Liver
Structure
The liver lies beneath the diaphragm and conforms to its shape, with a large diaphragmatic surface and a smaller visceral inferior surface. The sharp inferior border separates the diaphragmatic and inferior surfaces anteriorly. The porta hepatis, the entry for arterial and portal blood, lies at the junction of the inferior and diaphrgamatic surface posteriorly.
The diaphragmatic surface is interrupted medially by the falciform ligament, which attaches to the midline of the anterior abdominal wall.
The right posterior part of the liver is not covered by peritoneum and is connected to the diaphragm by loose connective tissue; this is referred to as the 'bare area' of the liver. More medially, the lesser omental sac intervenes. The lesser omentum extends from the liver to the stomach.
Gross Anatomical Structure
The liver was historically divided into right, left, caudate and quadrate lobes.
Right lobe
The right lobe consists of the liver right of the falciform ligament. It is the largest of the gross anatomical lobes. Two prominences exist on the inferior surface, adjacent to the porta hepatis - the caudate and quadrate lobes.
Left lobe
The left lobe lies to the left of the falciform ligament and is smaller than the right, tapering as it progresses left.
Caudate lobe
The caudate lobe is the prominence posterior to the porta hepatis. The inferior vena cava is to the right. It is functionally related to the left lobe but lies on the right side of the falciform ligament.
Quadrate lobe
The quadrate lobe is the prominence anterior to the porta hepatis, and medial to the gallbladder fossa.
Functional Anatomical Structure
The liver is functionally divided into 9 segments, based on drainage by hepatic veins. This is most important for liver surgery.
Segment I corresponds with the caudate lobe, and is located posterior to the porta hepatis. Biliary drainage is via both the right and left hepatic ducts.
Segment II is the leftmost of the segments, divided from segment III by the left portal fissure.
Segment III lies between segments II and III, typically draining via the left hepatic vein.
Segment IV lies right of Segment III, left of the central hepatic vein.
Segment V is inferior and right of segment IV. It drains to central and right hepatic veins.
Segment VI is the most right and inferior of the segments, draining via the right hepatic vein.
Segment VII is the most right and superiro of the segments, draining via the right hepatic vein.
Segment VIII is medial to segment VII and superior to segment V.
Segment IX is a newly described segment, and indicates the part of segment I that is posterior to segment VIII.
Macroscopic Appearance
The liver typically has a shiny appearance due to the coating peritoneum. Beneath this it is typically a dark red.
Microscopic Appearance
The liver is arranged into lobules surrounding tributaries of the hepatic veins. This lobules are classically viewed as hexagons in cross section, with the hepatic tributary lying in the centre. Branches of the portal vein, hepatic artery and biliary ducts travel in the connective tissue septa between each lobe. Blood passes from the portal vein and hepatic artery towards the central vein, whereas bile drains to the biliary ducts in the opposite direction. Hepatocytes are specialised epithelial cells that are usually polygonal in shape.
Relations
The diaphragm and right lung lie posterior, superior, lateral and anteriorly. The heart lies superior to the left lobe of the liver.
The anterior abdominal wall lies anterior and lateral.
The stomach is medial, separated by the lesser omentum and lesser omental sac.
The porta hepatis enters the liver on its postero-inferior surface.
The right kidney is inferior and posterior, as is the right adrenal gland.
The inferior vena cava is intimately related to the posterior surface.
Biliary system
The biliary system includes the right and left hepatic ducts (and their tributaries), the common hepatic duct, the cystic duct, the gallbladder, the common biliary duct and the sphincter of the bile duct.
Structure & Macroscopic Appearance
Gallbladder
The gallbladder is placed in the fossa for the gallbladder, a depression on the visceral surface of the liver. It has three parts:
- The neck, which unites the gallbladder with the cystic duct. It is initially narrow and may have an 'S' shape
- The body, the central part
- The fundus, the wide sac distant from the neck.
Cystic Duct
The cystic duct is about 4 cm long, and passes from the neck of the gallbladder to the common hepatic duct in the lateral border of the lesser omentum. It is accompanied by the vessels and nerves for the gallbladder. The cystic duct is notable for the spiral valve, a set of angular flaps that may prevent the flow of bile.
Common Hepatic Duct
The common hepatic duct is formed by the union of the right and left hepatic ducts, just beyond the porta hepatis in the lesser omentum. It is rapidly joined by the cystic duct to form the bile duct
Bile Duct
The bile duct (previously: common bile duct) runs from the union of the common hepatic duct and cystic duct. It runs inferiorly, posterior to the first part of the duodenum and anterior to the pancreas. It unites with the main pancreatic duct and then passes laterally to empty into the second part of the duodenum. This opening is the hepatopancreatic ampulla, surrounded by the sphincter of the bile duct.
Microscopic Structure
Gallbladder
The gallbladder mucosa is covered with small ridges in a honeycomb pattern. The mucosa is simple columnar with a rich capillary network. There are no goblet cells as the gallbladder has a purely absorbing function.
Ducts
The outer layers of the ducts contain smooth muscle in various arrangements. The mucosa has a simple columnar epithelium with small mucous ducts present in the submucosa.
