70 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



that in the area from which the portal vein collects blood — stomach, 

 intestine, spleen, and pancreas — the arterioles are all dilated, and the 

 capillaries are thus gorged with blood. Further, the active peristalsis 

 of the intestine and the pumping action of the spleen are additional 

 factors in driving more blood onwards to the liver. 



The bile is secreted from the portal blood at a much lower 

 pressure than one finds in glands, such as the salivary glands, the 

 blood supply of which is arterial. Heidenhain found that the 

 pressure in the bile duct of a dog averaged 15 mm. of mercury, 

 which is about double that in the portal vein. 



The second increase in the flow of bile — that which occurs some 

 hours after the arrival of the semi-digested food (chyme) in the intestine 

 — appears to be due to the effect of the digestive products carried by 

 the blood to the liver stimulating the hepatic cells to activity ; this 

 is supported by the fact that proteid food increases the quantity of 

 bile secreted, whereas fatty food which is absorbed, not by the portal 

 vein, but by the lacteals, has no such effect. 



The chemical processes by which the constituents of the bile are 

 formed are obscure. We, however, know 

 that the biliary pigment is produced by 

 the decomposition of haemoglobin. Bili- 

 rubin is, in fact, identical with the iron- 

 free derivative of haemoglobin called 

 haematoidin, which is found in the form 

 of crystals in old blood-clots such as 

 ^ '^,'*i ^i? occur in the brain after cerebral haemor- 



^^J^'J^-*^'" rhage (see fig. 24). 



F,G.*2r-HiBmatoidin crystals. ^u injectiou of haemoglobin iuto the 



portal vein, or of substances like water 

 which liberate haemoglobin from the red blood corpuscles, produces an 

 increase of bile pigment. If the spleen takes any part in the elabora- 

 tion of bile pigment, it does not proceed so far as to liberate haemo- 

 globin from the corpuscles. No free haemoglobin is discoverable in 

 the blood plasma in the splenic vein. 



The amount of bile secreted is differently estimated by different 

 observers ; the amount secreted daily in man appears to vary from 

 500 c.c. to 1 litre (1,000 c.c). 



THE CONSTITUENTS OF BILE 



The constituents of the bile are the bile salts proper (taurocholate 

 and glycocholate of soda), the bile pigments (bilirubin, biliverdin), a 

 mucinoid substance, small quantities of fats, soaps, cholesterin. 



