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The Liver - Part 1: anatomy.

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VOL: 101, ISSUE: 48, PAGE NO: 22

Helen L. Day, MSc, BSc, RGN, RSCN, DipN, is paediatric critical care clinical educator and outreach facilitator

Rachel M. Taylor, MSc, RGN, RSCN, DipRes, is nurse researcher, Paediatric Liver Centre; both at King's College Hospital, London

The liver performs vital tasks associated with synthesising proteins, clearing metabolic and toxic waste and assisting with immune function, and so liver failure has wide implications. For example, the liver is one of the first organs exposed to bacteria in the bloodstream, so if liver function is compromised the patient is at risk of sepsis, multi-organ dysfunction syndrome and death.

The liver performs vital tasks associated with synthesising proteins, clearing metabolic and toxic waste and assisting with immune function, and so liver failure has wide implications. For example, the liver is one of the first organs exposed to bacteria in the bloodstream, so if liver function is compromised the patient is at risk of sepsis, multi-organ dysfunction syndrome and death.

To deliver effective, evidence-based care to a patient with a liver disorder, it is of paramount importance that the practitioner has an understanding of normal anatomy and physiology, understands the investigations of liver function and is knowledgeable about acute and chronic liver disease.

This series of six articles will encompass these areas, including an examination of liver transplantation, which is the ultimate treatment for many cases of liver failure.

The anatomy
The liver is the largest and heaviest solid organ, weighing 1.2-1.5kg, and accounts for one-eighteenth of an infant's total body weight and one-fiftieth of an adult's. It is located under the diaphragm, partly protected by the ribs, occupies most of the right hypochondrium and extends across the epigastric region (Finkelstein, 1998).

The liver consists of two principal lobes, the large, right lobe and smaller left lobe. The lobes are separated anteriorly (Fig 1) by the falciform ligament and posteriorly (Fig 4) by the position and entry of the major blood vessels. Associated with the right lobe are the inferior quadrate lobe and the posterior caudate lobe.

The surrounding organs determine the shape of the liver - it retains the shape of a blunt wedge with its rounded base to the right. The liver has two surfaces: diaphragmatic and visceral. The gross anatomy does not correspond with the distribution of the blood supply to the liver or bile drainage. Therefore, for functional reasons and of particular relevance to hepatobiliary and liver transplant surgeons, different sectors of the liver are described that correlate well with blood supply and biliary drainage (Fig 2) (Sinnatamby, 2001).

The most obvious feature of the right lobe is the gallbladder. It is anterior to the descending and transverse colon. The gallbladder stores and concentrates bile. This leaves the liver via the right and left hepatic ducts and enters the common hepatic duct, which is joined at an acute angle by the cystic duct to form the common bile duct.

The hormone cholecystikinin controls the emptying of bile into the duodenum via the sphincter of Oddi. Pancreatic enzymes also enter the duodenum here through the ampulla of Vater (Sherlock and Dooley, 1997).

Blood supply
The liver receives approximately one-quarter of the total cardiac output. It is unusual because it has two blood supplies. Oxygenated blood is supplied via the hepatic artery, which arises from the superior mesenteric artery or aorta. However, it is interesting to note that the hepatic artery supplies only 25 per cent of the total blood supply to the liver. It divides at the transverse fissure (porta hepatis) into the left and right branches supplying the corresponding lobes.

There is no communication between the two sides of the healthy liver and the arteries are end arteries. In the presence of disease, anastomosis with phrenic vessels can occur to provide a collateral circulation.

Deoxygenated blood is supplied by the hepatic portal vein. It enters via the underside of the organ and contributes 75 per cent of its blood supply. It provides blood from the gastrointestinal tract that is rich in newly absorbed nutrients and waste products ready for absorption. Blood flows through large, endothelium-lined spaces called sinusoids, then drains into the central vein.

Finally it passes into the right, middle and left hepatic veins and into the inferior vena cava near the upper surface of the liver, just below the central tendon of the diaphragm. Several accessory hepatic veins commonly enter the inferior vena cava below this (Arias et al, 2001; Portmann, 2000).

Histology
The right and left lobes of the liver are made up of basic histological units called lobules that are hexagonal units comprising liver cells (hepatocytes) and sinusoids. These are grouped around a central vein, which opens into a branch of the hepatic veins. The lobule is surrounded by a capillary plexus in which branches of the hepatic artery run parallel with branches of the hepatic portal vein (interlobular veins) and small bile ducts - the 'peripheral portal triad' (Fig 3) (Ross et al, 2003). Blood flows from the interlobular veins through the sinusoids, which are lined with Kupffer cells (phagocytic cells that play a major role in the immune system) and fat storage cells. In the sinusoids, oxygen, nutrients and toxins are taken up from the blood by the hepatocytes while the products of hepatocyte function are excreted into the blood (Arias et al, 2001).

Nerve supply
The sympathetic nerve supply is provided by way of the coeliac ganglia. Nerves run with the vessels in the free edge of the lesser omentum and enter the porta hepatis. The vagal supply arises from the left vagal trunk, which reaches the porta hepatis along the lesser curvature of the stomach via the lesser omentum (Portmann, 2000).

Lymph drainage
Lymph leaves the liver from the hepatic nodes (in the porta hepatis) and the lymphatics of the gallbladder, draining downwards alongside the hepatic artery into the retropyloric and coeliac nodes. The bare area of the liver communicates with extraperitoneal lymphatics, which perforate the diaphragm and drain to nodes in the posterior mediastinum (Portmann, 2000; Sherlock and Dooley, 1997).

Summary
Understanding the anatomy of the hepatobiliary system provides the basis for the holistic care of patients with a liver disorder. In the second part of this series of articles, the functions of the liver will be examined.

This article has been double-blind peer-reviewed.

For related articles on this subject and links to relevant websites see www.nursingtimes.net

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