52 THE CIRCULATING LIQUIDS OF THE BODY 



is this property that enables haemoglobin to perform the part of an 

 oxygen-carrier to the tissues, a function of the first importance, 

 which will be more minutely considered when we come to deal with 

 respiration. 



The bright red colour of blood drawn from an artery or of venous 

 blood after free exposure to air is due to the fact that the haemo- 

 globin is in the oxidized state 

 in the state of oxyhaemo- 

 globin, as it is called. The 

 amount of oxygen with which 

 haemoglobin combines to form 

 oxyhaemoglobin is such that 

 one atom of iron corresponds to 

 two atoms of oxygen. If the 

 formula for haemoglobin given 

 on p. 50 be represented by 

 the symbol Hb, a molecule of 

 oxyhaemoglobin would be re- 

 presented as Hb0 2 . If the 

 oxygen is removed by means of 

 reducing agents, such as am- 

 monium sulphide, or by ex- 

 posure to the vacuum of an air- 

 pump, the colour darkens, the 

 blood-pigment being now in the 

 form of reduced haemoglobin. 

 In ordinary venous blood a 

 large proportion of the pigment 

 is in this condition, but there is 

 always oxyhaemoglobin present 

 as well. In asphyxia (p. 281), 

 however, nearly the whole of 

 the oxyhaemoglobin may dis- 

 appear. 



Crystallization of H cemoglobin. In the circulating blood the haemo- 

 globin is related in such a way to the stroma of the corpuscles that, 

 although the latter are suspended in a liquid readily capable of dissolv- 

 ing the pigment, it yet remains under ordinary circumstances strictly 

 within them. In a few invertebrates, however, it is normally in solution 

 in the circulating liquid. As a rare occurrence haemoglobin may form 

 crystals inside the corpuscles (p. 71). When it is in any way brought 

 into solution outside the body, it shows in many animals, but not in the 

 same degree in all, a tendency to crystallization ; and the ease with which 

 crystallization can be induced is in inverse proportion to the solubility 

 of the haemoglobin. Thus, it is far more difficult to obtain crystals of 

 haemoglobin from human blood than from the blood of the rat, guinea- 

 pig, or dog, whose blood-pigment is less soluble than that of man, and 

 for a like reason the oxyhaemoglobin of the bird, the rabbit, or the frog 

 crystallizes still less readily than that of human blood. 



Fig. 14. Oxyhaemoglobin Crystals (Frey). 

 a, b, from man ; c, from cat ; d, from guinea- 

 pig; e, from hamster; /, from squirrel. 



