The liver is the 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. 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.
The American Liver Foundation (ALF) is the nation’s leading nonprofit organization promoting liver health and disease prevention. ALF provides research, education and advocacy for those affected by liver-related diseases, including hepatitis. A wealth of material is available on the ALF website regarding liver function, liver diseases, and reserach programs to combat liver disease.
Liver disease is an important cause of mortality, affecting persons of all ages, but most frequently individuals between the ages of 40 and 60 years. Medical research on liver disease is critically important to bring under control the major toll of liver disease on human health and well-being. To address the burdens of liver disease in the United States, the National Institutes of Health has developed an Action Plan for Liver Disease Research.