Liver

The liver is the largest abdominal organ. It plays a major role in metabolism and has many functions, including glycogen storage, decomposition of red blood cells, plasma protein synthesis, hormone production, and detoxification. It is one of the very few organs that has the ability to regenerate.

Gross anatomy

The liver is an irregular, wedge-shaped organ that lies below the diaphragm in the right upper quadrant of the abdominal cavity and is in close approximation with the diaphragm, stomach and gallbladder. It is largely covered by the costal cartilages ⁹.

The liver is made of several functional units called lobules, which in turn can be subdivided into smaller units called sinusoids. The entire liver is covered by a fibrous capsule known as Glisson's capsule, which forms the innermost liver coverage.

The liver is almost entirely covered by visceral peritoneum and is associated with the peritoneal ligaments, including the falciform ligament anteriorly. The falciform ligament forms the boundary of the traditional anatomical division of right and left liver lobes ⁹.

The ligamentum teres (a remnant of the umbilical vein, also known as the round ligament of liver) starts from the inferior border of the liver at the meeting point with the falciform ligament, travels up into the porta hepatis and attaches to the left portal vein. The ligamentum venosum (a remnant of ductus venosum) continues from the ligamentum teres into the superior part of the porta hepatis ⁹.

The posterocranial aspect of the liver, adjacent to the dorsal body wall, is not completely covered by peritoneum, resulting in the so-called "bare area" (area nuda). 

The liver is described as having two surfaces, diaphragmatic and visceral, sharply demarcated anteriorly by the inferior margin:

• diaphragmatic surface: smooth peritoneal area that faces superiorly and anteriorly and includes the bare area ⁹

• visceral surface: faces inferiorly and posteriorly and is covered by peritoneum ⁶

  • marked by the structures of the porta hepatis ⁹

  • related to the oesophagus, stomach, and lesser omentum on the left, pancreas and duodenum in the midline, right kidney, adrenal, and hepatic flexure of the colon on the right ⁹

  • the surface contains impressions for the adjacent structures, including of the stomach, oesophagus, right suprarenal gland, right kidney, gall bladder, duodenum and hepatic flexure of the colon

The normal liver measures ⁷: 

• craniocaudal length: 10-12.5 cm

• transverse diameter: 20-23 cm

Segmental anatomy

Traditionally, the liver was divided into four anatomical lobes. However, this has been superseded by the use of the Couinaud classification which divides the liver into eight functional units (known as segments), supplied by individual segmental hepatic arteries, portal veins and bile ducts, which can be individually resected ⁹.

The middle hepatic vein (also known as principal plane or Cantlie's line) divides the liver into right and left lobes under Counaud classification ⁹. The line is located at 4 cm to the left of the falciform ligament ⁹.

The right hepatic vein divides the right liver lobe into anterior and posterior segments while the left hepatic vein divides the left liver into medial and lateral segments ⁹.

The portal vein and its branches divides the liver into upper and lower segments ⁹.

Arterial supply

The liver receives a dual blood supply from the portal vein and hepatic arteries. The hepatic portal vein supplies ~75% of the liver's blood supply by volume and carries venous blood drained from the spleen, gastrointestinal tract, and its associated organs (hence oxygen-poor and nutrient-rich).

The hepatic arteries supply arterial blood to the liver and account for the remainder of its blood flow (hence oxygen-rich and nutrient-poor). The hepatic arterial system supplies the biliary system.

Oxygen is provided from both sources; approximately half of the liver's oxygen demand is met by the hepatic portal vein, and half is met by the hepatic arteries.

Venous drainage

Most of the venous drainage from the liver passes into the three hepatic veins which drain into the inferior vena cava. These veins also helps to stabilise the liver. Hepatic veins do not have valves ⁹.

Lymphatic drainage

The majority of the lymph from the liver drains into nodes that lie in the porta hepatis. Drainage channels of these lymph nodes follow the hepatic artery to reach the retropyloric and then the coeliac lymph nodes ⁹.

The superior surface of the liver also has communications with extraperitoneal lymphatics that perforate the diaphragm and drain into mediastinal lymph nodes ⁹.

Innervation

The liver is supplied by sympathetic and parasympathetic autonomic fibres from the hepatic plexus via the coeliac plexus, which travel with branches of the hepatic artery and portal vein to the liver. Within the liver, the nerve fibres accompany the portal triad. Sympathetic fibres are derived from the splanchnic nerves and parasympathetic fibres are derived from the anterior and posterior vagal trunks ⁵. 

Radiographic features

MRI

Signal characteristics

• GRE

  • T1-weighted spoiled gradient echo is useful in investigating fatty liver

  • Magnetisation-prepared T1-weighted GRE (MPRAGE) is useful in minimising movement artifacts¹⁰.

• T2

  • useful to show water content in bile ducts, cysts, and focal lesions.

  • it is also used to show fat at high signal intensity¹⁰

• T1 C+ Gd

  • T1-weighted GRE fat-suppressed volume acquisition or spoiled GRE squences can be used.

  • fat suppression reduces motion artifact, increases dynamic range of the image, and increases signal to noise ratio of focal liver lesions¹⁰

Variant anatomy

• lobar variation

  • Riedel lobe

  • agenesis of the right hepatic lobe

  • agenesis of the left hepatic lobe

  • accessory hepatic lobes / ectopic liver tissue

  • beaver tail liver

  • supradiaphragmatic liver

• pseudolipoma of the Glisson capsule

• accessory hepatic sulci

Related pathology

• gamuts

  • differential diagnosis of hepatic nodule in cirrhotic liver

  • nutmeg liver

  • hypervascular liver lesions

  • hepatic attenuation on CT

• infections

  • acute hepatitis

  • emphysematous hepatitis

  • hepatic abscess

• neoplastic

  • see liver tumours

• traumatic

  • see liver trauma

• vascular

  • intrahepatic arteriovenous shunts

  • Budd-Chiari syndrome