#Anatomy

#Describe the functional anatomy of the liver.

#Macroscopic anatomy

• Adult liver weighs approximately 1,500 g
• Divided anatomically into 4 lobes and functionally into 8 segments based on branching of the portal vein, hepatic vein, and hepatic artery
• Largely intraperitoneal
• Borders
  • Superior: right hemidiaphragm
  • Anterior and lateral: chest wall
  • Posterior: IVC, back musculature
  • Inferior: gallbladder, stomach, pancreas, duodenum, transverse colon

#Blood supply

Explored in detail here.

#Biliary circulation

• Bile is produced by hepatocytes and secreted into bile canaliculi
• Drainage pathway
  • → peripheral interlobular bile ducts
  • → right and left hepatic ducts
  • → common hepatic duct
  • → common bile duct (CBD)
  • → duodenum via the ampulla of Vater

#Innervation

• Sympathetic innervation: via the coeliac plexus
• Parasympathetic innervation: via the anterior and posterior vagal trunks
• Both contribute to the hepatic plexus

#Microscopic anatomy

Liver parenchyma is organised into hexagonal lobules measuring 1–2 mm.

Each lobule contains:

• A central hepatic vein
• Peripheral portal tracts (portal triads):
  • Portal venule
  • Hepatic arteriole
  • Bile ductule

Blood flows from portal tracts to the central vein via hepatic sinusoids.

Lobules are functionally divided into zones based on oxygen delivery:

• Zone 1 (periportal)

  • Closest to portal triad
  • Highest oxygen tension
  • Predominantly high-energy metabolic processes occur here (e.g. gluconeogenesis, β-oxidation)

• Zone 2

  • Intermediate zone
  • A mix of high and low energy metabolic processes occur here

• Zone 3 (centrilobular)

  • Closest to central vein
  • Lowest oxygen tension due to consumption of oxygen in zones 1 and 2
  • Predominantly low-energy processes occur here (e.g. glycolysis, xenobiotic metabolism)

#Hepatic sinusoids

Composed of:

• Endothelium

  • Sinusoidal endothelial cells
  • Highly permeable and fenestrated

• Hepatocytes

  • The main functional cells of the liver, responsible for most functions of the liver
  • Have abundant microvilli projecting into the space of Disse

• Space of Disse

  • Perisinusoidal space between hepatocytes and endothelium
  • Contains hepatocyte microvilli and stellate cells

• Biliary epithelium

  • Composed of cholangiocytes
  • Cuboidal epithelium in small ducts
  • Columnar epithelium in larger ducts

• Supporting cells

  • Kupffer cells
    • Specialised liver macrophages
    • Responsible for phagocytosis of bacteria and toxins
      • An important defence against bacterial translocation from the gut into the portal system
    • Also reclaim iron from haem
  • Hepatic stellate cells (pericytes)
    • Regulate of sinusoidal blood flow, similar to how renal tubular pericytes regulate blood flow in the vasa recta
    • Storage functions (e.g. vitamin A)