| Liver segment | |
|---|---|
 Shape of human liver in animation. Eight segments by Couinaud labelled. | |
| Details | |
| Part of | Liver |
| Anatomical terminology | |
Aliver segment is one of eight segments of the liver as described in the widely used Couinaud classification (named afterClaude Couinaud) in theanatomy of theliver. This system divides thelobes of the liver into eight segments based on atransverse plane through thebifurcation of the mainportal vein,[1] arranged in a clockwise manner starting from the caudate lobe.
There are fourlobes of the liver. The Couinaud classification of liver anatomy then further divides the liver into eight functionally independent segments. Each segment has its own vascular inflow, outflow and biliary drainage.[2] In the centre of each segment there is a branch of the portal vein, hepatic artery and bile duct. In the periphery of each segment there is vascular outflow through the hepatic veins.[3] The division of the liver into independent units means that segments can be resected without damaging the remaining segments.[4] To preserve the viability of the liver following surgery, resections follow the vessels defining the peripheries of each segment. This means that resection lines parallel the hepatic veins, leaving the portal veins, bile ducts, and hepatic arteries intact.[1]
The classification system uses the vascular supply in the liver to separate the functional segments (numberedI toVIII)
SegmentI is the caudate lobe and is situated posteriorly. It may receive its supply from both the right and the left branches of portal vein. It contains one or more hepatic veins which drain directly into theinferior vena cava (IVC).[1]
Thecaudate lobe is a separate structure which receives blood flow from both the right- and left-sided vascular branches.[5][6]
The Caudate lobe includes: (1) the Spiegel lobe; (2) the paracaval portion; and (3) the caudate process portion. The Spiegel lobe has its portal venous and biliary branches ramified mainly from the left-side tract. The Paracaval portion has the portal venous branches ramified mainly from the left portal vein, while the biliary branches of this portion drained into the right and left biliary tracts at almost equal frequencies. The portal venous branches & Biliary branches of the caudate process portion has it ramified from the right-side tract.
SegmentsII andIII liemedial to the falciform ligament withII superior to the portal venous supply andIII inferior. SegmentIV lieslateral to the falciform ligament and is subdivided intoIVa (superior) andIVb (inferior).[7]
SegmentsV toVIII make up the right part of the liver:[3]
Thefissure for theround ligament of the liver (ligamentum teres) separates themedial and lateral parts of segmentIV. The inferior medial segment (IVb) is also called thequadrate lobe.[8]
The clockwise numbering of liver segments in Couinaud's classification is said to have been inspired by the numbering ofarrondissements (administrative districts) of Paris.[9]