CHAPTER XVII 



THE BIOLOGICAL SIGNIFICANCE OF HEMOGLOBIN 



X HE question which always recurs is : What is hsemoglobin ? It may 

 be asked from the chemical or the biological standpoint — one com- 

 mences with the chemical aspect, one concludes with the biological. 

 Haemoglobin, biologically, possesses the following properties : 



(1) It is capable of transporting a large quantity of oxygen from 

 one place to another. This it might be supposed to do either because 

 a small amount of haemoglobin could attach itself to a large amount 

 of oxygen, or because a large haemoglobin molecule can unite with a 

 relatively small oxygen molecule. The latter alternative, which is the 

 one actually in existence, demands that haemoglobin should be very 

 soluble, we therefore come to the second important property of 

 haemoglobin, namely, 



(2) Great solubihty. 



(3) Haemoglobin must unite with oxygen at a suitable velocity and 

 in sufficient amount under the circumstances in which it finds itself 

 in the blood, and must give it up in proper quantities imder the 

 circumstances which prevail in the tissues. This interaction is facih- 

 tated by the fom-th important property of haemoglobin, namely, that 



(4) Carbonic acid tends to expel oxygen from haemoglobin and, 

 conversely, oxygen tends to expel carbonic acid from alkaline solu- 

 tions which contain that gas. 



Let us turn now to see if we can trace some of the steps by which 

 a material with these remarkable properties has been produced. 



Of these the first is the fact of its forming a reversible compound 

 with oxygen. If one goes back to the simpler compounds from which 

 haemoglobin may be regarded as being synthesised, none of them 

 has this property. The facts which stand out are that haematin and 

 its compoimds are capable of oxidation and reduction, but that in 

 order to effect the reduction a chemical reducer is always necessary. 

 These substances then would be useless for a system such as prevails 

 in the body, in which the oxygen carrier is circulated in a blood-vessel, 

 whilst the actual oxidation takes place in a tissue situated at a little 

 distance. Over this distance the oxygen must travel as molecular 



