The liver is a large, complex organ with functions including detoxification, protein synthesis, and production of bile for digestion. While it appears as a single entity, the liver is internally organized into distinct functional sections. This internal architecture is defined by the network of blood vessels and bile ducts that supply the organ. Understanding this segmented nature is important for appreciating how the liver works and how medical professionals address diseases that affect it.
The Functional Anatomy of the Liver
The concept of liver segmentation is based on its vascular and biliary supply. The liver is divided into eight functional segments, each with its own blood inflow from the portal vein and hepatic artery, as well as its own biliary and venous outflow. This arrangement means that each segment operates as a self-contained unit. The portal vein brings nutrient-rich blood from the digestive system, while the hepatic artery supplies oxygenated blood.
Think of the liver as an apartment building where each apartment has its own independent plumbing; if one has an issue, it doesn’t necessarily affect the others. The three main hepatic veins—the right, middle, and left—run between the segments and divide the liver into vertical sections. The portal vein divides the liver into upper and lower segments, and this vascular network forms the basis for the functional divisions.
The Couinaud Classification System
The most widely accepted system for mapping the liver’s functional anatomy is the Couinaud classification, named after French surgeon Claude Couinaud. This system divides the liver into eight functionally independent segments, numbered I through VIII. The numbering of these segments generally follows a clockwise pattern when viewing the liver from the front. The segments are grouped into anatomical left and right lobes based on the vascular divisions.
Segment I, also known as the caudate lobe, is located posteriorly and is unique because it receives blood from both portal vein branches and drains directly into the inferior vena cava. Segments II and III make up the left lateral section, while Segment IV is the medial segment of the left lobe. Segments V, VI, VII, and VIII constitute the right lobe, with V and VIII being anterior and VI and VII being posterior.
Clinical Significance in Liver Surgery
The segmental anatomy of the liver is important in modern surgery. Understanding these segments allows surgeons to perform highly precise resections, known as hepatectomies. Because each segment has its own blood supply and drainage, a surgeon can remove a single diseased segment—a segmentectomy—while leaving healthy segments to function and regenerate. This approach is an advancement over older surgical techniques that often required removing a much larger portion of the liver. This preserves as much liver function as possible, which is a major factor in patient recovery.
The segmental anatomy is also a consideration in living donor liver transplantation. In this procedure, a portion of a healthy donor’s liver is removed and transplanted into a recipient. The surgeon can remove a specific lobe or set of segments, knowing the remaining segments will function for the donor and the transplanted portion will function for the recipient.
Visualizing Liver Segments in Medicine
While the Couinaud classification provides a conceptual map, there are no visible walls separating the segments within the organ itself. Clinicians rely on advanced medical imaging techniques to visualize these divisions. Computed tomography (CT) scans, magnetic resonance imaging (MRI), and ultrasound are the primary tools used to map the liver’s internal landscape. Radiologists and surgeons use the liver’s blood vessels and bile ducts as landmarks on these scans to determine the precise boundaries of each segment.
Contrast agents are often used with CT and MRI scans to enhance the visibility of these vascular structures, providing a clearer picture of the segmental anatomy. This ability to visualize the segments is important for diagnosing liver diseases and planning surgical procedures. By accurately identifying the location of a tumor within a specific segment, clinicians can develop a targeted treatment plan that minimizes the impact on the patient’s overall liver function. The use of color Doppler ultrasound can also help assess blood flow within the portal and hepatic veins, providing further diagnostic information.