Liver
The liver is the largest internal organ located in the upper right quadrant of the abdomen just below the diaphragm. It is a highly vascular organ containing 10-20% of total blood volume.
The basic functional units of the liver are the hexagonal lobules, which are surrounded by connective tissue. In the corner of each lobule is a portal triad composed of the portal vein, hepatic artery, and hepatic duct. Through the middle of the lobule runs a central vein surrounded by the hepatic cords composed of hepatocytes (liver epithelial cells). The spaces between the cords are filled by liver sinusoids connecting the portal triads with the central vein and containing endothelial cells as well as Kupffer cells (hepatic phagocytic cells). The lobules are organized into four lobes, two major (right and left) and two minor (caudate and quadrate).
Oxygenated blood comes into the liver through the hepatic artery and deoxygenated blood flows through the portal vein (coming from the gastrointestinal tract). An exchange of substances occurs in the liver sinusoids and the blood is collected in the central vein and then the hepatic vein. Due to the large number of sinusoids (capillaries) that can be filled up with blood, the liver can become a blood reservoir if required.
The liver plays an important role in metabolism, digestion, detoxification and removal of bacteria, debris, and waste products through phagocytosis and bile synthesis. The most important metabolic activities of the liver are the regulation of blood sugar levels through glycogen formation (high sugar) or gluconeogenesis (low sugar), protein synthesis (albumin, fibrinogen, heparin, clotting factors, and globulins) and recycling of amino acids, fat metabolism (synthesis of phospholipids, lipoproteins, and breakdown of fatty acids). Another important liver function is storage of copper, iron, vitamins A, B12, D, E, and K.
The detoxification role of the liver is centered on the conversion of toxic ammonia to urea, which is then secreted into the circulation and finally excreted by the kidneys. Drugs present in the circulation are inactivated in the liver at various rates lowering their availability in blood. Removal of particulate matter, such as dead blood cells, bacteria, and general debris is carried out by Kupffer cells.
An important role is played by the liver in the digestion process through bile production by hepatocytes. Bile is mainly an excretory compound but is also essential for the dilution of stomach acids and emulsification of fats.
Liver diseases are often fatal and can occur through different disease processes. One non-fatal disease is hemochromatosis, an iron storage defect leading to accumulation of iron in the liver. Problems with liver function can be manifested through appearance of jaundice that is yellowing of the tissues due to a high concentration of bilirubin in the blood. Jaundice is commonly associated with hepatitis (inflammation of the liver) caused mainly by viral infection. Some hepatitis viruses, (e.g., B and C) can cause chronic infection leading to the formation of scar tissue, loss of liver function, and often to liver cancer.
Cirrhosis is a process that can occur as a result of infection, prolonged inflammation, or as a result of alcohol abuse. It involves the death of hepatocytes and their replacement with the fibrous tissue. As a result, liver tissue becomes paler due to lower blood flow and has a nodular appearance due to accumulation of fibrous material.
The liver has a remarkable ability to regenerate tissue, provided that the circulation is not fully destroyed. However, in cases of severe damage, a liver transplant may be the only option for treatment of liver disease.
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