The liver is the largest internal organ and plays over 500 different roles in the body. Many functions are to do with the processing of various chemicals such as carbohydrates, amino acids and lipids. The liver also removes alcohol and other drugs from the bloodstream: this is why alcoholics often suffer from liver disease.
But one function of the liver that you need to understand in detail concerns its role in the digestive system. The liver cells produce a green liquid called bile which is stored in a sac underneath the liver called the gall bladder. Bile can pass from the gall bladder down the bile duct and as shown in the diagram below, it then mixes with the contents of the duodenum (small intestine) soon after the acidic chyme leaves the stomach.
What is in bile?
Bile contains a mixture of chemicals. It has an alkaline substance (hydrogencarbonate ions) which helps to neutralise the chyme as it leaves the stomach. Remember the pH of the stomach contents is around pH1-2 and in the duodenum, there is a pH of around pH7.5. The difference in pH is due to the alkali present in bile and pancreatic juice. Bile also contains excretory molecules called bile pigments. These are waste molecules from the liver that have been made from the breakdown of haemoglobin. And finally there are the bile salts. These play an important role in the digestion of lipids in the duodenum.
How do bile salts improve digestion of lipids in the duodenum?
The duodenum is where many digestive reactions happen in the body. This is because the pancreatic juice contains many enzymes, all of which catalyse a specific digestive reaction. One such enzyme is lipase and this enzyme catalyses the following reaction:
lipids + water ——> glycerol and fatty acids
Fatty acids and glycerol are small enough molecules to be absorbed into the villi in the ileum. They pass into the lacteal in the centre of each villus to be carried around the body in the lymphatic system.
But the problem is that in the duodenum, the lipid molecules will exist as large droplets. Large droplets of fat/oil will have a reduced surface area for lipase to bind to and so the rate of digestion of the lipid would be slow. But bile salts interact with the lipid droplet causing a few large droplets to be broken down into dozens of tiny droplets. This is called emulsification and while it does not chemically alter the lipid, it does make it easier for lipase to break it down. Lipase and bile salts together break down lipids much faster than lipase alone.
Final key point: there are no digestive enzymes in bile. But in spite of this, bile plays a crucial role in the digestion of lipid droplets in the duodenum.