The liver is found inferior to the diaphragm and occupies the majority of the right upper quadrant (RUQ) of the abdomen. It is mostly intraperitoneal, spanning from the fifth intercostal space in the midclavicular line to the right costal margin. The superior posterior aspect of the liver contains a bare area where the diaphragm and inferior vena cava are found. The rest of the liver is covered by visceral peritoneum which meets the diaphragm at the border of the bare area forming the coronary ligament. The inferior liver is closely associated with the gallbladder and right kidney. Another ligament of mention is the Falciform ligament which splits the liver into anatomic left and right along the anterior aspect. The free-form edge of the falciform ligament contains the round ligament of the liver which is the remnant of the embryonic umbilical vein. Anatomically the liver has four lobes: right, left, caudate, and quadrate. The caudate lobe is demarcated medially by the ligamentum venosum, posteriorly by the inferior vena cava and anteriorly by the porta hepatis. The ligamentum venosum is the remnant of the embryonic ductus venosus. The quadrate lobe is anterior to the porta hepatis and lateral to the round ligament of the liver and is closely associated with the gallbladder. However, these anatomic lobes do not correlate to the boundaries of the eight functional sub-divisions of the liver which are divided according to the blood supply.
The functional unit of the liver is the lobule. Liver lobules are collections of hepatocytes in a hexagonal shape with the center being a central vein. Within the lobules, the hepatocytes are arranged in cords, and in between the cords is a vascular space with a thin fenestrated endothelium and a discontinuous membrane called a sinusoid. These sinusoids contain Kupffer cells which are the resident macrophage of the liver and stellate cells which are hepatic lipocytes. At the vertices of the hexagon are a triad of a bile duct branch, a portal vein branch, and a hepatic artery branch referred to as a portal triad. Blood flows from the portal vein branch and hepatic artery branch across the lobule and finally into the central vein which is a branch of the hepatic vein. An alternative organization of hepatocytes places the line between 2 triads at the center of a rhomboid with the ends being central veins. This arrangement is called the portal acinus and is helpful in describing the functional zones of the liver. Zone 1 hepatocytes immediately surround the portal tracts and primarily are involved in oxidative energy metabolism. Zone 3 hepatocytes immediately surround the central veins and are the primary location for the biotransformation of drugs. Zone 2 hepatocytes lie in between zone 1 and 3 and have mixed functionality.
The liver protects the body from toxic substances absorbed from the gastrointestinal (GI) tract by processing and metabolism within the lobule. Phase-I reactions are catabolized by the cytochrome P-450 enzyme system while phase-II reactions conjugate substances with substrates such as glucuronide, glutathione, sulfate, among others.