we see that 100 volumes of blood at 38 C. might 

 contain 2*2 c.c. of oxygen in solution if the blood had been exposed to 

 oxygen at a pressure of one atmosphere. The blood in the lungs is, how- 

 ever, exposed to air which contains only about one-sixth of its volume 

 of oxygen, so that the total amount of oxygen present in arterial blood 

 in solution cannot be more than one-sixth of 2-2, i.e. about 0-36 c.c. 

 per cent. Since arterial blood, or blood saturated with oxygen by 

 shaking with air, will yield as much 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 combination with haemo- 

 globin, the plasma containing no more than could be accounted for 

 by simple solution. 



One gramme of crystallised haemoglobin can absorb about 14 c.c. 

 of oxygen (v. p. 929). If a solution of oxy haemoglobin be subjected 

 in an air-pump to gradually diminishing pressure at the temperature of 

 the body, very little oxygen is given of! until the partial pressure of the 

 oxygen is diminished to about 30 mm. Hg. (Fig. 494). At this point a 

 large evolution of gas begins, and continues at falling pressure until at 

 mm. pressure all the oxyhaemoglobin 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 oxyhaemo- 

 globin by equivalent amounts of CO or NO. Haemoglobin is also sup- 

 posed to form an unstable combination with carbon dioxide, since it 

 takes up much more of this gas than the corresponding bulk of water 



