Anatomy and Physiology of the Liver

🫀 Anatomy and Physiology of the Liver (with Drug Metabolism + Paracetamol)

The liver is the body’s central metabolic and detoxification hub. Clinically, many problems map to: hepatocellular injury (ALT/AST), cholestasis (ALP/GGT), or synthetic/metabolic failure (INR, albumin, glucose, ammonia), often alongside portal hypertension as fibrosis advances.

🧬 Introduction

• The liver weighs ~1.2–1.5 kg and performs hundreds of essential functions: metabolism, synthesis, storage, bile production, and detoxification.
• It lies mainly in the right upper quadrant beneath the diaphragm; pain is typically from capsular stretch (Glisson’s capsule), not hepatocytes themselves.
• Segmental anatomy (Couinaud segments) allows targeted resection and transplantation planning.

📍 Gross Anatomy and Couinaud Segments

Surface anatomy describes right and left lobes (falciform ligament). Functionally, the liver has 8 Couinaud segments, each with its own portal inflow, venous outflow, and biliary drainage.

• I: Caudate (independent inflow; drains directly to IVC; can hypertrophy in cirrhosis).
• II–III: Left lateral (superior/inferior).
• IVa–IVb: Left medial (superior/inferior).
• V–VIII: Right lobe (anterior/posterior; inferior/superior).

🔬 Microscopic Anatomy (functional microcirculation)

• Lobule: Hepatocyte plates draining to a central vein.
• Portal triad: Portal venule + hepatic arteriole + bile ductule at the periphery.
• Sinusoids: Fenestrated vessels enabling rapid exchange (nutrients, drugs, toxins).
• Kupffer cells: Macrophages filtering portal blood (bacterial/endotoxin clearance).
• Space of Disse: Exchange interface; lymph formation site.
• Stellate cells: Store vitamin A; activation → collagen deposition → fibrosis/cirrhosis.

Fibrosis begins when stellate cells activate → collagen in the Space of Disse → sinusoidal “capillarisation” → reduced exchange + increased portal resistance (portal hypertension).

💉 Blood Supply and Portal Flow

• Portal vein (~75% flow): nutrient-rich blood from gut/spleen; delivers absorbed drugs/toxins first (first-pass effect).
• Hepatic artery (~25% flow): oxygen-rich blood from coeliac axis.
• Outflow: hepatic veins → IVC.

🟡 Bile Production and Flow

• Bile flows from hepatocytes → canaliculi → ducts → hepatic ducts → common hepatic duct → CBD → duodenum.
• Bile acids emulsify fats and enable absorption of fat-soluble vitamins (A, D, E, K); conjugated bilirubin is excreted via bile.

🗺️ Acinar Zonation (why injury patterns differ)

• Zone 1 (periportal): highest oxygen and nutrients; first exposed to portal blood (including some toxins).
• Zone 2: intermediate.
• Zone 3 (centrilobular): lowest oxygen but highest CYP450 activity → most vulnerable to hypoxic injury and reactive drug metabolites (classically paracetamol).

Zone 3 sits “furthest” from oxygenated inflow and is CYP450-rich — perfect setup for ischaemic hepatitis (shock liver) and toxic-metabolite injury.

⚙️ Liver Physiology — Expanded and Clinically Useful

🍞 Carbohydrate Handling

• Fed state: insulin-driven glycogenesis (glucose → glycogen).
• Fasting: glucagon/adrenaline-driven glycogenolysis (glycogen → glucose).
• Prolonged fasting/illness: gluconeogenesis from lactate (Cori cycle), glycerol, and amino acids.
• Clinical: severe liver failure → impaired gluconeogenesis → hypoglycaemia, especially in sepsis/alcohol misuse.

🧈 Lipid Handling

• Synthesises cholesterol (precursor for steroid hormones + bile acids).
• Packages triglycerides into VLDL for export; handles HDL pathways.
• β-oxidation produces energy; ketogenesis supports fasting physiology.
• Clinical: insulin resistance → hepatic triglyceride accumulation → MASLD/NAFLD.

🥚 Protein and Nitrogen Handling

• Synthesises albumin (oncotic pressure + drug binding) and many transport proteins.
• Synthesises clotting factors (II, VII, IX, X) and regulators (protein C/S) → explains raised INR in liver failure.
• Urea cycle converts ammonia to urea (prevents neurotoxicity).
• Clinical: impaired ammonia clearance → astrocyte swelling + neurotransmitter imbalance → hepatic encephalopathy.

🛡️ Immunology and Inflammation

• Kupffer cells filter portal blood, clearing bacteria/endotoxin (key barrier to gut translocation).
• Liver produces acute phase proteins (e.g. CRP) and complement components.
• Clinical: cirrhosis = immune dysfunction (susceptibility to infections like SBP).

💊 Drug Handling and Hepatic Biotransformation (very high-yield)

Most drugs are either metabolised in the liver, excreted in bile, or both. Hepatic handling depends on: hepatic blood flow, protein binding, intrinsic metabolic capacity, and biliary transport. Cirrhosis reduces functional hepatocyte mass and distorts microcirculation, so drug effects become unpredictable.

🏭 First-pass metabolism (why oral doses behave differently)

• Drugs absorbed from the gut enter the liver via the portal vein before reaching systemic circulation.
• High first-pass drugs can have low oral bioavailability; liver disease may increase bioavailability → higher plasma levels at “usual” doses.
• Examples (conceptual): some opioids, beta-blockers, nitrates, and calcium channel blockers have significant first-pass effects (drug-specific).

🧪 Phase I and Phase II metabolism

• Phase I (CYP450): oxidation/reduction/hydrolysis → may create reactive intermediates (important in toxicity).
• Phase II (conjugation): glucuronidation, sulphation, acetylation, methylation, glutathione conjugation → makes compounds water-soluble and safer for excretion.
• Cholestasis can impair biliary excretion → drug accumulation (especially with biliary elimination).

🔁 What changes in liver disease (practical prescribing physiology)

• ↓ Albumin → ↑ free fraction of highly protein-bound drugs (more effect/toxicity at same total level).
• Portosystemic shunting reduces first-pass clearance → oral doses behave “stronger”.
• ↓ CYP activity (variable) + ↓ hepatocyte mass → slower clearance for many drugs.
• Cholestasis → reduced biliary excretion.
• Ascites/edema → increased volume of distribution for hydrophilic drugs (complex dosing effects).

✅ Rule of thumb: in decompensated cirrhosis, start low, go slow, reassess frequently, and avoid hepatotoxic medicines where possible. Always consider renal function too (hepatorenal physiology).

🚑 Paracetamol (Acetaminophen) — Physiology of Toxicity

Paracetamol is usually safe at therapeutic doses because most is metabolised by glucuronidation and sulphation (Phase II pathways). A small fraction is metabolised via CYP450 (especially CYP2E1) to a reactive metabolite called NAPQI. NAPQI is normally detoxified by glutathione.

• Normal dose: conjugation pathways dominate; minimal NAPQI; glutathione easily neutralises it.
• Overdose: conjugation pathways saturate → more paracetamol diverted to CYP450 → excessive NAPQI → glutathione depletion → NAPQI binds hepatocyte proteins → oxidative injury and cell death.
• Where? Injury is typically centrilobular (Zone 3) due to high CYP activity and low oxygen reserve.

⚠️ Who is at higher risk of toxicity at lower doses?

• Chronic alcohol use (induces CYP2E1; may reduce glutathione stores).
• Malnutrition / fasting (reduced glutathione precursor availability).
• Enzyme-inducing drugs (some antiepileptics; depends on agent).
• Chronic liver disease (risk depends on reserve and nutrition; dosing must be cautious and individualised).

🧯 Why NAC works (mechanism, not just “because guidelines say so”)

• N-acetylcysteine (NAC) replenishes glutathione stores and provides sulphydryl groups to detoxify NAPQI.
• It also improves microcirculatory blood flow and acts as an antioxidant, which helps even when presentation is delayed.

Paracetamol toxicity is a classic example of liver physiology: Phase II saturation + Zone 3 CYP activity + glutathione depletion → predictable injury pattern. This is why timing, risk factors, and early antidote matter.

🩺 Clinical Relevance (how physiology shows up)

• Shock liver (ischaemic hepatitis): profound hypotension/hypoxia → marked ALT/AST rise; Zone 3 vulnerability.
• Cholestasis: pruritus + ALP/GGT rise; drug accumulation if biliary elimination impaired.
• Hepatic encephalopathy: ammonia and neurotoxin handling failure; precipitated by infection, GI bleed, constipation, sedatives.
• Cirrhosis: synthetic failure (INR, albumin), portal hypertension (varices/ascites), immune dysfunction.

✅ Makindo Exam Pearls

• Zone 3 is most vulnerable to hypoxia and paracetamol toxicity.
• INR is an early marker of reduced hepatic synthesis; albumin reflects chronic reserve.
• Paracetamol overdose: conjugation saturates → ↑ NAPQI → glutathione depletion → centrilobular necrosis; NAC restores glutathione.
• First-pass metabolism and reduced albumin explain why “standard doses” can be too strong in decompensated cirrhosis.

📌 Conclusion

The liver’s structure (segments, sinusoids, zonation) is inseparable from its function (metabolism, synthesis, bile, detox). Drug handling depends on blood flow, CYP activity, conjugation capacity, and biliary transport — all of which change in liver disease. Paracetamol toxicity neatly demonstrates these principles: Phase II saturation plus Zone 3 CYP metabolism creates NAPQI, and glutathione depletion drives predictable hepatocyte injury.