ENDOGENOUS UROBILIN METABOLISM 555 



{199 If), proved by McMaster and Elman {673,1827,1828) and con- 

 firmed by Royer {490,2379). 



The urobilin reabsorbed from the intestine is carried by the blood 

 of the portal vein to the liver. The greater part is evidently taken 

 up by the liver and re-excreted in the bile, while a small amount 

 passes the liver, enters the general circulation, and is excreted in the 

 urine. This endogenous metabolism of urobilin is very complicated 

 and is not yet entirely understood. 



9.2.2. Absorption from the Intestine. Urobilin is mainly absorbed from 

 the ascending colon, less from the sigmoid colon {2382). Urobilinuria is 

 increased by injection of urobilin into the portal vein {2381). The amount of 

 the reabsorption is thought to be considerable, since in liver disease more 

 urobilinogen is occasionally excreted in the urine than in the feces {2989, 

 p. '2500; 2988): it is usually estimated to be between 30 and 70%, but this is 

 based on guesswork rather than on experiment. If the pathological liver is 

 able to form mesobilane, some of it may find its way directly into the general 

 circulation. Watson {2990) assumes that the preponderance of mesobilane 

 over tetrahydromesobilane in some urines has to be explained by an unusu- 

 ally rapid absorption of the former from the intestine, but so far there is 

 no evidence tliat the feces in these cases do not also contain an abnormal 

 proportion of the two urobilinogens. This problem is not easy to solve experi- 

 mentally, particularly since mesobilane is rather unstable and may undergo 

 further alterations in the intestine {cf. below). 



9.2.3. Re-excretion of Urobilin by the Liver. By far the greater part of 

 the urobilin is normally reabsorbed by the liver. Its later fate is still far from 

 certain. Some of it may again reach the intestine with the bile; Garrod 

 {981) always found some urobilin in the bile (of. also 24-17). Watson found 

 urobilin and urobilinogen in the bile half an hour after intravenous injection 

 of /-tetrahydromesobilene. Royer {2386) found about half of the biliary 

 urobilin to disappear from the gall bladder, but since he determined only 

 urobilin, not urobilinogen, this may have been due quite as well to reduction 

 to urobilinogen, as to absorption or to destruction. The fact, however, that 

 strong urobilinuria is observed only if the gall bladder remains connected 

 indicates that absorption of urobilin from it occurs. 



We have seen above (Section 2.2.2.) that the total amount of 

 urobilin excreted in feces and urine is distinctly smaller than the 

 amount of bilirubin formed by the breakdoy^^n of hemoglobin, or the 

 amount which can be calculated from total circulating hemoglobin 

 and the lifetime of the erythrocyte. Although Heilmeyer {1206) 

 came to the conclusion that after ingestion of bilirubin the corre- 

 sponding amount of urobilin is excreted with a delay of three to five 

 days, there is little doubt that a part of the urobilin is destroyed 

 (792,1219,1736,2281 ,2969,3046) . 



