Normal bilirubin metabolism
Understanding the pathogenesis of jaundice is impossible without knowing the stages of pigment metabolism. The process proceeds in three key phases:
- Prehepatic phase (in the blood). The main source (85%) is the physiological breakdown of erythrocytes in the reticuloendothelial system. The resulting unconjugated (indirect) bilirubin is insoluble in water, so it binds tightly to albumin for transport. In this form, it cannot pass through the renal filter.
- Intrahepatic phase. At the hepatocyte membrane, the complex dissociates. Free bilirubin enters the cell, where it is conjugated with glucuronic acid by the action of UDP-glucuronosyltransferase. This produces conjugated (direct) bilirubin, which is non-toxic, water-soluble, and ready for excretion.
- Intestinal phase (excretion). The conjugated fraction enters the intestine with bile, where bacteria convert it into urobilinogens. The bulk is oxidized to stercobilin (which colors the stool brown). About 1% is reabsorbed back into the blood and excreted by the kidneys as urobilin.
Clinical evaluation and semiotics
As bilirubin levels rise, tissues become discolored not simultaneously, but in a strict sequence:
- First, the sclera turns yellow.
- Then the color of the mucous membrane of the soft palate and the lingual frenulum changes.
- Next, the face, palms, and soles become discolored.
- Finally, total discoloration of the skin occurs.
It is important to remember the factors affecting visualization. Edema associated with hypoproteinemia, anemia, and pronounced obesity can mask jaundice. Conversely, in lean patients with well-developed musculature, pigmentation is much easier to notice. Long-standing jaundice can change shade: from greenish (due to biliverdin accumulation) to bronze-black.
Differential diagnosis of acquired jaundice
Clinical practice most commonly encounters acquired forms, which are usually divided into three pathogenetic classes.
| Sign | Prehepatic (hemolytic) | Hepatocellular (parenchymal) | Posthepatic (obstructive) |
|---|---|---|---|
| Skin color | Lemon-yellow | Yellow | Dark yellow, earthy |
| Pruritus | Absent | Possible | Intense, with excoriations |
| Blood (bilirubin) | Unconjugated increased | Mixed (conjugated predominates) | Conjugated increased |
| Stool | Dark (pleochroism) | Hypocholic or discolored | Acholic (like window putty) |
| Urine | Dark (high urobilin) | Dark (conjugated bilirubin + urobilin) | «Dark beer» color, without urobilin |
In hemolytic jaundice, the liver simply cannot keep up with processing the massive volume of pigment from destroyed erythrocytes. In parenchymal jaundice (viruses, alcohol), the hepatocytes themselves are damaged, leading to a backflow (regurgitation) of direct bilirubin into the blood. In obstructive jaundice (stones, tumors), bile outflow is physically blocked, which also leads to regurgitation of the conjugated fraction.
Briefly: answers to topical questions
What other hereditary syndromes, besides Crigler-Najjar, present with elevated unconjugated bilirubin?
A syndrome that presents with elevated unconjugated (indirect) bilirubin, in addition to Crigler-Najjar syndrome, is Gilbert syndrome.
- Gilbert syndrome is a hereditary enzymopathy (autosomal dominant inheritance pattern) characterized by a moderate increase in unconjugated bilirubin levels in the blood.
This disorder is caused by decreased hepatic uptake of bilirubin and a subsequent defect in its conjugation due to reduced enzyme activity.
Which blood enzymes (markers of cholestasis) are elevated in obstructive jaundice?
In obstructive (posthepatic) jaundice, the activity of enzymes that serve as markers of cholestatic syndrome increases in the blood.
- Alkaline phosphatase (ALP) — its maximal elevation is characteristic of bile duct damage during mechanical obstruction.
- Gamma-glutamyl transferase (GGT, or gamma-glutamyl transpeptidase) is an enzyme whose activity increases significantly in conditions leading to bile stasis.
With pronounced cholestasis in obstructive jaundice, ALP and GGT levels rise sharply.
How does urobilinogen from the intestine get back into the blood?
Urobilinogen gets back into the blood via absorption from the intestine into the portal venous system.
After being absorbed in the intestine, part of the urobilinogen enters the portal vein (v. portae) blood and is directed to the liver, where it can be re-excreted into bile; this corresponds to enterohepatic circulation. A small fraction enters the general circulation, is filtered by the kidneys, and is excreted in the urine as urobilin.
To easily remember the sequence in which jaundice appears, use the rule "from top to bottom and from inside out": Sclera → Oral mucosa → Face and palms → Total skin.
Class and exam questions
Why is urine dark in obstructive jaundice, yet contains no urobilin?
Due to duct blockage, bile does not enter the intestine, so urobilinogen is simply not formed and cannot be absorbed into the blood. The dark color of the urine is caused exclusively by conjugated (direct) bilirubin, which easily passes the renal filter due to its water solubility.
How to differentiate true jaundice from carotenoderma?
The key differential diagnostic feature is the color of the sclera. In alimentary (false) jaundice due to excess carotene, only the skin is discolored, while the sclera remain completely white.
What is the danger of Crigler-Najjar syndrome?
This is a hereditary absence of the enzyme glucuronyltransferase. Because of this, from the first days of life, the level of toxic unconjugated bilirubin rapidly rises, crossing the blood-brain barrier and causing severe brain damage (kernicterus).
What else to study on this topic
- Mechanisms of intracellular cholestasis and the phenomenon of regurgitation.
- Hereditary functional hyperbilirubinemias (Gilbert, Dubin-Johnson, Rotor syndromes).
- The role of enzymes (ALT, AST, ALP, GGT) in the differential diagnosis of cytolysis and cholestasis.
- Instrumental methods (ultrasound, CT, ERCP) for determining the level of distal and proximal block in obstruction.
- Etiology of hemolytic jaundice: from vitamin B12-deficiency anemia to infections (malaria).
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