The Liver: The Body's Chemical Factory

The liver is the largest internal organ, weighing 1.2–1.5 kg in adults, and is arguably the most metabolically versatile organ in the body. It performs over 500 distinct biochemical functions and is located in the right upper quadrant of the abdomen, tucked beneath the right hemidiaphragm. The liver is unique among abdominal organs in receiving a dual blood supply and in its extraordinary capacity for regeneration. This guide is for educational purposes only.

## Gross Anatomy and Ligaments

The liver is divided into a larger right lobe and smaller left lobe by the falciform ligament on its anterior surface. The falciform ligament contains the round ligament (ligamentum teres hepatis) — the remnant of the umbilical vein — in its free inferior edge. The posterior aspect of the liver features the porta hepatis (hepatic hilum), through which the portal vein and hepatic artery enter and the hepatic ducts exit. The bare area on the posterior superior surface is in direct contact with the diaphragm without peritoneal covering.

Additional ligaments include the coronary ligament (reflections of peritoneum onto the diaphragm, enclosing the bare area), the triangular ligaments (left and right, condensations of the coronary ligament at the lateral margins), and the lesser omentum (hepatoduodenal and hepatogastric ligaments connecting the liver to the duodenum and lesser curvature of the stomach respectively). The hepatoduodenal ligament contains the portal triad: portal vein (posterior), hepatic artery (left), and common bile duct (right) — structures that can be compressed between the thumb and forefinger at the epiploic foramen (Pringle manoeuvre) to temporarily control haemorrhage.

## Hepatic Lobules

The microscopic structural unit of the liver is the classic hepatic lobule: a hexagonal arrangement of hepatocyte plates radiating outward from a central vein, with portal triads (portal venule, hepatic arteriole, bile ductule) at each corner. Blood flows from the portal triads through the sinusoids towards the central vein; oxygen and nutrient concentrations fall as blood traverses from zone 1 (periportal, most oxygenated) to zone 3 (centrilobular, most susceptible to ischaemia and toxins such as paracetamol/acetaminophen).

The sinusoids are lined by specialised fenestrated endothelial cells (allowing passage of large molecules) and Kupffer cells (resident macrophages that phagocytose bacteria, damaged red cells, and foreign particles arriving from the portal blood). Between the endothelial lining and the hepatocytes lies the space of Disse, where hepatocytes exchange substances with plasma.

Hepatic stellate cells (Ito cells) reside in the space of Disse and store vitamin A. When activated by liver injury, they transform into myofibroblasts that deposit collagen — the cellular basis of liver fibrosis and, ultimately, cirrhosis.

## Portal Hepatic System

Approximately 75% of the liver's blood supply comes from the portal vein, which drains the entire gastrointestinal tract (stomach to rectum), spleen, and pancreas. This arrangement allows the liver to process absorbed nutrients, hormones, and potential toxins before they enter the systemic circulation — the first-pass effect that markedly reduces the bioavailability of many orally administered drugs. The remaining 25% of hepatic blood flow comes from the hepatic artery (a branch of the coeliac trunk), which is the primary oxygen supply to the bile ducts and liver parenchyma.

Portal hypertension — elevated pressure in the portal venous system (normal < 10 mmHg; clinically significant > 12 mmHg) — results most commonly from cirrhosis impeding blood flow through the liver. It leads to the development of portosystemic collateral vessels (varices) at the gastro-oesophageal junction, umbilicus (caput medusae), and anorectum, with risk of life-threatening haemorrhage from oesophageal varices.

## Bile Production

The liver produces 500–1,000 mL of bile per day. Bile is composed of bile salts (conjugated bile acids — essential for emulsification and absorption of dietary fats and fat-soluble vitamins), bilirubin (the conjugated product of haem catabolism), cholesterol, phospholipids, and electrolytes. Bile flows through canaliculi between adjacent hepatocytes into bile ductules, then into the intrahepatic bile ducts, and is collected at the porta hepatis in the right and left hepatic ducts, which unite to form the common hepatic duct. The common hepatic duct joins the cystic duct (from the gallbladder) to form the common bile duct, which traverses the head of the pancreas and opens at the ampulla of Vater (hepatopancreatic ampulla) into the second part of the duodenum through the sphincter of Oddi.

Bilirubin metabolism is clinically important: prehepatic jaundice results from excessive haemolysis (unconjugated bilirubin), hepatic jaundice from hepatocellular failure (mixed), and posthepatic (obstructive) jaundice from bile duct blockage — by gallstones, carcinoma of the pancreatic head, or stricture — producing dark urine, pale stools, and pruritus.

## Metabolic Functions

The liver's metabolic roles are vast. It is the primary site of gluconeogenesis (synthesising glucose from amino acids, lactate, and glycerol during fasting) and glycogenesis/glycogenolysis (storing and releasing glycogen). It synthesises the majority of plasma proteins including albumin (oncotic pressure, drug binding), clotting factors (I, II, V, VII, VIII, IX, X, XI — hence coagulopathy in liver failure), and acute phase reactants. Lipid metabolism in the liver includes fatty acid synthesis, beta-oxidation, ketone body production, lipoprotein assembly (VLDL, HDL), and cholesterol synthesis (the target of statin drugs). The liver also detoxifies ammonia (produced by gut bacterial metabolism of amino acids) by converting it to urea via the urea cycle.

## Surgical Segments (Couinaud)

For surgical planning, the liver is divided into eight functionally independent segments (Couinaud classification) based on the distribution of portal and hepatic venous supply. Each segment has its own portal pedicle (portal vein branch, hepatic artery branch, bile duct) and is drained by a hepatic vein. The three hepatic veins (right, middle, and left) divide the liver into four sectors; portal branches further divide these into eight segments. The middle hepatic vein — running in the principal plane from the gallbladder fossa to the inferior vena cava — marks the true anatomical division between the right and left hemilivers, which does not correspond to the external appearance of right and left lobes.

## Liver Pathologies

Non-alcoholic fatty liver disease (NAFLD) — ranging from simple steatosis to non-alcoholic steatohepatitis (NASH), fibrosis, and cirrhosis — is the most common chronic liver disease globally, driven by obesity, insulin resistance, and metabolic syndrome. Viral hepatitis (B and C) remains a major cause of cirrhosis and hepatocellular carcinoma worldwide. Hepatocellular carcinoma most commonly arises in cirrhotic liver, and alpha-fetoprotein is a useful tumour marker. Liver failure manifests as coagulopathy, hypoalbuminaemia, jaundice, encephalopathy, and ascites.