366 



RESPIRATION 



[CH. XXVI. 



Haemoglobin owes its value as a respiratory pigment to two 

 principal facts. (1) It can unite with a large quantity of oxygen, 

 and therefore blood can carry about thirty times as much oxygen to 

 the tissues as plasma would under the same circumstances. (2) The 

 interaction between haemoglobin and oxygen is a reversible one ; 

 the two unite in the lungs, where the pressure of oxygen is high; 

 but when oxygen is absent or at a low pressure, as in the tissues, the 

 haemoglobin parts with its store of oxygen. 



The reaction between haemoglobin and oxygen is a chemical one. 

 At most, one gramme of haemoglobin can unite with 1/34 c.c. of 

 oxygen. This figure is not quite constant, probably on account of 

 slightly different forms of globin (the protein constituent of haemo- 

 globin) united with the haematin (the iron containing constituent) 

 in different animals. The relation between the respiratory oxygen 

 and the iron of the haemoglobin is, however, quite constant, and is 

 called the "specific oxygen capacity" Each gramme of iron in 

 haemoglobin unites with 400 c.c. of oxygen ; these figures are in the 

 relation of one atom of iron to two atoms of oxygen. The reversible 

 nature of the reaction may therefore be expressed by the equation 

 H6 + 2 ^^ H&0 2 . A reversible reaction means that it will go in 

 either direction according to the concentration of the substances 

 present ; thus if the concentration of oxygen in solution is increased, 



\ 



FIG. 294. Barcroft's Tonometer, suspended horizontally in warm bath in which it is rotated. 



more of the haemoglobin will become oxyhaemoglobin ; and if it 

 is diminished, oxyhaemoglobin will break up into haemoglobin (some- 

 times called reduced haemoglobin) and oxygen. 



The reader must be clear that when we speak of the concentration 

 of oxygen in solution, we mean in physical solution, that is, not 

 united chemically with the haemoglobin. This quantity varies in 

 the direct ratio of the oxygen pressure to which the haemoglobin 

 solution is exposed ; therefore the problem before us is to determine 

 the relative quantities of oxy- and reduced haemoglobin when a 

 haemoglobin solution is shaken up with oxygen at different pressures. 



