The main digestive function of the liver is to produce bile, a fluid consisting of water, bile acids, cholesterol, bile pigments, certain other lipids, and electrolytes. The liver uses cholesterol to make the primary bile acids, cholic acid and chenodeoxycholic acid. Bile salts are bile acids that have lost a hydrogen ion and gained a sodium or potassium ion, and the terms can be used interchangeably. Bile acids combine with either the amino acid glycine or an amino acid derivative taurine to form conjugated bile acids, which travel in bile to the small intestine, most bile salts are deconjugated ( undergo loss of glycine or taurine ) and recycled to the liver for reuse.

Bile contains several substances with no digestive function. The bile pigments biliverdin and bilirubin, hemoglobin breakdown products, are merely transported in the bile to the small intestine where they can be excreted. Bile is mostly water and has an excretory function that it carries bilirubin and excess cholesterol to the intestines for elimination in feces. The digestive function of bile is accomplished by bile salts, which emulsify fats in the small intestine. Emulsification means that large fat globules are broken into smaller globules. This is mechanical, not chemical, digestion; the fat is still fat but now has more surface area to facilitate chemical digestion. Production of bile is stimulated by the hormone secretin, which is produced by the duodenum when food enters the small intestine.


METABOLIC FUNCTIONS OF THE LIVER

The liver synthesizes non-essential amino acids from other amino acids, glucose and fatty acids. The enzymes alanine and aspartate transaminases convert amino acids that are in abundance to others that are needed by the body. A high concentration of these enzymes in the blood indicates liver damage. The liver makes most plasma proteins including albumin, and produces coagulation factors. The liver breaks down proteins and removes the toxic ammonium ion by converting it to urea.

Reported in the May 2004 issue of the "International Journal of Biochemistry and Cell Biology," the liver plays a prominent role in regulating blood glucose levels. After a meal, carbohydrates are broken down into glucose in the small intestine, and glucose is absorbed into the bloodstream. After absorption, glucose is directly transported to the liver. The liver removes excess glucose from the blood and stores it as glycogen, which is a polymer of glucose units. In between meals, hepatocytes break down the glycogen and release glucose back into the bloodstream for use by other cells of the body. If the body requires more glucose than what is stored as glycogen, the liver will make glucose from other molecules like fatty acids and amino acids.

After digestion, fats are also transported to the liver. These fats are packaged into protein complexes in the liver, and then transported to other cells in the body including the fat-storing cells, also known as adipocytes. The liver also synthesizes triglycerides from excess carbohydrates and proteins. Because fats in the form of triglycerides are not water soluble, they must be transported to other cells by proteins. The lipoprotein transport molecules are made in the liver. These lipoproteins are the VLDLs that are measured in a lipid profile. The liver additionally synthesizes cholesterol from fatty acids.

Hepatocytes make bile, a yellowish-brown fluid that aids in fat digestion. The gall bladder, which is located just beneath the liver, stores bile until it is released into the small intestine. Bile salts, the main component of bile are made from cholesterol in the liver. The bile salts emulsify fats and break the fat down into smaller units. This increases the surface area of the fat so that enzymes called lipases can further digest the fat.

In addition to removing ammonia from the body, the liver also processes bilirubin, an insoluble breakdown product of hemoglobin. Hemoglobin, the main protein in red blood cells, is metabolized to bilirubin in the liver, and then secreted in bile and removed from the body in feces or urine. The total level of bilirubin in the blood is marker for liver function.

The liver is the primary organ involved in the detoxification of drugs and poisons. Hepatocytes have multiple enzyme systems that breakdown foreign molecules, called xenobiotics, into water soluble compounds that can be excreted. In most cases, the liver metabolizes the foreign molecules into less toxic compounds. However, as pointed out by the authors of an article on the liver published in the April 2004 issue of "Pediatrics," sometimes the xenobiotics are metabolized to toxic intermediates and the liver becomes a target for this toxicity.

The liver stores the fat-soluble vitamins, A, B12, D, E, and K, and the minerals, iron and copper. The liver stores the vitamins and minerals after digestion until they are required by other cells to conduct a specific biological function.