The human liver is second in size after the skin, which is considered as an organ. The liver is situated in the right upper side of the abdomen. It is attached to the gallbladder through canals, which are used to transport bile from the liver to the gallbladder for storage, and for use by the intestine for digesting food. The liver is supplied with blood from two sources.
The first source is via the portal hepatic vein which transports veinous blood from the intestine, pancreas and the spleen to the liver. This blood contains degraded food products from the intestine and blood cells products from the spleen as well as products from the pancreas. This is in addition to waste products such as metabolized medicines and other compounds such as ammonia which are processed in the liver.
The second source of blood supply to the liver comes from the heart using the hepatic artery. This artery carries oxygenated blood from the heart to the liver. The oxygen is used for liver tissue meatoblism, and in particular for the generation of ATP in the process of oxidative phosphorylation.
The cells of the liver include three types which are: kupfer cells, which are phagocytic cells derived from monocytes. The have immunologic function and help degrade end products of damaged red blood cells. Hemosiderin, a product of ferritin metabolism is stored in these cells. Iron is stored in kupfer cells as well.
The second type of liver cells is called stellate cells or Ito cells. These cells are responsible for storage of vitamin A. As is well known vitamin A is important in the physiology of vision. It is transported from the liver to the retina of the eye, where it is converted to Rhodopsin, which is the visual pigment of Rods and cons of the retina. During pathologic conditions such as in cirrhosis stellate cells are transformed into myofibroblasts. Myofibroblasts are important for their synthesis and deposition of collagen in the liver sinuses, which results in fibrosis of the liver.
The third type of liver cells is the hepatocytes. Hepatocytes are the main type of cells in the liver and constitute approximately 80% of the total cells in the liver. These cells are large and polygonal with many large organelles inside it. The liver has many function and one of them has already been mentioned with the storage of vitamin A and processing of vitamin A. Vitamin D is also processed in the liver. The dietary supplement of vitamin D is called vitamin D3, which is also produced in the skin by the impact of UV radiation.
Vitamin D is transported after processing in the liver to other tissues for use in absorption of calcium. Vitamin D3 is converted to the active form vitamin D in the liver as well as in the kidneys. The liver is responsible for the synthesis of prothrombin, which is an important factor in blood clotting. Prothrombin is synthesized from vitamin K which is obtained in the diet. Vitamin K deficiency and hence prothrombin deficiency causes bleeding disorders and hypoprothrobinemia.
The liver functions also in storing and metabolizing iron. Excess iron in the body and in the liver is associated with hemochromatosis. Another function of the liver is glucose metabolism. If glucose levels in the blood are reduced, glycogen which is stored in the liver is degraded and additional glucose is formed, and transported to blood for use. The liver can also produce glucose when its levels are low using gluconeogenesis from amino acids.
When glucose levels are high, insulin is secreted from the pancreas to restore the glucose levels to its original level by making glycogen polymer from the glucose molecules. In addition, the liver hepatocytes metabolize fatty acids to generate ATP molecules. They also synthesize cholesterol in as much as 50% of the total amount of cholesterol in the body. The rest of the cholesterol comes from the diet. Another function of hepatocytes is amino acids metabolism. They do so by removing the amino group from the amino acid, a process called deamination.
The toxic product ammonia of this process is then converted to urea a less toxic compound. A pathologic condition in which the liver is unable to remove ammonia or convert it to urea is called hepatic encephalopathy which affects the function of the brain by the excess ammonia which reaches the brain from the liver via the blood stream. Another function of the liver is bile salts synthesis and secretion. Bile salts are synthesized by hepatocytes and transported to the gallbladder for storage and for use by the intestine to digest food.
The main component of bile is bilirubin which is a pigment and is a product of red blood cells degradation. It is absorbed in the liver from the intestine and is secreted to the gallbladder for storage. Bilirubin is responsible for the yellow color in jaundice as its accumulation in the blood due to obstruction of bile flow causes jaundice. In addition to the afore mentioned functions of the liver if functions by storing vitamins and minerals such as iron and copper. These compounds are released to the blood when needed.