Many potentially toxic substances enter the body via the gastrointestinal tract (gut). As the blood supply from the gastrointestinal tract (through the portal vein) drains into the liver, the liver comes into contact with the potentially toxic substances, and this exposure will often be at a higher concentration than in other tissues. The liver is essential for the metabolic disposal of virtually all xenobiotics (foreign substances). This process is mostly achieved without injury to the liver itself or to other organs. A few compounds such as carbon tetrachloride are toxic themselves or produce metabolites that cause liver injury in a dose-dependent fashion. However, most agents cause liver injury only under special circumstances when toxic substances accumulate.