Liver
The wedge-shaped liver is the largest--and in some ways the most complex--organ in the body. It serves as the body's chemical factory, performing many vital functions, from regulating the levels of chemicals in the body to producing substances that make the blood clot during bleeding.
Functions of the Liver
The liver manufactures about half of the body's cholesterol; the rest comes from food. Most of the cholesterol made by the liver is used to make bile, a greenish yellow, viscid fluid that aids in digestion. Cholesterol is also needed to make certain hormones, including estrogen, testosterone, and the adrenal hormones, and is a vital component of every cell membrane. The liver manufactures other substances as well, especially proteins, which the body uses to carry out its functions. Clotting factors, for example, are proteins needed to stop bleeding. Albumin is a protein needed to maintain fluid pressure in the bloodstream.
Sugars are stored in the liver as glycogen and then broken down and released into the bloodstream as glucose when needed--for example, when sugar levels in the blood become too low, such as during sleep when a person spends many hours without eating.
Another one of the liver's main functions is to break down harmful or toxic substances absorbed from the intestine or manufactured elsewhere in the body and then to excrete them as harmless by-products into the bile or the blood. By-products excreted into the bile enter the intestine, then leave the body in the stool. By-products excreted into the blood are filtered out by the kidneys, then leave the body in the urine. The liver also chemically alters (metabolizes) drugs (see Section 2, Chapter 11), rendering them inactive or allowing them to be more easily excreted from the body.
Disorders of liver function can be divided broadly into two groups: those caused by a malfunction of the cells in the liver itself (such as cirrhosis and hepatitis) and those caused by an obstruction of bile flow from the liver through the biliary tract (such as bile stones and cancer).
Blood Supply of the Liver
The liver receives blood from the intestines and directly from the heart. Tiny capillaries in the intestinal wall drain into the portal vein, which enters the liver. The blood then flows through a latticework of tiny channels inside the liver, where digested nutrients and any harmful substances are processed. The hepatic artery brings blood to the liver from the heart. This blood carries oxygen for the liver tissue itself as well as cholesterol and other substances for processing. Blood from the intestines and heart then mix together in the tissues of the liver and flow back to the heart through the hepatic vein.
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