io A MANUAL OF VETERINARY PHYSIOLOGY 



Haemoglobin is a protein, but is distinguished from the majority 

 of the other members of its class by the comparative ease with 

 which it may be obtained in a crystalline form ; while, on the 

 other hand, its behaviour in a dialyser is not that of a crystalloid, 

 but a colloid. 



Haemoglobin, under the influence of heat, acids, or alkalies, is 

 broken up in the presence of oxygen into a simple protein, globin, 

 and a pigment, hcematin : it is therefore a compound of a protein 

 body with haematin ; the protein portion of the molecule repre- 

 sents the greater part, the pigment being only 4 per cent, of the 

 total. In the protein portion is found all the sulphur, and in the 

 pigment all the iron of the molecule. The pigment haematin 

 exists in the living blood as haemoglobin, the great difference 

 between these two substances being that haematin forms a firm 

 while haemoglobin forms a feeble combination with oxygen. 



The union of oxygen with haemoglobin is a true chemical com- 

 pound, a definite weight of haemoglobin uniting with a fixed 

 volume of the gas, and forming oxyhsemoglobin. This is not a 

 stable compound ; it readily gives off its oxygen either in the 

 presence of oxidisable substances, or in an atmosphere free from 

 oxygen, and by so doing becomes reduced haemoglobin. It is 

 oxyhaemoglobin which gives the bright colour to arterial blood, 

 and it is the presence of partly reduced haemoglobin which gives 

 the darker tint to venous blood. The change in colour which 

 venous blood undergoes on exposure to the air is due to the 

 absorption of oxygen by haemoglobin. Similarly the blood is 

 charged with oxygen in the lungs, brought back to the heart, and 

 distributed all over the body to the tissues ; here it gives up the 

 bulk of its oxygen, and as partially reduced haemoglobin is brought 

 back by the veins to the heart for distribution to the lungs, where 

 it renews its oxidised condition. Excepting in the latest stages 

 of asphyxia, haemoglobin is never completely reduced in the body. 



Oxygen is not the only gas with which haemoglobin is capable 

 of forming a chemical compound. Carbon monoxide unites with 

 it, forming the definite compound carbonic oxide haemoglobin. 

 In great contrast to oxyhaemoglobin, this is a remarkably stable 

 compound, the carbon monoxide holding so tenaciously to haemo- 

 globin that oxygen cannot displace it. This explains the highly 

 poisonous nature of this gas. The spectrum of CO haemoglobin 

 closely resembles that of oxyhaemoglobin (see Fig. 2). A still 

 more poisonous compound is nitric oxide haemoglobin, since this 

 gas is attached to the haemoglobin even more tenaciously than 

 carbon monoxide. 



Haemoglobin also forms a compound with carbon dioxide, 

 producing carbohaemoglobin ; this compound may be formed 

 even when the haemoglobin is already nearly saturated with 



