LIVER

largest glandular organ of the body, weighing about 3 lb (1.36 kg). It is reddish brown in color and is divided into four lobes of unequal size and shape. The liver lies on the right side of the abdominal cavity beneath the diaphragm. Blood is carried to the liver via two large vessels: the hepatic artery carries oxygen-rich blood from the aorta, and the portal vein carries blood containing digested food from the small intestine. These blood vessels subdivide in the liver repeatedly, terminating in minute capillaries. Each capillary leads to a lobule. Liver tissue is composed of thousands of lobules, and each lobule is made up of hepatic cells, the basic metabolic cells of the liver. One of the liver's major functions is the manufacture and secretion of bile, which is stored in the gall bladder and released in the small intestine. Bile salts emulsify fats, a process that prepares the latter for digestion by the intestinal enzymes (see digestive system). The hepatic cells assimilate carbohydrates, fats, and proteins. They convert glucose to its stored form, glycogen, which is reconverted into glucose as the body requires it for energy. The ability of the liver to maintain the proper level of glucose in the blood is called its glucose buffer function. The end products of fat digestion, fatty acids, are used to synthesize cholesterol and other substances needed by the body. Excess carbohydrates and protein are also converted into fat by the liver. Digested proteins in the form of amino acids are broken down further in the liver by deamination. Part of the amino acid molecule is converted into glycogen and other compounds. Urea, a waste product of protein breakdown, is produced by the liver, a process which removes poisonous ammonia from the body fluids. The liver is also capable of synthesizing certain amino acids (the so-called nonessential amino acids) from other amino acids in a process called transamination. Some essential components of blood are manufactured by the liver, including about 95% of the plasma proteins and the blood-clotting substances (fibrinogen, prothrombin, and other coagulation factors). The liver also filters harmful substances from the blood. Phagocytic cells in the liver, called Kupffer cells, remove large amounts of debris and bacteria. In addition, the liver stores important vitamins and minerals, including vitamins A, D, K, and B₁₂. Several diseases states can affect the liver, such as hepatitis (an inflammation of the liver) and cirrhosis (a chronic inflammation that progresses ultimately to organ failure). Alcohol alters the metabolism of the liver, which can have overall detrimental effects over long periods of abuse. In 1994, a bioartificial liver, part machine, part cloned living liver cells, was used for the first time. Functioning somewhat like a kidney dialysis machine, the bioartificial liver can support patients with acute liver failure until their own livers regenerate, or it can be used by patients while waiting for a liver transplant.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

-28469-