## Hepatic Segmentation — Couinaud Classification **Key Point:** The Couinaud classification divides the liver into 8 functional segments based on vascular and biliary anatomy, not gross anatomical lobes. ### Left Lobe Segments | Segment | Location | Blood Supply | Biliary Drainage | |---------|----------|--------------|------------------| | **II** | Posterolateral, left lobe | Left hepatic artery & portal vein | Left hepatic duct | | **III** | Anterolateral, left lobe | Left hepatic artery & portal vein | Left hepatic duct | | **IV** | Medial, left lobe (quadrate lobe) | Left & right hepatic arteries | Left & right hepatic ducts | ### Right Lobe Segments | Segment | Location | Blood Supply | Biliary Drainage | |---------|----------|--------------|------------------| | **V** | Anteroinferior, right lobe | Right hepatic artery & portal vein | Right hepatic duct | | **VI** | Posteroinferior, right lobe | Right hepatic artery & portal vein | Right hepatic duct | | **VII** | Posterosuperior, right lobe | Right hepatic artery & portal vein | Right hepatic duct | | **VIII** | Anterosuperior, right lobe | Right hepatic artery & portal vein | Right hepatic duct | | **I** | Caudate lobe (posterior) | Hepatic artery & portal vein (variable) | Hepatic ducts (variable) | **High-Yield:** Segment II is the only purely posterolateral left lobe segment supplied exclusively by left-sided vascular branches. It is clinically significant in left hepatectomy and living donor liver transplantation. **Mnemonic:** LEFT LOBE = Segments II, III, IV (2, 3, 4). RIGHT LOBE = Segments V, VI, VII, VIII (5, 6, 7, 8). Segment I (caudate) is separate. **Clinical Pearl:** In hepatic resection planning, segment II is often preserved because it has independent vascular supply and can be spared even during extended left hepatectomy, improving residual liver function. [cite:Standring Anatomy 42e Ch 65] 
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