The human liver is the largest internal organ, situated in the upper right quadrant of the abdomen, tucked beneath the diaphragm. It is a complex biological filter and chemical factory, responsible for hundreds of functions, including detoxification, protein synthesis, and metabolism. The organ’s structure is intricate, leading to different ways of defining its internal divisions. Understanding how the liver is divided is a source of confusion because there are two distinct ways to count its parts—one based on external appearance and another based on internal blood supply. This duality explains why the liver is described as having either four lobes or eight segments, depending on the context.
The Four Primary Anatomical Lobes
The traditional, or classical, view of the liver’s structure divides the organ into four distinct lobes. This anatomical classification is based largely on the visible surface features, such as the location of the falciform ligament and various fissures. The largest division is the Right Lobe, which occupies the majority of the organ’s mass and volume. The Right Lobe is separated from the smaller Left Lobe by the falciform ligament on the anterior surface.
When viewed from the underside, or visceral surface, two smaller, accessory lobes become apparent. These are the Caudate Lobe and the Quadrate Lobe, both of which are situated on the anatomical right side of the organ. The Caudate Lobe is located superiorly and posteriorly, nestled near the inferior vena cava. The Quadrate Lobe sits on the inferior surface, lying adjacent to the gallbladder.
These four parts represent the classical morphological description taught in basic anatomy. This system provides a simple, gross description of the liver’s external shape and its relationship to surrounding structures. While this four-lobe model is straightforward, it does not accurately reflect the organ’s internal plumbing or its functional independence. This purely descriptive anatomical view is less relevant to modern surgical practice.
The Functional Segmentation System
Modern medicine, particularly liver surgery, relies on a more sophisticated structural model known as the Couinaud classification. This system divides the liver not into four anatomical lobes, but into eight functionally independent segments, numbered I through VIII. This classification is based on the distribution of the liver’s inflow vessels: the hepatic artery and the portal vein.
Each of the eight segments is considered a self-contained unit because it possesses its own dedicated blood supply, biliary drainage, and lymphatic drainage. The boundaries between these segments are defined by the three main hepatic veins, which provide the outflow for the blood. These veins run between the segments, while the portal vein branches, hepatic artery branches, and bile ducts run within the center of each segment.
This functional model re-defines the liver’s main division, separating it into a functional right liver and a functional left liver along an imaginary line called Cantlie’s line, which includes the middle hepatic vein. This division separates the liver based on the primary branches of the portal vein and hepatic artery, which provide the functional blood supply. Segment I, the Caudate Lobe, is often considered functionally distinct because it can receive blood supply from both the right and left branches of the portal vein.
Connecting Structure to Function
The shift from the four-lobe anatomical view to the eight-segment functional system has had profound implications for patient care. The functional segmentation allows surgeons to plan highly precise procedures, known as segmental resections. By understanding which vessels supply which segment, a diseased part can be removed while preserving the blood supply and drainage of the adjacent, healthy segments.
This precision is important because the liver possesses a remarkable ability to regenerate, meaning the remaining healthy segments can grow to compensate for the lost mass. Removing only the affected segment along the lines of the hepatic veins minimizes blood loss and preserves the functional integrity of the remaining liver tissue. The liver’s functions, such as filtering toxins and regulating blood components, can continue effectively with significantly reduced mass, provided the remaining segments are healthy and structurally intact. The segmented structure ensures that removing one section does not compromise the viability of the others, facilitating optimal patient recovery and sustained function.