The liver is the largest gland in the body and one of the most important accessory organs of the gastrointestinal tract. The liver lies under the diaphragm and is divided into four lobes known as the right, left, cordate, and quadrate lobes. The lobes of the liver are further divided into lobules, which contain the liver cells (hepatocytes) and cells of the reticuloendothelial system called Kupffer’s cells.
The liver receives material absorbed from the digestive system through a subdivision of the circulatory system called the hepatic portal system. This system is formed by venules and veins draining blood from various regions of the digestive system and merging to form a single hepatic portal vein. The hepatic portal vein carries blood from the capillary beds of the digestive system to the liver, where it branches to form a second capillary bed. Materials are also delivered from the circulatory system to the liver by the hepatic vein, which joins the inferior vena cava. The liver is located under the diaphragm and occupies most of the right hypochondrium of the abdominal cavity. Its secretions reach the small intestine by the hepatic duct, which leads to the common bile duct.
The liver has many vital functions related to the digestive process. One of the important functions is the production of bile, a yellow, brownish, or olive-green liquid. Bile consists of water and bile salts, cholesterol, a phospholipid called lecithin, bile pigments and ions. The principal pigment of bile is bilirubin, which is later digested in the intestine by bacteria. Bilirubin is an orange-yellow pigment in the bile formed as a breakdown product of haemoglobin. Excess amounts in the blood produce the yellow appearance associated with jaundice. The bile is stored in a pear-shaped sac called the gall bladder. The cystic duct delivers bile to the duodenum through the common bile duct, where it aids the emulsification and absorption of fats. The common bile duct is formed by the union of the cystic duct from the gall bladder and the hepatic duct from the liver.
Another function of the liver concerns carbohydrate metabolism. When the level of glucose in the blood is high, enzymes in the liver convert glucose to glycogen. This process is called glycogenesis. When the supply of blood glucose is low, enzymes in the liver cells convert glycogen into glucose. This process is called glycogenolysis. The enzymes of the liver are also able to convert amino acids into carbohydrate molecules for energy use when the level of blood carbohydrate is low, a process known as gluconeogenesis. The liver is able to break down fatty acids to smaller molecules.
In protein metabolism, enzymes of the liver perform a process called deamination. Deamination involves the removal of the amino groups from amino acids. The resulting molecules can then be used for energy metabolism or converted to carbohydrates or fats. The amino groups that result from amino acids are used to synthesize a toxic substance called urea. Urea is eventually removed from the bloodstream in the kidney and is the major component of urine.
Other forms of protein metabolism are also performed in the liver. For example, cells of the liver synthesize most plasma proteins such as albumin, globulins, and prothrombin and fibrinogen used in blood clotting.
Cells of the liver also remove drugs and hormones from the blood. For example, liver cells can remove drugs such as penicillin and sulpha drugs from the blood and excrete them to the bile. Liver enzymes can also alter the chemical structure of certain steroid hormones such as oestrogen and aldosterone.
Vitamin storage is another function of the liver. The liver stores fat-soluble vitamins such as A, B12, D, E, and K, as well as minerals such as iron and copper. In liver cells, a protein called apoferretin combines with iron ions to form ferretin, and iron is stored in the liver in this way. The Kupffer’s cells of the liver perform phagocytosis and remove old red and white blood cells from the bloodstream for destruction. Finally, the liver participates in the activation of vitamin D for use in the body.