94 A TEXTBOOK OF PHYSIOLOGY 



On this reaction depends the chemical method of determining the 

 amount of loosely combined oxgyen in the blood. 



The solution of MetHb is greenish-brown in colour. The spectrum 

 of this solution suitably diluted shows, in addition to the two bands 

 characteristic of OHb, another band in the red near the line C. These 

 three bands are well marked and of almost equal intensity. The O 2 

 in methsemoglobin is not yielded to the vacuum pump, and is not 

 available for respiratory purposes. By treatment, however, with 

 ammonium sulphide MetHb can be easily reduced to Hb, which in 

 turn can be converted again to OHb by shaking well in air. MetHb 

 is sometimes passed in the urine after the administration of excessive 

 doses of potassium chlorate, and antipyretics such as phenacetin, 

 antifibrin, etc. It is formed when the red corpuscles are hsemolyzed 

 and a considerable amount of Hb is set free in the blood-stream. 

 The condition is known as methaemoglobinuria. The deep brown 

 colour gives the urine a peculiar look. The spectroscopic test serves 

 to identify it. 



Haemoglobin is split into globin and haematin by the action 

 of heat, acids and alkalies, etc., and there are a number of derivatives 

 of haematin which give characteristic spectra. 



Chief of these are acid haematin, alkaline hsematin, reduced alkaline 

 haematin (hsemochromogen), and haematoporphyrin (iron-free haematin). 



Acid Hsematin is readily prepared by shaking up a small amount 

 of defibrinated blood with a few drops of 20 per cent, acetic acid, 

 and then suitably diluting the mixture with 60 per cent, alcohol. It 

 forms a brownish solution, giving a well-marked absorption band in 

 the red near the line C, nearer than the similar band of MetHb. 

 Sometimes two bands are seen between D and E, but they are feeble, 

 and not of the same intensity as the band in the red. Acid haematin 

 cannot be reduced to Hb by the action of ammonium sulphide. 

 This serves to distinguish acid haematin from methaemoglobin. 



Alkaline Haematin is prepared similarly by shaking up a small 

 amount of defibrinated blood with 20 per cent, potash and diluting 

 with weak alcohol. It gives one broad absorption band between 

 C and D, which is in contrast to the band of Hb between D and E. 



Reduced Alkaline Haematin (Haemochromogen). This gives an 

 extremely characteristic spectrum in suitable dilutions ; one very dark 

 band between D and E and another less dark band between E and b. It 

 is only the spectrum which has a band in this postion. Haemo- 

 chromogen is prepared by adding potash to the solution of blood, 

 or dissolving an old dried blood-stain in potash, and reducing it by 

 ammonium sulphide. As it can be prepared from old blood-stains, 

 it affords us a test for blood of great medico-legal value. A solution 

 of haemochromogen and globin, if mixed and allowed to stand, will 

 reunite and form haemoglobin. 



Acid Haematoporphyrin. By the addition of acid to haemo- 

 chromogen the iron is split off from the compound with the production 



