As far as the digestive system is concerned the most important thing that liver does is produce bile.
A. BILE
Bile is produced at a fairly constant rate (500 - 1000 ml.day-1) and to enable the increased flow rates that are required during a meal this is stored in a concentrated form in the gall bladder. Bile is a dark yellow fluid that contains a number constituents:
(i) Bicarbonate.
Bile contains relatively high concentrations of bicarbonate ions and this gives rise to its alkaline nature (pH ~ 8) and helps neutralise the acidic chyme arriving from the stomach.
(ii) Bile salts.
The major bile salts are cholic acid and chenodeoxycholic acid and these are synthesised by the liver from cholesterol. Bile salts are unusual molecules in that they have both hydrophobic and hydrophilic components in their structure. This means that when in an aqueous medium (such as chyme) they form aggregates known as micelles with the hydrophobic ends of the molecules all pointing towards the centre.
As we will see, these micelles play a very important role in the digestion and absorption of fats. Almost all of the bile salts secreted into the small intestine are absorbed and then recycled into bile as the blood flows back through the liver in the portal vein.
(iii) Bile Pigments.
These are metabolic by-products that are secreted into the digestive tract and are responsible for the characteristic colour of bile and faeces. Bile pigments constitute about 0.2% of bile and consist mostly of bilirubin which is the result of the haeme component from haemoglobin metabolism following red blood cell death. The secretion of bilirubin is a convenient way for the body to eliminate this large insoluble metabolite.
(iv) Cholesterol.
Bile is the major route for the excretion of cholesterol by the body and plays an important role in cholesterol balance.
(v) Lecithin.
Lecithin is a phospholipid that helps in the process of fat digestion.
B. CONTROL OF BILE SECRETION.
In the fasting state the sphincter of Oddi is closed so very little bile reaches the small intestine. This encourages the flow of bile along the cystic duct where it is concentrated and stored in the gall bladder. The arrival of food in the duodenum produces a marked elevation in bile secretion through a couple of mechanisms:
The presence monoglycerides and fatty acids (and to a lesser extent peptides) in chyme stimulates the production of cholecystokinin from the I cells in the duodenum. An increase in circulating CCK concentrations cause contraction of the gall bladder and relaxation of the sphincter of Oddi and results in a marked increase in bile flow into the small intestine.
The low pH of the chyme in the duodenum causes the release of secretin from the S cells. As well as inhibiting gastric motility and stimulating pancreatic juice secretion (described previously) secretin causes the secretion of bicarbonate and water by the bile ducts that helps dilute the concentrated bile leaving the gall bladder and thus increases the flow rate.
Collectively these two hormones enhance the volume of bile flowing into the small intestine in proportion to the size of the meal.