In this article we will discuss about the mechanism of bile with the help of suitable diagram.
Bile secretion by liver is an active and continuous process, but its expulsion in the duodenum is intermittent. These facts necessarily prove that after secretion bile must be stored somewhere. The gall-bladder is the chief place for bile storage. The common bile duct, which is shut off from duodenum by the sphincter of oddi, may hold some bile.
When the pressure of bile in the common bile duct rises to about 70 mm of water, bile begins to pass into gall-bladder. The gall-bladder has got a special capacity of absorbing water and slightly the inorganic salts (Fig. 9.43). Due to this property it can concentrate bile nearly ten times.
The average capacity of gall-bladder is about 50 ml since bile is ten times concentrated 50 ml of bladder bile is equivalent to 500 ml of duct bile. Thus gall-bladder acts as a very efficient reservoir. It can store large amount of bile without any appreciable rise of pressure.
Experiment:
It is best studied in animals by the experiment of triple intubation as devised by McMaster and Elman (Fig. 9.44). In man it can be studied by giving tetra-iodophenolphthalein, (which is excreted in bile and is opaque to X-ray) and then noting the alteration of size of gall-bladder under X-ray—cholecystography.
During fast there is no expulsion of bile, because the gall-bladder contractions are weak and the tone of the sphincter is high. When food is seen or taken, contraction of gall-bladder starts and at the same time, the resistance of the sphincter is reduced. The pressure in the bile duct rises to about 200 mm of H2O and bile is momentarily expelled. This expulsion of bile is probably assisted by the contraction of the ampullary part of the common bile duct.
Under normal conditions the sphincter can withstand a pressure of about 100 – 120 mm of H2O. After 24 hours fasting or with a carbohydrate diet the resistance may become very high,—as great as 300 mm of H2O. Usually, the pressure in the gallbladder is about 100 mm of H2O during fasting. After food there is immediate lowering of the sphincter resistance and strong contraction of gall-bladder starts. During contraction of gallbladder the pressure may be raised to about 250 mm of H2O.
Once expulsion starts, it goes on intermittently at frequent intervals for the next few hours as long as digestion and absorption continue. After this, expulsion ceases.
Expulsion of Bile:
It is obvious that for expulsion of bile two things are required:
1. Increased pressure of bile.
2. Relaxation of the sphincter of Oddi.
Increased pressure of bile can be brought about by:
(a) Increased rate of secretion of bile, and
(b) Contraction of gall-bladder.
This latter is brought about by stimulation of the vagus (both conditioned and unconditioned reflexes), high fat and protein diet, cholecystokinin and salts like magnesium sulphate, calomel, etc. Relaxation of the sphincter is also brought about by the same factors from these facts it will be obvious that expulsion of bile takes place only during digestion and absorption of food. This is due to the fact that it is during these processes that the various factors, which stimulate contraction of gall-bladder and relaxation of the sphincter, come into operation.