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 in the right upper abdomen and midabdomen and extends to the left upper abdomen. The liver has the general shape of a prism or wedge, with its base to the right and its 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.  Confusion surrounds the nomenclature of liver anatomy. The International Hepto-Pancreato-Biliary Association (IHPBA) terminology of liver anatomy and resections is followed by most liver surgeons.
Embryologically, the liver grows as a ventral diverticulum from the junction of foregut and the 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 portal vein; the ligamentum venosum is the obliterated ductus venosus, which joins the left portal vein to left hepatic vein.
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 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 process 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 portal vein behind, enclosed in the hepatoduodenal ligament (HDL), composed of 2 layers of lesser omentum.
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.
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 portal vein (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.
Each lobe is divided into 2 sectors. The right hepatic vein (RHV) divides the right lobe into anterior and posterior sectors; the left hepatic vein (LHV) divides the left lobe into medial (quadrate) and lateral sectors. While the falciform ligament and 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 a 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); 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 similarly.
NOTE Caudate 'lobe' is not a lobe but a segment (I); left lateral 'segment' is not a segment but a sector including two segments (II and III).
On computed tomography (CT), the portal vein branches (with the left being higher than right) divide the right and left lobes of the liver into superior and inferior halves. The superior half of 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).
The falciform ligament (which divides the liver into a larger anatomical right lobe and a smaller anatomical left lobe) has 2 layers of peritoneum; it attaches the anterosuperior surface of 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 2 leaves of the coronary ligament to the right of the IVC is the bare area of the liver, which is in contact with the inferior vena cava 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 hepatogastric and hepatoduodenal ligaments.
Inferior vena cava ligament is a bridge of tissue between posterior surface of right lobe and caudate lobe behind the inferior vena cava.
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).
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 proximal body of the pancreas. After giving off the gastroduodenal artery (GDA) 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 bile duct and in front of the portal vein. In the hepatic hilum, it divides in a Y-shaped manner into the RHA and the LHA, with the RHA ascending behind the CHD; the cystic artery is usually a branch of the RHA.
The portal vein, formed by union of the superior mesenteric vein (SMV) and the splenic vein behind the neck of the pancreas, collects blood from the gastrointestinal (GI) tract (SMV and inferior mesenteric vein [IMV]) and from the spleen and pancreas (splenic vein). It then ascends in 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 sleevelike sheaths.
The left portal vein connects to the umbilical vein through the ligamentum teres hepatis and to the left hepatic vein through the ligamentum venosum. The portal venous system (2 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 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.
Sinusoids are large-diameter capillaries lined by endothelial cells between rows of plates or cords of hepatocytes. Sinusoids also contain Kupffer cells of the reticuloendothelial system (RES). Each hexagonal lobule has a central portal tract with branches of the hepatic artery, the portal vein, and bile ducts, as well as a peripheral tributary of the hepatic vein. Bile canaliculi between hepatocytes 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.
Natural variants in liver anatomy are as follows (These variants 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.):
Anomalous right hepatic artery (RHA) from superior mesenteric artery (SMA)
Anomalous left hepatic artery (LHA) from left gastric artery (LGA)
Aberrant right posterior sectoral duct joining the left hepatic duct (can be damaged during left hepatectomy)
Aberrant right segmental, sectoral or even main hepatic duct joining the common hepatic duct below the biliary ductal confluence in the Calot triangle (can be injured during cholecystectomy)
In cirrhosis, the superoinferior span (between the upper percussible border and the lower palpable border) of the liver, which is normally 12-16 cm, is reduced. Caudate lobe hypertrophies can occur in cirrhosis.
Lobar, sectoral, and segmental liver resection (ie, lobectomy, sectorectomy, and segmentectomy) can be performed (eg, 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 (eg, hepatic artery, portal vein, and hepatic vein) and bile ducts and is a very useful tool for liver resections.
Liver cancer (hepatocellular carcinoma) drains into hepatic lymph nodes at the porta hepatis and into the lymph nodes in the hepatoduodenal ligament.
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.