160 ESSENTIALS OF PHYSIOLOGY. 



the prosthetic group of the molecule, and the value of hsematin in this 

 respect is determined by the presence of iron. The actual amount of 

 iron is small. Hsematin forms about 4 per cent, of the haemoglobin 

 molecule, and iron accounts for about 1 1 per cent, of the hsematin, or 

 about 0*05 per cent, of blood itself. Iron has on this account been 

 described as the gold currency of the body, and it is carefully preserved. 

 Red corpuscles are being continually destroyed in the liver, and the 

 broken-down material is excreted to a large extent in the bile. Hsematin 

 appears in the bile in an iron-free form as bilirubin, the primary bile 

 pigment, the iron being retained by the liver cells. The iron is eventu- 

 ally used in the formation of new erythrocytes, this taking place in the 

 red marrow. 



Owing to the high atomic weight of iron, the transport of a 

 substance like hsematin would be attended with difficulty if it were 

 not combined with a large protein molecule. By means of its com- 

 bination with globin the weight of the iron is distributed, and the 

 resulting compound can be floated along in the blood stream without 

 difficulty. 



Haemoglobin has an affinity for carbon monoxide 130 times as great 

 as its affinity for oxygen, and when it is exposed to air containing even 

 a minute quantity of the former gas, it forms with it a stable compound, 

 carboxyhcemoglobin (HbCO). HbCO differs slightly in colour from Hb0 , 

 strong solutions having a more florid appearance, and weak solutions 

 retaining a pink colour, whereas the same dilution of Hb0 9 has a yellow 

 tinge. Solutions of HbCO, when examined spectroscopically, exhibit 

 two bands in the green, slightly nearer the violet end than those 

 presented by Hb0 9 . HbCO is unaffected by the addition of ammonium 

 sulphide. 



Haemoglobin forms a still more stable compound with nitric oxide, 

 HbNO, but this is purely a laboratory product, and is only of theoretical 

 interest. 



When a solution of oxyhsemoglobin is treated with ferricyariide of 

 potassium, a volume of oxygen is given off corresponding with the dissoci- 

 able oxygen of the oxyhaemoglobin molecule, and the solution becomes 

 brown, its spectrum showing a characteristic band in the red in addition 

 to other bands (fig. 51). The brown substance is called methcemoglobin. 

 It contains the same amount of oxygen as oxyhsemoglobin, but more 

 firmly combined, and attached in all probability to a different part of 

 the molecule. Haldane suggests that in the case of oxyhsemoglobin the 



/o 



dissociable oxygen is in the molecular form Hb<( | and in meth&mo- 



N) 



