liver

The human digestive system. When food is swallowed, it is moved down the oesophagus by the action of muscles (peristalsis) into the stomach. Digestion starts in the mouth and continues in the stomach as the food is mixed with enzymes and strong acid. After several hours, the food passes to the small intestine. Here more enzymes are added and digestion is completed. After all nutrients have been absorbed, the indigestible parts pass into the large intestine and thence to the rectum. The liver has many functions, such as storing minerals and vitamins and making bile, which is stored in the gall bladder until needed for the digestion of fats. The pancreas supplies enzymes. The appendix appears to have no function in human beings.

The structure of the human liver, front view (top) and cross-section.

In vertebrates, large organ with many regulatory and storage functions. The human liver is situated in the upper abdomen, and weighs about 2 kg/4.5 lb. It is divided into four lobes. The liver receives the products of digestion (food absorbed from the gut and carried to the liver by the bloodstream), converts glucose to glycogen (a long-chain carbohydrate used for storage), and then back to glucose when needed. In this way the liver regulates the level of glucose in the blood (see homeostasis). This is partly controlled by a hormone, insulin. The liver removes excess amino acids from the blood, converting them to urea, which is excreted by the kidneys. The liver also synthesizes vitamins, produces bile and blood-clotting factors, and removes damaged red cells and toxins such as alcohol from the blood.

If more protein is eaten than is needed to make different proteins for the body, the excess is broken down. This breakdown takes place in the liver. One product of this breakdown is urea and this has to be lost from the body in the urine. The liver also stores some vitamins, produces bile, and breaks down red blood cells.

In the USA, 30,000 people die annually (1999) as result of liver failure.

Unlike the kidney, the liver function is so complex that it cannot at present be taken over by a machine, hence acute liver failure results in certain death unless a donor liver is available for transplantation. In 2007, however, researchers made progress towards keeping livers alive outside of the body. If successful, the ‘extracorporeal liver’ will not only make more donor livers available (as they can be more easily kept and transported), but also open up new treatment options, such as using an extracorporeal liver temporarily to give the patient's own liver time to regenerate.