黄疸

Jaundice becomes clinically apparent when plasma bilirubin levels reach approximately 2.5 to 3 milligrams per deciliter. At this threshold, bilirubin deposits in tissues, causing yellow discoloration of the skin, sclerae, and mucous membranes. This visible yellowing indicates significant hyperbilirubinemia requiring clinical evaluation.

Macrophages degrade hemoglobin into globin and heme. Globin is converted into amino acids for reuse, while heme is transformed into biliverdin by heme oxygenase. Biliverdin reductase then reduces biliverdin to lipid-soluble unconjugated bilirubin, which circulates bound to albumin in the bloodstream.

Hepatocytes absorb unconjugated bilirubin from the bloodstream and conjugate it with glucuronic acid using the enzyme uridine diphosphate-glucuronosyl transferase. This produces water-soluble conjugated bilirubin, which the liver secretes into bile for excretion into the intestine, completing normal bilirubin elimination and preventing accumulation.

Prehepatic jaundice results from excessive red blood cell destruction, overwhelming the liver's conjugation capacity. This leads to elevated unconjugated bilirubin accumulation in blood. Since unconjugated bilirubin is lipid-soluble, it does not appear in urine. Common causes include sickle cell anemia, hemolytic anemias, and transfusion reactions.

Intrahepatic jaundice involves hepatocyte dysfunction or intrahepatic obstruction, causing elevated unconjugated and conjugated bilirubin with possible bilirubinuria. Posthepatic jaundice results from extrahepatic bile duct obstruction, preventing conjugated bilirubin from reaching the intestine, causing dark urine, pale stools, and elevated alkaline phosphatase.

Posthepatic jaundice presents with elevated conjugated bilirubin, high alkaline phosphatase, dark urine, pale stools, and pruritus. These findings reflect bile duct obstruction from gallstones, pancreatic cancer, or cholangiocarcinoma. The conjugated bilirubin enters the bloodstream and is filtered into urine, causing its dark appearance.

Jaundice develops when any stage of bilirubin metabolism is disrupted: increased red blood cell destruction, impaired hepatic uptake or conjugation, or obstruction of bile excretion. These disruptions cause elevated serum bilirubin levels. Understanding viral hepatitis introduction and these three pathways helps clinicians identify underlying causes.