The liver is the largest gland of the body. It has multiple and complex functions, including exocrine secretion (via bile ducts into the duodenal lumen) and maintenance of optimal concentrations of various components of blood, which it receives via the portal vein from the digestive tract and spleen.
The liver parenchyma is divided into many small lobules shaped like polygonal cylinders (Figure 1-7-4).
• Each cylinder is composed of plates of cells arranged radially around a central vein. Between the plates are radial blood sinusoids.
■ At the periphery of the lobules, branches of the portal vein, hepatic artery, bile ducts, and lymphatics course together.
Hepatocytes are 20- to 30-mm polyhedral cells (Figure 1-7-4). Liver regeneration can occur rapidly under some circumstances. As much as 90% can be replaced in about 2 weeks.
• Their six or more surfaces may either contact another cell to form gap junctions and bile canaliculi or form a free surface with microvilli exposed to the perisinusoidal space of Disse.
• Abundant glycogen in these cells takes the form of electron-dense granules that are clustered near the SEEL
■ There are several hundred mitochondria per liver cell.
■ The hepatocyte produces proteins for export (e.g., albumin, prothrombin, fibrinogen), secretes bile, stores lipids and carbohydrates, converts lipids and amino acids into glucose via the enzymatic process of gluconeogenesis, and detoxifies and inactivates drugs by oxidation, methylation, and conjugation.
The liver contains sinusoids (Figures 1-7-4,1-7-5, and 1-7-6) that are lined with fenestrated endothelial cells and scattered phagocytic Kupffer cells, which are part of the mononuclear phagocyte system.
• Kupffer cells phagocytize red blood cells and particles and contain cytoplasmic residual bodies of iron and pigments.
• Lipocytes also lie in the perisinusoidal space.
The liver contains a biliary system consisting of:
■ Bile canaliculi—tubular spaces Limited by the plasma membrane of several hepatocytes (Figures 1-7-4,1-7-5, and 1-7-6). These ducts empty into Hering canals, which are small ducts composed of cuboidal cells.
■ Hepatic ducts—receive Hering canals and eventually form the right and left hepatic ducts, which join to form the common hepatic duct.
• Common bile duct—receives the common hepatic and cystic ducts.
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