Bile Physiological Functions

Bile salts are composed of a hydrophilic side, and a hydrophobic side. This means that they are more likely to aggregate to form micelles, with the hydrophobic sides towards the centre and hydrophilic towards the outside. In the centre of these micelles are triglycerides, which are separated from a larger globule of lipid.

Pancreatic lipase is able to get to the molecules of triglyceride through gaps between the bile salts, providing a largely increased surface area for digestion. Ordinarily, the micelles in the duodenum have a diameter of around 14-33 ?m. However, it is possible for these to be much smaller, as small as 160 nm when using artificial means. Should bile not be present in the duodenum, not all of the lipid is able to be digested during digestion, and a lot of it is passed out in feces. As a result the time taken for the lipid to be broken down would be greatly increased if there was no bile present in the duodenum. This is how the body is able to efficiently digest and absorb lipids for metabolism.

Bile acts to some extent as a surfactant, helping to emulsify fats (increasing surface area to help enzyme action), and thus aids in their absorption in the small intestine. The most important compounds are the salts of taurocholic acid and deoxycholic acid. Bile salts combine with phospholipids to break down fat globules in the process of emulsification by associating its hydrophobic side with lipids and the hydrophilic side with water. The phospholipids have a charged �head� which prevents two fat droplets from recombining. Emulsified droplets then are organized into many micelles which increases absorption. Pancreatic lipase acts on the triglyceride material of the fat in the small intestine and breaks them down into two fatty acids and a monoglyceride, which are absorbed by the villi, rebuilt into triglycerides, arranged into chylomicrons, and enter the lacteals inside the intestinal villus. Since bile increases the absorption of fats, it is an important part of the absorption of the fat-soluble vitamins D, E, K and A. Besides its digestive function, bile serves as the route of excretion for the hemoglobin breakdown product (bilirubin) created by breakdown of erythrocytes, which are conjugated by glucuronidation in the liver; it also neutralises any excess stomach acid before it enters the ileum, the final section of the small intestine. Bile salts are also bacteriocidal to the invading microbes that enter with food.