- The functional unit of the liver is the acinus, the array of cells radiating from a portal triad that contains the terminal branches of the bile ductule(s), portal vein and hepatic artery.
- Portal blood pressure is primarily determined by the vascular radii of vessels in the liver and flow of blood into the portal venous system. Portal hypertension ensues when hepatic vessels are constricted and/or inflow from the splanchnic circulation increases.
- Commonly used liver tests are not quantitative tests of liver function; they are a reflection of interrelated physiological and pathological events, but are useful in determining the diagnosis, course, severity and prognosis of hepatobiliary diseases.
- The liver has a central role in the metabolism of nutrients and xenobiotics absorbed from the gut; such biotransformation of toxic compounds neutralizes their activity and allows their excretion in bile. The liver synthesizes essential proteins, clotting factors, and lipoproteins, which circulate in the blood, and bile acids, which are key detergents in the bile and intestine.
- The Kupffer cells shield the body from exposure to foreign antigens, endotoxins and bacteria absorbed from the gut.
- Over 95% of the bile salts secreted in bile are reabsorbed from the small intestine, where they are modified and returned to the liver for re-excretion into bile (enterohepatic circulation).
- About 80% of the bilirubin produced each day is derived in the spleen from the hemoglobin in senescent red blood cells. Mechanisms that protect against the toxicity of unconjugated bilirubin include binding to plasma albumin, and rapid hepatic uptake and conjugation, which allows its excretion by hepatocytes into bile. Bilirubin conjugates are water-soluble, and not absorbed by enterocytes in the gut, but their excretion in the urine limits their accumulation in the body when hepatocellular or cholestatic jaundice occurs. Unconjugated bilirubin also accumulates in the plasma due to associated hemolysis and to hydrolysis of retained conjugates.
- In cholestasis, conjugated bile salts and alkaline phosphatase regurgitate into the lymph through leaky tight junctions in bile canaliculi and biliary ductules, as well as into plasma by carrier-mediated reflux across the sinusoidal membrane of the hepatocytes and the basolateral membrane of cholangiocytes.
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