The human liver is divided into eight functionally independent segments, each supplied by its own branch of the portal vein, hepatic artery, and bile duct, and each drained by a branch of the hepatic veins. This segmental map, developed by the French surgeon Claude Couinaud in the 1950s, replaced older descriptions that simply split the liver into right and left lobes. The Couinaud system matters because it gives surgeons a way to remove diseased tissue while leaving the rest of the liver intact and fully functional, and it remains the most widely used classification in hepatobiliary medicine today.
How the Eight Segments Are Defined
The Couinaud system works by using internal blood vessels as landmarks rather than the liver’s outer surface. Three hepatic veins divide the liver vertically into four sections, and the plane where the portal vein splits into its left and right branches divides the organ horizontally. The result is eight wedge-shaped segments, each with its own inflow (portal vein branch and hepatic artery branch), its own bile drainage, and its own outflow through a hepatic vein. This arrangement means each segment behaves like a miniature liver, capable of functioning on its own as long as its vascular supply is intact.1PubMed Central. Liver segmentation: practical tips
The segments are numbered 1 through 8, roughly counterclockwise when you look at the liver from the front. Segment 1 is the caudate lobe, tucked behind the other segments against the spine. Segments 2 and 3 make up the left lateral section. Segment 4 sits between the falciform ligament and the middle hepatic vein and is sometimes subdivided into 4a (upper) and 4b (lower). Segments 5 and 8 form the right anterior section, and segments 6 and 7 form the right posterior section. The portal vein’s horizontal branching plane separates the upper segments (2, 4a, 7, 8) from the lower ones (3, 4b, 5, 6).
One detail that trips people up: the visible surface anatomy of the liver does not match the internal segmental anatomy. The falciform ligament, which looks like it divides the liver into a right lobe and a left lobe, actually runs through segment 4, not along the true functional divide. The real split between the right and left hemilivers follows the middle hepatic vein, a vessel you cannot see from the outside. This is why cross-sectional imaging is essential before any surgical procedure on the liver.
Segment 1 Stands Apart
The caudate lobe, segment 1, is anatomically unique. Unlike every other segment, it receives portal blood from both the left and right portal vein branches, and its venous drainage does not go through one of the three main hepatic veins. Instead, several small hepatic veins from the caudate lobe drain directly into the inferior vena cava, the large vein that runs behind the liver on its way to the heart.2Scientific Reports. Human liver segments: role of cryptic liver lobes and vascular physiology in the development of liver veins and left-right asymmetry
This independent drainage pattern has a clinically important consequence. In diseases that block the main hepatic veins, such as Budd-Chiari syndrome, the caudate lobe often compensates by enlarging while the rest of the liver shrinks. Surgeons and radiologists watch for caudate lobe hypertrophy as a diagnostic clue. The developmental origin of this arrangement is also interesting: during embryonic life, the main hepatic vein of the caudate lobe transforms into the intrahepatic portion of the inferior vena cava itself, leaving the smaller veins as the caudate’s drainage channels.2Scientific Reports. Human liver segments: role of cryptic liver lobes and vascular physiology in the development of liver veins and left-right asymmetry
Anatomical Variations Are Surprisingly Common
Textbook diagrams of the liver’s eight segments suggest a tidy, consistent layout. In practice, the branching patterns of the portal vein, hepatic arteries, and bile ducts vary considerably from person to person. Vascular and biliary anatomy differs in up to half of all cases, which is why preoperative imaging and collaboration between surgeons and radiologists are considered so important.3PubMed Central. Radiology for surgeons: liver anatomy
Portal vein branching is one of the best-studied areas. In one imaging study of 200 patients, only about 65% had the conventional branching pattern in which the main portal vein splits into a left and a right branch. In roughly 10% of patients, the portal vein trifurcated into three branches at once. In about 24%, the right posterior branch came off the main portal vein separately before the left and right anterior branches divided.4PubMed. Prevalence and types of main and right portal vein branching variations on MDCT Other reviews put the overall variant rate at 20 to 35% of the population.5PubMed Central. Portal vein normal anatomy and variants: implication for liver surgery and portal vein embolization
Biliary anatomy is at least as variable. Studies have found variant rates exceeding 40% for the bile duct system.6PubMed Central. Biliary Anatomy These variants rarely cause symptoms on their own, but they become critical during surgery. A bile duct that crosses unexpectedly from one segment into another’s territory can be accidentally cut or clipped if the surgeon does not know it is there. The practical message is that no two livers are quite the same on the inside, even if they look similar on the outside.
Where Segment Boundaries Actually Fall
Surgeons performing anatomic liver resections need to know exactly where one segment ends and the next begins. Traditionally, hepatic veins served as the primary landmarks for these boundaries. But more recent work using 3D reconstruction and fluorescence imaging with indocyanine green dye has shown that the true portal territory boundaries often deviate from hepatic vein trajectories.7PubMed Central. Revisiting the Intersegmental Plane in Anatomical Liver Resection: A Three-Zone Framework Beyond Hepatic Vein Exposure In other words, the zone fed by a given portal vein branch does not always line up neatly with the zone drained by the adjacent hepatic vein.
This mismatch is especially pronounced in segments 5 and 8, the right anterior section. Newer computational methods that map both portal inflow and hepatic vein outflow simultaneously have shown better agreement with a patient’s actual vascular anatomy in these tricky segments.8PubMed Central. Patient-specific functional liver segments based on centerline classification of the hepatic and portal veins The practical takeaway is that the Couinaud model is an excellent general framework, but real boundaries are patient-specific and sometimes irregular.
Why Surgeons Think in Segments
The whole point of the segmental system is to make liver surgery safer and more precise. When a tumor sits within segment 6, a surgeon can remove that segment alone, preserving the blood supply and bile drainage of the remaining seven segments. This is called an anatomic resection: the line of transection follows the segmental boundary rather than cutting arbitrarily through liver tissue. In theory, anatomic resection should reduce bleeding during surgery (because you are not cutting across major vessels) and might lower the chance of local cancer recurrence (because microscopic tumor deposits that have spread along the portal vein branches within that segment are removed along with it).
Whether anatomic resection actually improves long-term cancer outcomes compared to simply cutting out the tumor with a margin of normal tissue has been debated for years. A European study of patients with hepatocellular carcinoma, including patients with and without cirrhosis, found no significant difference in recurrence-free survival or overall survival between anatomic and non-anatomic resections.9PubMed Central. Anatomic versus non‐anatomic liver resection for hepatocellular carcinoma—A European multicentre cohort study in cirrhotic and non‐cirrhotic patients The evidence on this question remains mixed across the broader literature. In practice, the decision between anatomic and non-anatomic resection depends on tumor location, liver function, how much healthy liver needs to be preserved, and surgeon preference. The segment map is indispensable for planning either approach.
Segments in Living-Donor and Split-Liver Transplantation
Liver transplantation has pushed the clinical importance of segment anatomy further than perhaps any other field. In living-donor transplantation, a healthy person donates a portion of their liver to a recipient. The graft must include enough segments to sustain the recipient’s metabolic needs, while leaving enough behind for the donor to recover safely. The most common adult-to-adult donation uses the right lobe (segments 5 through 8), while pediatric recipients often receive the left lateral section (segments 2 and 3).
Smaller grafts are sometimes possible. In very small infants, even a single segment can be sufficient. A study of children who underwent living-donor liver transplant found that a modified segment 2 graft could work for the smallest recipients, with the graft volume adjusting over time toward the ideal size for the child’s body weight.10PubMed. Liver graft volumetric changes after living donor liver transplantation with segment 2 graft for small infants On the other end of the spectrum, when a standard right or left lobe graft is not suitable because of donor anatomy, the right posterior sector (segments 6 and 7) has been used as an alternative graft. Research has suggested that roughly 7% of potential donors who were initially deemed unsuitable could become candidates through this approach.11American College of Surgeons. Living Donor Right Posterior Sector Liver Transplant Guided by Right Hepatic Artery: First Report in the United States
One anatomic pitfall in split-liver transplantation involves the bile duct of the left lateral section. In about half of specimens studied, the duct for the left lateral section originates not within segments 2 and 3 but crosses over into the territory of segment 4. This means that splitting the liver along the standard plane can inadvertently leave separate small bile ducts from segments 2 and 3 that each need to be reconnected, rather than a single duct.12PubMed Central. Surgical Anatomy of the Left Lateral Segment as Applied to Living-Donor and Split-Liver Transplantation: A Clinicopathologic Study Details like this illustrate why segment anatomy is not just a teaching exercise but a matter of surgical safety.
Segments That Shrink and Segments That Grow
The liver is famously regenerative, but that regeneration does not happen evenly across all eight segments. When blood flow through the portal vein to a particular segment is blocked, that segment shrinks (atrophies), and the remaining segments enlarge (hypertrophy) to compensate. This atrophy-hypertrophy complex is one of the liver’s most striking adaptive behaviors, and it plays out segment by segment.
Animal experiments have shown that portal vein obstruction, not bile duct obstruction, is the main driver of this phenomenon. When researchers selectively ligated either the portal vein or the bile duct to portions of rat livers, a significant atrophy-hypertrophy response developed only after portal ligation.13PubMed. Experimental atrophy/hypertrophy complex (AHC) of the liver: portal vein, but not bile duct obstruction, is the main driving force for the development of AHC in the rat In humans, the most pronounced atrophy usually occurs when both portal vein and biliary obstruction are present together on the same side.14PubMed. Hepatic lobar atrophy: association with ipsilateral portal vein obstruction
A clinical example of this is cavernous transformation of the portal vein, a condition in which a blood clot in the portal vein is replaced over time by a tangle of small collateral vessels. A CT study of patients with this condition found that over 90% showed the atrophy-hypertrophy complex. The left lateral section (segments 2 and 3) atrophied in about three-quarters of patients, while the caudate lobe hypertrophied in 86%.15PubMed. Atrophy-hypertrophy complex in patients with cavernous transformation of the portal vein: CT evaluation Segment 4 hypertrophied in half of them. These patterns matter because a shrunken segment is functionally compromised, while a hypertrophied one may be handling more than its usual share of the liver’s workload.
Portal Vein Embolization and Preoperative Planning
Surgeons have learned to exploit the atrophy-hypertrophy response deliberately. When a patient needs a major liver resection but the expected future liver remnant is too small to sustain life, the portal vein branches feeding the segments that will be removed can be intentionally blocked in a procedure called portal vein embolization. This redirects portal blood to the segments that will remain, causing them to grow larger over a period of weeks before the actual surgery.16PubMed Central. Portal vein embolization: rationale, technique, and current application
The decision about which portal vein branches to embolize depends entirely on the segmental anatomy. If a patient needs segments 5 through 8 removed, the right portal vein branches to those segments are embolized while leaving the branches to segments 1 through 4 open. The remaining segments then hypertrophy. This technique has expanded the number of patients who can safely undergo major hepatic surgery, including patients with large tumors who would previously have been considered inoperable because too little healthy liver would remain.
3D Reconstruction and the Future of Segment Mapping
Traditional imaging interprets the liver’s three-dimensional anatomy from flat, two-dimensional CT or MRI slices. This requires significant mental gymnastics, and even experienced surgeons can misjudge segment boundaries or miss vascular variants on conventional scans. 3D reconstruction software addresses this by building a virtual model of the patient’s liver from the same imaging data, showing the segments, their vessels, and any tumors as a rotatable, color-coded object.
A systematic review of the use of 3D reconstruction in liver surgery found that preoperative planning was performed with 3D models in about 87% of the studies analyzed. Over half of the studies reported improved navigation and accuracy during the actual resection, and nearly a fifth reported more accurate measurement of the future liver remnant volume. 3D reconstruction was also found useful in living-donor transplantation for graft selection and vascular preservation.17PubMed. 3-D reconstruction in liver surgery: a systematic review
Immersive virtual reality is the next step. Rather than viewing a 3D model on a flat screen, surgeons can explore the liver in a VR headset, walking around the virtual organ and examining vascular relationships from any angle. Current VR applications still have limitations, such as the challenge of performing accurate volume calculations within the VR environment itself, but the technology is advancing quickly.18PubMed Central. Using virtual 3D-models in surgical planning: workflow of an immersive virtual reality application in liver surgery
A Low-Tech Trick for Learning Segment Anatomy
For all the high-tech imaging available, one of the most enduring teaching tools for liver segment anatomy is a closed fist. A method described in surgical education literature uses the right hand as a model: make a fist with the thumb tucked behind the other fingers, then face the fist toward yourself. The four visible fingers represent segments 2 through 5 (left to right from the viewer’s perspective), the thumb behind them represents the caudate lobe (segment 1), and the knuckles above the proximal finger joints represent segments 6 through 8. The line of the proximal interphalangeal joint mimics the horizontal plane of the portal vein, dividing upper from lower segments, and the spaces between fingers represent the planes where the hepatic veins run.19PubMed Central. A Handy Tool to Teach Segmental Liver Anatomy to Surgical Trainees
When the fist is viewed from the palm side, the fingers representing the right lobe sit lower than those representing the left, mimicking the way the right lobe sits more posteriorly in the body. The mnemonic has been used across generations of surgical trainees and is especially useful in the operating room, where pulling up a 3D model is not always practical. It does not replace imaging, but it gives trainees a spatial framework they can carry with them at all times.
Pig Livers and Human Segments
Researchers studying liver transplantation, surgical techniques, and new devices often need an animal model, and pigs are the most common choice. Despite the obvious differences in external shape, a comparative anatomy study concluded that pig livers and human livers share the same number of segments and have similar vascular and biliary systems. This similarity is what makes the pig liver useful for experimental surgery and xenotransplantation research.20PubMed Central. Comparative Anatomical Study Between the Human and Swine Liver and Its Importance in Xenotransplantation
The relevance extends beyond the lab. As xenotransplantation research moves toward clinical application, understanding how well the pig’s segmental anatomy maps onto the human system becomes increasingly important. If a pig liver graft were to be used in a human recipient, surgeons would need to know which pig segments correspond to which human segments, where the vascular pedicles align, and where they diverge. The segmental framework that Couinaud built for the human liver turns out to be the shared anatomical language for this cross-species work.