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Structure

In the embryo the liver appears as an outgrowth from the upper portion of the duodenum, just below the stomach. Unlike any other organ, the liver has two sources of blood supply: the hepatic artery carrying oxygenated blood from the heart and the portal vein carrying food substances from the stomach and intestines. These vessels enter the glandular tissue of the liver and break up into minute sinusoids (microscopic spaces between rows of liver cells).

The liver derives its own supply of oxygenated blood from the hepatic artery, which branches off the aorta. Blood leaving the liver is collected in the hepatic veins, which join together into a single hepatic vein that empties the blood into the inferior vena cava; from here it is passed back to the right side of the heart, to be pumped to the lungs. The substance of the liver is composed of minute divisions called lobules, separated from each other by connective tissue. These lobules are made up of columns of cells surrounded by tiny channels known as canaliculi, into which the bile secreted by the liver cells is poured. These channels unite to form progressively larger ducts, culminating in the hepatic duct. The hepatic duct and the duct from the gallbladder form the common bile duct that discharges into the duodenum. In primates and carnivores, the pancreatic duct usually joins the common bile duct before it enters the intestine.

Function

Blood passes through the liver at a rate of about 1.4 litres per minute; at any instant, the liver contains about 10 per cent of all the blood in the body. It also carries blood from the pancreas and spleen. The liver cells help the blood to assimilate food substances and to excrete waste materials and toxins, as well as products such as steroids, oestrogen, and other hormones.

The liver is a highly versatile organ. It stores glycogen, iron, copper, vitamin A, many of the B-complex vitamins, and vitamin D. It produces albumin and other proteins, including many of those essential in normal blood clotting, such as prothrombin and fibrinogen, and the anticoagulant substance heparin. In the liver, digested amino acids are deaminated; that is, their nitrogen is removed for use in the body. The liver also can use nitrogen to manufacture new protein from carbohydrate or fat. In addition, it can manufacture many other substances, including carbohydrate, from fat or from protein. From carbohydrate or protein the liver can also make fat that it stores and later releases into the blood as free fatty acid, which can be burned for energy. The liver also synthesizes cholesterol.

Special phagocytes in the liver remove foreign substances and bacteria from the blood. The liver also detoxifies many drugs and secretes cholesterol, bilirubin (the breakdown product of haemoglobin), and many other substances, including enzymes. The activities of the liver generate a great deal of heat, affecting body temperature. The liver of mammals contains stores of B vitamins; one of these, vitamin B12, is used in treating pernicious anaemia. The liver also stores other antianaemic agents produced elsewhere in the body.

Diseases of the Liver

Hepatitis, the term used to denote any inflammation of the liver, is derived from the Greek for liver, hepar. The commonest cause of hepatitis is a viral infection of the liver. Hepatitis may also be caused by chemical agents or poisons; drugs; bacteria or bacterial toxins; amoebic disease; and certain parasitic infestations. Hepatitis can progress to a chronic state that may lead to cirrhosis, a condition of progressive scarring. Most cases of cirrhosis, however, are associated with excessive ingestion of alcohol, usually in conjunction with a poor diet. In acute hepatitis, the disease is occasionally so severe that virtually all the liver cells are destroyed and the patient dies of liver failure or by obstruction of the blood vessels leading from the liver. Jaundice, a common symptom of hepatitis and other liver diseases, is caused by the accumulation of abnormally large quantities of bilirubin in the blood.

Certain diseases such as diabetes mellitus are associated with an excess deposition of fat in the liver; pituitary disorders and such poisons as alcohol and chloroform, which interfere with proper oxidation in the liver, may also cause such deposition. As the accumulation of fat increases, the liver cells are replaced by adipose tissue, and fatty degeneration of the liver sets in. Deposition of fat in the liver is seen temporarily in pregnancy and after maintaining a diet rich in fat. Other diseases afflicting the liver include abscess, which may be due to bacteria or amoebas; tumours, including cancer, which is most often secondary to cancer that has spread from elsewhere in the body; infiltrations of foreign substances; and granulomas, masses of chronically inflamed tissue. Operations to replace severely diseased livers with healthy ones had a fairly low rate of success until recent years.