Hepatic Architecture, Zonation & Liver Tests

Microscopic architecture: lobule, acinus, and portal triad

Classic lobule. Histologically the liver is tiled by hexagonal lobules: a central vein in the middle, one-cell-thick hepatocyte plates radiating from it, and portal triads at the corners. Blood flows inward along the sinusoids to the central vein, then to the hepatic veins and inferior vena cava.

Acinus. The hepatic acinus (of Rappaport) is a diamond whose axis is the terminal portal venule and hepatic arteriole running between two lobules, with a central vein at each tip. Hepatocytes are ranked by distance from that inflow, so the acinus, not the lobule, explains zonal injury. A third scheme, the triangular portal lobule, centers on a bile ductule and emphasizes bile drainage.

Portal triad. Each portal tract holds a hepatic artery branch, a portal vein branch, and a bile ductule, plus lymphatics and nerves. The portal vein brings ~75% of inflow (nutrient- and toxin-rich gut blood); the hepatic artery supplies ~25% of flow but about half the oxygen.

Sinusoids, space of Disse, and bile canaliculi

Sinusoids and Kupffer cells. Sinusoids have fenestrated endothelium without a continuous basement membrane, so plasma proteins reach hepatocytes freely. Kupffer cells, resident macrophages of the mononuclear phagocyte system, line them and clear gut-derived bacteria, endotoxin, and aged red cells; when activated they release TGF-β, TNF-α, and other profibrotic cytokines.

Stellate cells. The space of Disse lies between the endothelium and hepatocyte microvilli, and hepatic lymph begins here. Its stellate (Ito) cells store vitamin A as retinyl esters in lipid droplets, the body's main vitamin A reservoir. In chronic injury, TGF-β and PDGF activate them: they lose their droplets, become contractile myofibroblasts, and deposit type I collagen, while the sinusoids lose fenestrations and gain a basement membrane (capillarization).

Bile canaliculi. Apical membranes of adjacent hepatocytes form canaliculi sealed by tight junctions; bile salts exit via BSEP and conjugated bilirubin via MRP2. Bile drains through the canals of Hering (home to hepatic progenitor cells) into ductules and interlobular ducts, flowing toward the triad, opposite to blood.

Zonation: the oxygen and enzyme gradient

Why zones exist. Blood at the portal end is richest in oxygen, nutrients, and absorbed toxins. Zone 1 (periportal) cells favor gluconeogenesis, β-oxidation, and urea synthesis; zone 3 (centrilobular) cells favor glycolysis, lipogenesis, glutamine synthesis, and drug metabolism, with the most CYP450 (especially CYP2E1).

Zone 1 meets absorbed substances and viruses first: viral hepatitis (portal and interface inflammation) and ingested toxins, classically cocaine, plus eclampsia (periportal hemorrhagic necrosis) and acute iron overdose. As the best-oxygenated zone, it resists hypoperfusion.

Zone 2 is the target of yellow fever, a flavivirus carried by Aedes aegypti: midzonal necrosis with Councilman (acidophilic) bodies, which are apoptotic hepatocytes, plus fever, jaundice, and hemorrhage ("black vomit"). Acidophil bodies occur in any acute viral hepatitis; the midzonal distribution is the clue.

Zone 3 has the least oxygen and the most CYP450, so it dies first in hypoperfusion and with drugs activated to reactive metabolites (acetaminophen to NAPQI, carbon tetrachloride, halothane). Alcoholic injury (steatosis, alcoholic hepatitis, perivenular fibrosis) starts here too.

Shock versus nutmeg liver. Ischemic hepatitis after shock or cardiac arrest causes zone 3 coagulative necrosis: ALT/AST in the thousands with a very high LDH within 1–3 days, falling quickly once perfusion returns. Congestive hepatopathy (right heart failure, constrictive pericarditis, hepatic vein obstruction) engorges zone 3 sinusoids, giving dark red centers against pale periphery (nutmeg liver) and, if chronic, cardiac cirrhosis.

Liver tests: patterns of injury

Aminotransferases. ALT is cytosolic and fairly liver-specific; AST, largely mitochondrial, is also in heart, skeletal muscle, kidney, and red cells, so an isolated AST rise with a high CK suggests muscle. Hepatocellular injury raises ALT/AST out of proportion to ALP; the R ratio, (ALT/ULN) ÷ (ALP/ULN), is >5 in hepatocellular, <2 in cholestatic, and 2–5 in mixed injury.

AST:ALT ratio. Alcohol gives AST:ALT >2:1 (a Scotch and toAST), usually with AST below 300–500 U/L: alcohol-related pyridoxal phosphate (B6) deficiency depresses ALT more than AST, and mitochondrial injury releases AST. GGT (alcohol-induced) and MCV are often high. Viral hepatitis and fatty liver disease (NAFLD/MASLD) are ALT-predominant until cirrhosis.

Magnitude. ALT/AST >1,000 U/L signals massive necrosis: ischemic hepatitis, acute viral hepatitis, or toxins (acetaminophen, Amanita mushrooms); less often autoimmune hepatitis, acute Budd-Chiari, or a transiently obstructing bile duct stone. Alcohol alone almost never does this. Ischemia rises and falls fast with an LDH very high relative to ALT; viral hepatitis resolves over weeks.

Liver tests: cholestasis and synthetic function

Cholestasis. ALP (± conjugated bilirubin) rises out of proportion to aminotransferases with canalicular injury or duct obstruction. Because bone (growth, Paget disease, blastic metastases) and placenta (third trimester) also make ALP, a raised GGT or 5′-nucleotidase is needed to confirm a hepatobiliary source; an isolated ALP rise with normal GGT points to bone or placenta (cholestasis of pregnancy, by contrast, brings pruritus and raised bile acids). GGT alone is nonspecific, induced by alcohol and drugs such as phenytoin and barbiturates.

Synthetic function. PT/INR and albumin measure function. Factor VII has the shortest half-life (~4–6 h), so the INR rises within a day of acute liver failure; albumin (half-life ~3 weeks) falls mainly in chronic disease and is also low in malnutrition, nephrotic syndrome, and inflammation. A PT that corrects with parenteral vitamin K indicates vitamin K malabsorption (e.g., cholestasis), not hepatocyte failure. Factor VIII, also made by endothelium, stays normal or high in liver disease but falls in DIC.

Aminotransferases measure injury, not function: they may be normal in burnt-out cirrhosis, and in acute liver failure a falling ALT with a rising INR and bilirubin means lost hepatocyte mass, not recovery.