LIVER

the largest gland in the vertebrate body, composed of a spongy mass of wedge-shaped lobes that has many metabolic and secretory functions. The liver secretes bile, a digestive fluid; metabolizes proteins, carbohydrates, and fats; stores glycogen, vitamins, and other substances; synthesizes blood-clotting factors; removes wastes and toxic matter from the blood; regulates blood volume; and destroys old red blood cells. It is essential to life. To perform many of its varied functions, the liver is uniquely and closely connected with both the intestine and the gallbladder (q.v.). From the intestine the portal vein carries venous blood to the liver to be processed before returning to the heart. From the liver a duct system carries bile to the common bile duct, which empties into the duodenum of the intestine and which connects with the gallbladder for storage and subsequent release of bile. Liver tissue consists of a mass of cells tunnelled through with bile ducts and blood vessels. Hepatic cells make up about 60 percent of the tissue and carry on more metabolic functions than any other group of cells in the body. A second group of cells, called Kupffer cells, line the smallest channels of the liver's vascular system and play a role in blood formation, antibody production, and ingestion of foreign particles and cellular debris. The human liver secretes about 800 to 1,000 millilitres (about a quart) each day of bile, which contains bile salts needed for the digestion of fats in the diet. Bile is also the medium for excretion of certain metabolic waste products, drugs, and toxic substances. Bile secreted into the common bile duct enters the gallbladder, where it is concentrated and stored. The presence of fat in the duodenum stimulates the flow of bile out of the gallbladder and into the intestine. Worn-out red blood cells are destroyed in the liver, spleen, and bone marrow. A pigment, bilirubin, formed in the process of hemoglobin breakdown, is released into the bile, creating its characteristic greenish-orange colour, and is excreted from the body through the intestine. The liver cells synthesize a number of enzymes. As blood flows through the liver, both from the portal vein and from the hepatic artery, the cells and enzymes filter out and modify many substances and particles. Nutrients entering the liver from the intestine are changed into forms usable by the body cells or are stored for future use. Fats are converted into fatty acids and then into carbohydrates or ketone bodies and transported by the blood to the tissues, where they are further metabolized. Sugars are converted into glycogen, which remains stored in the liver until it is needed for energy production, when it is reconverted into glucose and released into the bloodstream. The liver manufactures blood serum proteins, including albumin and several clotting factors, and supplies them to the blood. In its role as a blood purifier, the liver metabolizes nitrogenous waste products from body processes and detoxifies poisonous substances, preparing them for elimination in the urine or feces. A common sign of impaired liver function is jaundice, a yellowness of the eyes and skin arising from excessive bilirubin in the blood. Jaundice can result from an abnormally high level of red blood cell destruction (hemolytic jaundice), defective uptake or transport of bilirubin by the hepatic cells (hepatocellular jaundice), or a blockage in the bile duct system (obstructive jaundice). Failure of hepatic cells to function can result from hepatitis, cirrhosis, tumours, vascular obstruction, or poisoning. Symptoms may include weakness, low blood pressure, easy bruising and bleeding, tremor, apathy, brain wave changes, and accumulation of fluid in the abdomen. Tests of liver function help in identifying disease, estimating liver damage, and evaluating treatment. Blood tests can reveal abnormal levels of bilirubin, cholesterol, serum proteins, urea, ammonia, and various enzymes. Injecting the dye bromsulfalein (BSP) and measuring its time of retention in the blood is another gauge of liver function. The condition of liver tissue can be examined microscopically by performing a needle biopsy. Acute viral hepatitis is an inflammation of the liver that includes hepatitis A, usually spread by fecal contamination of food or drink, and hepatitis B, commonly transmitted by injection with unsterile instruments or by transfusion of contaminated blood or plasma. A third category of viral hepatitis, called non-A, non-B hepatitis, is caused by at least one (and probably two) other viral agents. Non-A, non-B infection is a common cause of posttransfusion hepatitis. In all forms of viral hepatitis, the illness may last two to six weeks, and fatigue often persists for as long as six months. The disease is ordinarily self-limiting; treatment includes bed rest and abstention from alcohol. Prevention of outbreaks requires careful sanitation as well as sterile injection and transfusion procedures. Cirrhosis of the liver is an irreversible chronic disease characterized by the replacement of functioning liver tissue with bands and lumps of scar tissue. It can be brought about by viral hepatitis, chronic alcoholism, obstruction of the bile channels or the hepatic vein, heart failure, deposition of iron or copper, or some forms of metabolic disease. Cirrhosis causes hepatic cell failure and abnormally high pressure in the portal vein, leading to enlargement of the spleen and rupture of blood vessels. Treatment requires eliminating the primary cause, if possible. An alcoholic with cirrhosis, for example, can show striking improvement with bed rest and total avoidance of alcohol. The liver is subject to a variety of other disorders and diseases. Abscesses can be caused by acute appendicitis; those occurring in the bile ducts may result from gallstones or may follow surgery. The parasite that causes amebic dysentery in the tropics can produce liver abscesses as well. Various other parasites prevalent in different parts of the world also infect the liver. Cancers of the liver are common, most of them secondary tumours originating elsewhere in the body. Glycogen-storage diseases, a group of hereditary disorders, generate a buildup of glycogen in the liver and an insufficient supply of glucose in the blood. Certain drugs may damage the liver, producing jaundice.