The liver is the largest solid organ in the human body. It has a unique structure with a dual blood supply, being approximately one-third from the hepatic artery and two-thirds from the portal venous system. Within the liver substance blood flows through sinusoids between plates of hepatocytes to drain into central veins, which in turn join the hepatic veins draining into the vena cava. The liver is a major site of protein synthesis exporting plasma proteins to maintain oncotic pressure and coagulation factors. Acute phase proteins that act as antiproteases, opsonins and metal ion carriers are synthesised by the liver in response to injury or infection. Numerous immune cells populate the liver and the resident tissue macrophages, the Kupffer cells, form an important component of the innate immune system. Nutrients are extracted from portal blood by the liver and processed, and the liver acts as an important reservoir for glycogen. Waste products are either modified in the liver for excretion by the kidneys or are excreted into bile. Many drugs are taken up by the liver and metabolised, giving either active metabolites or inactive metabolites for excretion. In humans, as in many vertebrates, the liver's capacity for metabolism and clearance far exceeds what is required for day-to-day life. It is possible that this ability offers a significant advantage in terms of survival from poisoning, starvation or trauma.