More information about the liver

It fulfils many functions, and can be regarded as the chemical factory of the body (though you’d need a large chemical factory to perform all its tasks. One covering several acres, in fact). It is concerned among other things with the breakdown of haemoglobin into a substance called bilirubin. The liver cells join this with a chemical called glucuronic acid, a process called conjugation, making it water-soluble, and secrete it with other substances as bile. In the gut this so-called conjugated bilirubin is converted by the action of bacteria to urobilinogen which may be absorbed and appear in the urine, or be converted to stercobilin, which colours the faeces brown. Besides the breakdown of haemoglobin and the formation of bile and bile salts the liver is intimately involved with the breakdown and storage of fats and the synthesis of lipids, with the storage of glucose as glycogen, with the break down of proteins, as amino acids, to produce urea - as well as the synthesis of proteins such as albumin. It is the source of a number of the components of the blood-clotting cascade, so people with severe liver disease tend to bleed badly if they are injured or undergo surgery, and it is the site of synthesis of vitamin A, which it stores together with vitamin B12, vitamin D and vitamin K.

The liver is also responsible for the metabolism of many drugs and medicines, which means that a proportion of the medicines that we take by mouth never reach the circulation - they are consumed in the liver, a phenomenon known as the "first pass effect". For this reason techniques have been developed to allow treatments to be delivered directly to those parts of the circulation which do not initially pass through the liver, such as through the skin (called transdermal, commonly used for hormone replacement therapy), under the tongue (called sublingual, commonly used in the treatment of angina) or rectally (used it to deliver, for example, anti fever medication by suppository).

The functional unit of the liver is called the lobule, and the liver contains millions. Think of a six-sided hexagon. At each angle there is a structure called a portal tract, while in the middle there is a central vein. In between is a network of capillaries, called sinusoids, bordered by liver cells, called hepatocytes. Each portal tract contains a branch of the portal vein, the vein which drains the small intestine and carries all the carbohydrate, protein, fats, salts and minerals that have been absorbed during digestion (together with lots of other things, such as medicines). These enter the lobules and terminate in the small gaps between the liver cells - the sinusoids. The liver cells absorb the constituents carried by the blood, and at the same time secrete bile, which drains through the sinusoids into the bile ducts, which run with the portal veins and the branches of the hepatic artery (which brings oxygenated blood to the liver) in the portal tracts. The blood drains into the veins which lie in the centre of each lobule (branches of the hepatic vein). The bile from the bile ducts is stored in the gall bladder which lies underneath the liver. It is secreted, together with secretions from the pancreas, into the duodenum in response to the presence of food in the stomach.

Unlike other organs of the body the liver has the ability to regenerate itself, so that if one of the lobes of the liver is removed it will regrow. In some diseases, however, this process goes awry and leads to the development of cirrhosis.