Liver Anatomy

The liver is the second largest (after the skin) organ in the human body and the largest gland (weighing an average of 1500 g). It lies under the diaphragm, occupying most of the right hypochondriac and epigastric regions and frequently extending into the left hypochondriac region as far as the left anterior axillary line. ^[2] The liver has the general shape of a prism or wedge, with its base to the right and apex to the left (see the image below). It is pinkish brown in color, with a soft consistency, and is highly vascular and easily friable. ^[1]  The liver has a yellowish tinge and a more rounded appearance of its edges due to fat accumulation (steatosis). It plays a central role in various important functions required for homeostasis, nutrition, and immune defense. ^[2] Confusion surrounds the nomenclature of liver anatomy. Most liver surgeons follow the International Hepato-Pancreato-Biliary Association terminology for liver anatomy and resections.

Liver and gallbladder, anterior view.

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Embryologically, the liver grows as a ventral diverticulum from the junction of the foregut and midgut into the ventral mesogastrium (the caudal part of the septum transversum; the cranial part forms the diaphragm). The same diverticulum forms the gallbladder and bile ducts as well. The ligamentum teres hepatis is the obliterated umbilical vein, which joins the left hepatic portal vein (HPV); the ligamentum venosum is the obliterated ductus venosus, which joins the left portal vein to left hepatic vein (LHV).

The upper surface of the liver is percussed at the level of the fifth intercostal space. Superior, anterior, posterior, and right surfaces of the liver are continuous with each other and are related to the diaphragm and anterior abdominal wall. The superior surface of the liver is the largest and lies right under the respiratory diaphragm. A layer of peritoneum separates the superior surface from the diaphragm, except for a small triangular area where the two layers of the falciform ligament diverge. ^[2]

The anterior surface is separated from the inferior (visceral) surface by a sharp anterior (inferior) border that is clinically palpable on deep inspiration. The inferior surface is related to the hepatic flexure (the area where the vertical ascending (right) colon takes a right-angle turn to become the horizontal transverse colon), right kidney, transverse colon, duodenum, and stomach. The gallbladder straddles the undersurfaces of liver segments IVB and V.

There is an H-shaped fissure on the inferior surface of the liver. The right vertical arm of the H is formed by the gallbladder anteriorly and the inferior vena cava (IVC) posteriorly; it is incomplete, with the caudate lobe between the two. The left vertical arm of the H is formed by the ligamentum teres hepatis in front and the ligamentum venosum behind.

The transverse limb of the H is the porta hepatis (hilum), a 5-cm transverse fissure (slit) on the undersurface of the liver with the quadrate lobe in front and the caudate lobe behind. It contains the common hepatic duct (CHD) in front and to the right, the proper hepatic artery in front and to the left, and the HPV behind, enclosed in the hepatoduodenal ligament (HDL), composed of two layers of lesser omentum.

Most of the posterior surface of the liver is attached to the diaphragm by loose connective tissue, forming the triangular "bare area." The IVC lies in a groove in the medial end of the bare area. ^[3]

Anatomic Divisions

Anatomically, the liver is divided into a larger right lobe and a smaller left lobe by the falciform ligament (see the image below). This division however is of no use surgically.

Liver and gallbladder, anterior view.

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From a surgical point of view, the liver is divided into right and left lobes of almost equal (60:40) size by a major fissure (Cantlie's line) running from the gallbladder fossa in front to the IVC fossa behind. This division is based on the right and left branches of the hepatic artery and the HPV (see the image below), with tributaries of bile (hepatic) ducts following. The middle hepatic vein (MHV) lies in Cantlie's line. The left pedicle (left hepatic artery [LHA], left branch of the portal vein, and left hepatic duct) has a longer extrahepatic course than the right.

Arteries and veins of liver, anterior view.

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Each lobe is divided into two sectors. The right hepatic vein (RHV) divides the right lobe into anterior and posterior sectors; the LHV divides the left lobe into medial (quadrate) and lateral sectors. While the falciform ligament and the umbilical fissure mark the division between left lateral and left medial sectors on the surface of the liver, no surface marking is observed between right anterior and right posterior sectors. The posterior sector of the right lobe and the caudate lobe are not seen on the frontal view of the liver; the anterior sector of the right lobe forms the right lateral border in this view.

The sectors are further divided into segments (after Couinaud) based on the distribution of portal venous branches in the parenchyma. ^[2] Each segment has its own blood supply and biliary drainage. The anterior sector of the right lobe contains superior (VIII) and inferior (V) segments. The posterior sector of the right lobe has superior (VII) and inferior (VI) segments. The medial sector of the left lobe (quadrate lobe, segment IV) is part of the left lobe from a surgical perspective but lies to the right of the midline; it is further divided into a superior subsegment (A) and an inferior subsegment (B) (note: Japanese surgeons call the superior subsegment B and inferior subsegment A). The lateral sector of the left lobe contains segments II and III.

The caudate lobe (segment I) lies in the lesser sac on the inferior surface of the liver between the IVC on the right, the ligamentum venosum on the left, and the porta hepatis in front (see the image below). The caudate lobe has three parts: a left spigelian lobe, a paracaval part, and a caudate process that connects the caudate lobe to the right lobe. The caudate lobe receives numerous small branches from the right hepatic artery (RHA), the LHA, the portal vein, and the confluence; bile ducts drain in a similar manner. 

Note: Caudate "lobe" is not a lobe but a segment (I); the left lateral "segment" is not a segment but a sector including two segments (II and III).

Liver and gallbladder, posterior view.

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On computed tomography (CT), the HPV branches (with the left being higher than the right) divide the right and left lobes of the liver into superior and inferior halves. The superior half of the liver is composed of (from right to left) segments VII, VIII, IVA, and II; the inferior half is composed of (from right to left) segments VI, V, IVB, and III.

Accordingly, the right portal vein divides the posterior sector of the right lobe into segments VII (superior) and VI (inferior) and the anterior sector of the right lobe into segments VIII (superior) and V (inferior). The left portal vein divides the medial sector of the left lobe (quadrate lobe) into subsegments A (superior) and B (inferior) and the lateral sector of the left lobe into segments II (superior) and III (inferior).

Anatomic divisions are further refined based on emerging research and clinical findings. According to the Committee of the International Hepato-Pancreato-Biliary Association, the first-order division is the right and left hemi-liver. The right hemi-liver consists of segments V-VIII and the left consists of segments II-IV. The second-order division further classifies the two hemi-livers into sections. ^[4]

• Right hemi-liver -

  • Anterior section (segments V and VIII)
  • Posterior section (segments VI and VII).
• Left hemi-liver -

  • Medial section (segments IVa and IVb)
  • Lateral section (segments II and III)

Takasaki's Glissonean pedicle approach (Glissonean pedicle approach by extrafascial approach) involves isolating Glissonean pedicles extrahepatically using Laennec's capsule. This method divides the liver into four functional segments, each supplied by secondary portal branches. It enables precise segmental resections with minimal parenchymal damage, making it particularly valuable in minimally invasive liver surgery. ^[5]

The hepatic lobule is recognized as the functional unit of the liver, with distinct zones. Zone I is near the portal triad and is highly oxygenated and regenerative. Zone II is the intermediate zone between the portal triad and the central vein. Zone III is near the central vein and is hypoxic but metabolically active. This zonal distinction is critical for understanding liver pathology and regeneration. ^[5]

Ligaments

The falciform ligament (which divides the liver into a larger anatomical right lobe and a smaller anatomical left lobe) has two layers of peritoneum; it attaches the anterosuperior surface of the liver to the anterior abdominal wall and diaphragm. The free edge of the falciform ligament contains the ligamentum teres hepatis (round ligament of the liver): the obliterated umbilical vein, which is attached to the inferior surface of the liver between segment IV on the right and segment III on the left. The ligamentum venosum (the obliterated ductus venosus) is attached to the inferior surface of the liver between the caudate lobe and the left lateral sector.

The superoposterior surface of the liver has coronary and left triangular ligaments; between the two leaves of the coronary ligament to the right of the IVC is the bare area of the liver, which is in contact with the IVC and inferior surface of the diaphragm. The falciform ligament is continuous with the anterior layer of the coronary ligament. On the left, the anterior and posterior layers of the coronary ligament unite to form the left triangular ligament. On the right, the anterior and posterior layers of the coronary ligament unite to form the right triangular ligament.

The posterior layer of the coronary ligament on the right side is called the hepatorenal ligament. The hepatorenal pouch is the area below the posterior layer of the right triangular and coronary ligament over the right kidney. The lesser omentum connects the liver with the lesser curvature of the stomach and the first part of the duodenum via the hepatogastric ligament and HDL.

The IVC ligament is a bridge of tissue between the posterior surface of the right lobe and the caudate lobe behind the IVC.

Blood Supply

The liver has a unique dual blood supply (about 1500 mL/min) both from the proper hepatic artery (20-40%) and from the portal vein (60-80%; see the image below).

Arteries and veins of liver, anterior view.

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The celiac trunk (axis) comes off the anterior surface of the abdominal aorta at the level of T12-L1 between the right and left crura of the diaphragm. It is a short structure (about 1 cm) that trifurcates into the common hepatic artery (CHA), the splenic artery, and the left gastric artery (LGA).

The CHA runs toward the right on the superior border of the body of the pancreas. After giving off the gastroduodenal artery behind the first part of the duodenum above the neck of the pancreas, it continues as the proper hepatic artery in the HDL (the free edge of the lesser omentum) to the left of the common bile duct and in front of the HPV. In the hepatic hilum, it divides in a Y-shaped manner into the RHA and LHA, with the RHA ascending behind the CHD; the cystic artery is usually a branch of the RHA.

The HPV, formed by union of the superior mesenteric vein (SMV) and the splenic vein behind the neck of the pancreas, collects blood from the gastrointestinal tract (SMV and inferior mesenteric vein) and from the spleen and pancreas (splenic vein). It then ascends within the HDL behind the CBD and the proper hepatic artery and divides in a T-shaped manner into right and left portal vein branches in the hepatic hilum. The right portal vein divides within the liver parenchyma into a vertical right anterior sectoral branch (which then divides into segmental V and VIII branches) and a horizontal right posterior sectoral branch (which then divides into segmental VI and VII branches). The left portal vein runs below the base of segment IV to which it gives off several small branches; it then enters the liver parenchyma where it divides into branches to segments IV, III, and II.

The hilar plate is a condensation of fibroareolar tissue that lies on the undersurface of the hilum of liver, separating it from the biliovascular pedicle at the porta hepatis; it continues along the right and left portal pedicles as sleeve-like sheaths.

The left portal vein connects to the umbilical vein through the ligamentum teres hepatis and to the LHV through the ligamentum venosum. The portal venous system (two groups of capillaries, one in the organ being drained and the other in the liver) has no valves.

Portosystemic connections are present in the gastroesophageal area (between the esophageal tributary of the left gastric vein and the esophageal tributaries of the azygos vein), in the rectum (between the superior, middle, and inferior rectal veins), around the umbilicus (between the left portal, umbilical, and paraumbilical veins and the superficial and deep epigastric veins), and in the retroperitoneum (between the colic and splenic veins and renal and posterior parietal veins).

The three hepatic veins (RHV, MHV, and LHV) are largely intrahepatic and lie on the posterior surface of the liver. The MHV and the LHV may join to form a common trunk before draining into the IVC. The IVC lies on the posterior surface of the liver in a groove (or sometimes a tunnel) between the bare area on the right, the caudate lobe on the left, and the caudate process in front.

The hepatic arterial buffer response plays a key role in maintaining total hepatic blood flow. This unique mechanism involves the ability of the hepatic artery to cause compensatory blood flow changes in response to changes in portal venous flow. Thus, when portal venous flow decreases, the hepatic artery compensates by increasing its flow, and when portal venous flow increases, the hepatic artery constricts. This mechanism ensures stable perfusion despite fluctuations in splanchnic circulation or systemic hemodynamics. ^[6]

CT Anatomy

On contrast-enhanced CT scanning, the RHV (horizontal) lies in the right portal fissure and separates the right posterior sector (segments VI and VII) from the right anterior sector (segments V and VIII) ^[7] ; the MHV (vertical), located in the main portal fissure (Cantlie's line), ^[7] divides the right anterior sector and segment IV; the LHV lies between the left medial sector and the left lateral sector. The HPV bifurcation creates a horizontal plane that divides the liver into cranial and caudal segments. The right portal vein bifurcates into anterior and posterior branches, supplying the right anterior and posterior sectors, respectively. Similarly, the left portal vein bifurcates into transverse and umbilical portions, supplying segments II-IV. Bifurcation of the portal vein separates the cranial segments (VII, VIII, IVa, II) from the caudal segments (VI, V, IVb, III). ^[7]

The surface of the liver is covered by visceral peritoneum (serosa), with a Glisson capsule underneath. At the porta hepatis, the Glisson capsule travels along the portal tracts (triads), carrying branches of the hepatic artery, the portal vein, and the bile ducts into the liver substance.

Microscopically, the liver is organized into hexagonal lobules, each centered around a central vein. ^[3] Each hexagonal lobule has a central portal tract with branches of the hepatic artery, the HPV, and bile ducts, as well as a peripheral tributary of the hepatic vein. Sinusoids are large-diameter capillaries lined by fenestrated endothelial cells (lack basement membrane) between rows of plates or cords of hepatocytes. These fenestrations facilitate exchange between blood and hepatocytes through the space of Disse. ^[3] The space of Disse, located between hepatocytes and sinusoidal endothelial cells, contains stellate cells (Ito cells) that store vitamin A and play a role in fibrosis during liver injury. This space also facilitates nutrient and waste exchange between hepatocytes and blood plasma. ^{[3, 8]} Sinusoids also contain Kupffer cells of the reticuloendothelial system. Kupffer cells are resident macrophages that line the sinusoids and play an important role in immune surveillance and pathogen clearance. ^[9]

Hepatocytes are arranged in plates or cords that radiate outward from the central vein. Between adjacent hepatocytes are bile canaliculi, small channels formed by tight junctions that collect bile produced by hepatocytes. ^[10] Bile canaliculi drain into bile ductules in the portal triad. Bile ductules then form several orders of intrahepatic bile ducts, in an arrangement resembling the twigs and branches of a tree.

Blood from the hepatic artery and HPV flows through the sinusoids, facilitating the exchange of substances between the blood and hepatocytes. This blood is then collected by the central vein of each lobule, which drains into the hepatic veins, ultimately leading to the IVC. ^[3]

Natural variants in liver anatomy can be replaced, in which case no normal artery is present, or they can be accessory, in which case an anomalous artery is present in addition to a normal artery. ^[11]  They are as follows:

• Anomalous RHA from superior mesenteric artery
• Anomalous LHA from LGA
• Aberrant right posterior sectoral duct joining the left hepatic duct (can be damaged during left hepatectomy)
• Aberrant right segmental, sectoral, or even the main hepatic duct joining the CHD below the biliary ductal confluence in Calot's triangle (can be injured during cholecystectomy)
• A rare but critical variant where the right posterior sectoral duct drains into the cystic duct rather than the CHD. This anomaly is prone to injury during gallbladder surgery. ^[12]

The left portal pedicle lies at the base of segment IV and has a long extrahepatic course. The right portal pedicle has a short extrahepatic course; it divides into a right anterior sectoral pedicle that lies in the gallbladder fossa and a right posterior sectoral pedicle that lies in the Rouviere's sulcus. 

In patients with cirrhosis, the superoinferior span (between the upper percussive border and the lower palpable border) of the liver, which is normally 12-16 cm, is reduced. Caudate lobe hypertrophies can occur in such patients with cirrhosis.

Lobar, sectoral, and segmental liver resection (e.g., lobectomy, sectorectomy, and segmentectomy, respectively) can be performed (e.g., right hepatic lobectomy [segments V-VIII], left hepatic lobectomy [segments II-IV], right posterior sectorectomy [segments VI, VII]). Liver lobes (right or left) can be removed from a live donor and transplanted to another person. Intraoperative ultrasonography may delineate intrahepatic blood vessels (e.g., hepatic artery, HPV, and hepatic vein) and bile ducts and is a very useful tool for liver resections. The Glissonean pedicle approach allows for systematic isolation of vascular structures without parenchymal destruction, enhancing safety during anatomical resections. ^[13]

The liver has enormous capacity for regeneration; a normal liver can tolerate major liver resections involving up to 70-75% of liver parenchyma. Regeneration is driven by hepatocyte proliferation and can be augmented by techniques such as HPV embolization, which induces contralateral lobe hypertrophy to increase the future liver remnant before major resections. ^[14]

Liver cancer (hepatocellular carcinoma) drains into hepatic lymph nodes at the porta hepatis and into the lymph nodes in the HDL.

A hepatocellular carcinoma is supplied mainly by the hepatic artery. Unresectable tumors can be treated using transarterial embolization, transarterial chemoembolization, and transarterial radioembolization. Ongoing clinical trials and the development of new therapeutic agents, including immunotherapy combinations, are aimed at improving outcomes for patients with hepatocellular carcinoma. ^[15]

The liver has a dual (arterial and portal) blood supply. The hepatic artery can be ligated or embolized; the liver then gets its arterial blood supply from the diaphragm and abdominal wall through its ligaments and the bare area.

Unilateral portal vein embolization results in atrophy of the ipsilateral lobe and hypertrophy of the contralateral lobe. This is useful before major liver resections to increase the functional liver remnant.