114 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



mixing blood with strong sulphuric acid ; the iron is taken out as 

 ferrous sulphate. This substance is also found sometimes in nature ; 

 it occurs in certain invertebrate pigments, and may also be found in 

 certain forms of pathological urine. It shows well-marked spectro- 

 scopic bands, and so is not identical with the iron-free derivative of 

 haemoglobin called baematoidin which is formed in extravasations of 

 blood in the body (see p. 90). The two substances are possibly 

 isomeric. 



Haemopyrrol is methyl-propyl-pyrrol, with the formula 



H.C-C CH 2 .C 2 H 3 



II II 

 H.C C CH 3 



N.H 



7 



and is obtained by reduction from haematoporphyrin. It is also 

 similarly obtained from the derivative of chlorophyll called phyllo- 

 porphyrin, a fact which illustrates the near relationship of the 

 principal animal and vegetable pigments. 



COMPOUNDS OF HAEMOGLOBIN WITH GASES 



Haemoglobin forms at least four compounds with gases : 



f 1. Oxyhaemoglobin, 

 oxygen J _ 



( 2. Methaemoglobm. 



With carbonic oxide 3. Carbonic oxide haemoglobin. 



With nitric acid 4. Nitric oxide haemoglobin. 



These compounds have similar crystalline forms : each probably 

 consists of a molecule of haemoglobin combined with one of the gas- 

 They part with the combined gas somewhat readily, and are arranged 

 in order of stability in the above list, the least stable first. 



Oxyhaemoglobin is the compound that exists in arterial blood. The 

 oxygen linked to the haemoglobin, which is removed by the tissues 

 through which the blood circulates, may be called the respiratory 

 oxygen of hcemoglobin. The processes that occur in the lungs and 

 tissues, resulting in the oxygeriation and deoxygenation respectively 

 of the haemoglobin, may be imitated outside the body, using either 

 blood or pure solutions of haemoglobin. The respiratory oxygen can 

 be removed, for example, in the Torricellian vacuum of a mercurial 

 air-pump, or by passing a neutral gas like hydrogen through the 

 blood or by the use of reducing agents such as ammonium sulphide or 



