Liver Function and Anatomy

Liver Introduction and Function

• The liver is the largest gland in the body, weighing approximately 1.4 kg.
• It receives all materials absorbed by the gastrointestinal tract except some lipids, which are carried by lymphatics to enter the general circulation.
• Blood from the portal vein, which drains the entire gastrointestinal tract and spleen, passes through the liver before entering the systemic circulation.

Blood Supply and Hepatic Portal System

• The liver has a dual blood supply, with both venous (portal) and arterial (hepatic) bloods percolating through the liver sinusoids and exiting by way of the hepatic vein.
• The hepatic portal system is a unique arrangement of sinusoids between veins that enables the liver to filter blood from the gastrointestinal tract.

Histology

• The liver is surrounded by a delicate connective tissue capsule called Glisson's capsule.
• The parenchyma of the liver consists of large polyhedral epithelial cells, hepatocytes, arranged in plates that radiate from the central vein.
• Hepatocytes are separated by blood sinusoids and have surfaces that bear numerous well-developed microvilli.
• The plates of hepatocytes are supported by a delicate stroma consisting primarily of reticular fibers.

Hepatic Lobule

• The classic hepatic lobule is a hexagonal unit consisting of a central vein, portal areas, and plates of hepatocytes.
• Each lobule contains a portal triad, which consists of a branch of the portal vein, a branch of the hepatic artery, and an interlobular bile duct.
• The liver consists of approximately one million hepatic lobules.

Bile Production and Secretion

• Bile is produced by hepatocytes and poured into bile canaliculi, which are spaces between the plasma membrane of adjacent cells.
• Bile canaliculi become continuous with intralobular ductules, which in turn become continuous with interlobular ducts at the portal triad.

Other Cells and Structures

• Hepatic macrophages (Kupffer cells) are actively phagocytic cells that form part of the sinusoidal lining.
• Lipocytes are cells that accumulate lipid and are most numerous in the peripheral and intermediate zones of the hepatic lobule.
• The liver acinus is a smaller unit of liver structure defined by the hepatic tissue supplied by a terminal branch of the hepatic artery and portal vein and drained by a terminal branch of the bile duct.