The human liver is a large, complex organ that performs hundreds of functions, including detoxification and metabolism. While the liver appears divided into a large right lobe and a smaller left lobe externally, this surface division does not reflect its internal organization. Modern medical understanding relies on functional segmentation, which views the liver as a highly organized structure composed of distinct functional units. This segmentation is the standard for understanding liver anatomy and planning surgical procedures.
Why Functional Segmentation is Necessary
The traditional anatomical division of the liver into right and left lobes, separated by the falciform ligament, is misleading for surgical planning. This external division does not account for the organ’s internal plumbing. Cutting based on surface appearance would sever the blood supply and bile drainage systems, causing significant damage to the remaining liver tissue. A functional division, in contrast, ensures that remaining segments maintain independent blood flow and bile drainage after partial removal.
The universally accepted standard is the Couinaud classification system, sometimes called the Brisbane 2000 terminology. This system divides the liver based on the distribution of its internal vessels, which determine functional independence. Each segment is recognized as a self-sufficient unit with its own vascular inflow, outflow, and biliary drainage. The ability to isolate and remove these independent segments without disrupting others is the foundation of precise liver surgery and transplantation.
The Vascular Basis of Liver Segmentation
The functional division of the liver hinges on the arrangement of two major vessel systems: the hepatic veins and the portal veins. The hepatic veins, which drain deoxygenated blood from the liver into the inferior vena cava, run between the segments and serve as the primary divisional landmarks.
The three main hepatic veins (right, middle, and left) create three vertical planes that separate the liver into four sectors. The plane containing the middle hepatic vein, known as Cantlie’s line, divides the liver into the true functional right and left hemilivers. Surgeons plan cuts along these planes because the surrounding tissue is relatively free of portal vein branches and bile ducts, minimizing blood loss.
The portal veins, which deliver nutrient-rich blood from the digestive tract, run within the center of the segments they supply. This allows surgeons to operate along the hepatic vein planes without cutting the portal vein supply of an adjacent segment. The portal vein’s main branches further divide the liver horizontally into superior (upper) and inferior (lower) tiers of segments. Each of the eight functional segments is wedge-shaped, with the apex pointing toward the center of the liver where the portal vein, hepatic artery, and bile duct enter.
The Eight Functional Units
The Couinaud system assigns Roman numerals I through VIII to the eight functionally independent segments. Segment I, known as the caudate lobe, is situated posteriorly and holds a unique position. It often receives blood supply from both the right and left portal vein branches. Unlike the others, the caudate lobe frequently drains its blood directly into the inferior vena cava through its own small hepatic veins, bypassing the three main hepatic veins.
The remaining segments are organized into the functional left and right hemilivers, separated by the middle hepatic vein plane. The functional left hemiliver consists of Segments II, III, and IV. Segment IV is located medially, while Segments II (superior) and III (inferior) are located laterally.
The functional right hemiliver comprises Segments V, VI, VII, and VIII. Segments V (anterior-inferior) and VIII (anterior-superior) are in the front portion of the right liver. Segments VI (posterior-inferior) and VII (posterior-superior) are situated in the back portion.