530 DIGESTION 



as iron-free hematin or hematoporphyrin, which is produced by treating 

 hemoglobin with concentrated sulphuric acid. 



Chemical investigation has shown that bilirubin is built up from sub- 

 stituted pyrrols, probably four such being contained in the molecule. 

 The pyrrol group is also present in indole and tryptophane, and con- 

 sists of four carbon atoms and an NH group linked together as a ring 

 (see page 639). Similar pyrrol derivatives can be produced by decom- 

 posing chlorophyl, the green coloring matter of plants. It is important 

 to remember that bilirubin is acid in nature, and, therefore, can com- 

 bine with alkalies to form salts. The relative amounts of bilirubin and 

 biliverdin vary in the bile of different animals. 



When these pigments enter the intestine they are reduced to urobilin, 

 part of which passes out with the feces, another part being absorbed into 

 the blood and excreted in the urine. Part of that excreted in the urine 

 exists, however, as a so-called chromogen named urobilinogen. The 

 urobilinogen is converted into urobilin by the action of oxygen. 



The method by which bile pigments are produced from blood pigment has 

 been studied by histological examination of the liver particularly of birds 

 and amphibia, in which destruction of blood pigment goes on rapidly. 

 Increased destruction of blood pigment can be induced by poisoning 

 with certain substances such as arseniureted hydrogen. After extra- 

 vasation of blood in the subcutaneous tissues, as in a bruise, for example, 

 a decomposition of hemoglobin proceeds quite like that occurring in 

 the liver, and leads to the production of blue and brown and green 

 pigments like those of the bile. When hemolysis is produced, as by 

 inhalation of arseniureted hydrogen or the injection of inorganic 

 or biological hemolysins, there is an immediate increase in the amount 

 of bile pigment in the bile. Even the injection of hemoglobin solutions 

 has this effect. Under these conditions of hemolysis, besides an increase 

 in urobilin, there may be considerable quantities of hemoglobin secreted 

 in the urine. From such studies it is usually believed that the bile pig- 

 ments are a peculiar product of hepatic activity, being produced from 

 blood pigments that are derived from erythrocytes which have been 

 broken down either in the liver itself or in some other viscus (e.g., the 

 spleen). Whipple and Hooper 25 have shown that bile pigments may also 

 be formed in other tissues than the liver. They have found, for example, 

 that the bile pigments are formed just as readily in animals in which the 

 circulation of the liver was greatly curtailed, by anastomosing the portal 

 vein with the vena cava (Eck fistula), as in normal animals. Even when 

 the circulation was limited to the anterior end of the animal (head and 

 thorax) bile pigment appeared in the blood when hemolyzed erythrocytes 

 were injected, and it was also formed when hemoglobin was placed in the 



