Liver
From Encyclopædia
The liver, an
organ found in all vertebrates, is the second-largest
organ in the human body, after the
skin. It is a spongy, reddish brown gland that lies just below the diaphragm in the abdominal cavity. It serves to metabolize
carbohydrateS and store them as
glycogen; metabolize LIPIDS (fats, including
cholesterol and certain vitamins) and PROTEINS; manufacture a digestive fluid, bile; filter impurities and toxic material from the blood; produce blood-clotting factors; and destroy old, worn-out red blood
cells (see
circulatory system).Two large lobes, the right and the left, make up most of the liver; attached to the right lobe are the smaller quadrate and caudate lobes. The lobes are made up of lobules--six-sided
cells arranged in sheets one
cell thick--that are closely arranged around blood vessels, bile ducts, lymph vessels, and nerves. Certain reticuloendothelial
cells (Kupffer
cells) line these lobules and play a role in immunity.Approximately three sides of each
cell are in contact with a blood vessel, and three are adjacent to a bile duct. The lobules are grouped in clusters so that the bile manufactured by each lobule passes
Down a common duct, which connects to larger ducts that
lead to the common hepatic duct. This duct joins with the cystic duct of the GALLBLADDER and enters the duodenum along with the pancreatic duct of Wirsung. Once in the intestines, bile salts aid in emulsifying fats and enhancing the
metabolism of fats and proteins by enzymes.The liver is a highly vascular tissue. It receives 25% of its blood from the hepatic
artery. The other 75% of the liver's blood supply comes from the portal vein, which transports digested nutrients and hormones from the intestines, hormones from the pancreas, and old red blood
cells and bilirubin--a component of bile--from the
spleen. Blood leaves the liver by the hepatic veins and thence to the inferior vena cava and heart.The liver is able to regenerate itself after being injured or diseased; if a disease progresses beyond the tissue's capacity to regenerate new
cells, the body's entire
metabolism is severely affected. Two common liver diseases are HEPATITIS (inflammation of the lobules) and CIRRHOSIS, scarring of the lobules. Many disorders can affect the liver and interfere with the blood supply, the hepatic and Kupffer
cells, and the bile ducts. Bile consists of such substances as lecithin, bile salts, and
cholesterol, the last two of which can form GALLSTONES under certain conditions and result in obstruction of bile ducts. JAUNDICE, yellow
skin discoloration, is a symptom of a variety of liver disorders. Liver cancer is fairly rare but generally incurable. Severely impaired livers are sometimes replaced, and in the early 1990s the one-year survival rate was 76 percent. In 1984 the U.S. Department of Health and Human Services shifted liver transplantation in children to a nonexperimental basis (see
TRANSPLANTATION, ORGAN).
bibliography: Chen, T. S. and P. S., Understanding the Liver: A History (1984); Johnson, L. R., Gastrointestinal Physiology, 3d ed. (1985); Sherlock, S. V., The Human Liver (1978); White, T. T., Sarles, H., and Benhamou, J. P., Liver, Bile Ducts, and Pancreas (1977); Wright, Ralph, et al., Liver and Biliary Diseases, 2d ed. (1985).
