RESPIRATION 71 



by various reducing agents. With potassium ferricyanide it 

 yields up all its oxygen, but the haemoglobin is speedily re- 

 oxidised by the reagent to form a brown isomer metha^moglobin 

 with a conspicuous absorption band in the red. Haemoglobin 

 is a compound of a protein and a nitrogenous pigment called 

 haemochromogen. The nature of the protein differs in 

 different animals. Haematin, which is the oxidised form of 

 haemochromogen, is known to contain four pyrol rings and 

 one atom of iron in its molecule. The constant relation 

 between the iron-content and the oxygen- capacity of a solution 

 of haemoglobin, established by Peters, shows that the formation 

 of oxy-haemoglobin is an essentially chemical union. Haematin 

 is separated from its conjugate globulin by dilute alkalis and 

 acids. The brown solutions formed in the two cases have 

 slightly different absorption spectra. On reduction of the 

 alkaline derivative with ammonium sulphide the red pig- 

 ment haemochromogen is found. Haematoporphyrin is the 

 purple substance formed by splitting off the iron from the 

 molecule with strong acids, and is isomeric with the bile pig- 

 ment bilirubin. 



Haemoglobin and oxy-haemoglobin respectively take up or 

 give up oxygen according to the partial pressure of the gas 

 in the medium with which they are in contact. In the case 

 of man 100 c.c. of blood take up about 18*5 c.c. of oxygen 

 when fully saturated. Human blood is fully saturated at 

 a partial pressure of 100 mm., which is less than the 

 partial pressure of oxygen in the atmosphere. The curve 

 relating oxygen tension to oxygen content (or percentage 

 saturation) in a haemoglobin solution is a rectangular hyperbola. 

 Complete saturation of the blood occurs at the same partial 

 pressure, but the initial part of the curve is steeper in a pure 

 haemoglobin solution, so that at low tensions its oxygen-content 

 is higher than in blood at the same partial pressure of oxygen. 

 Among factors which influence the form of the oxy-haemoglobin 

 dissociation curve are those which affect the physical chemistry 

 of proteins generally — neutral salts, hydrogen-ion concentra- 

 tion, and temperature. Increased hydrogen-ion concentration 

 facilitates the dissociation of oxygen at low tensions — /.^.flattens 



