Bile: Liver Secretion, Composition & Role in Digestion

Bile: The Liver's Digestive Secretion

Bile is a greenish-yellow fluid (pH 7.6–8.6) produced continuously by hepatocytes (~600–1000 mL/day).

Composition

• Water (~95%)
• Bile salts (cholate, chenodeoxycholate) – synthesized from cholesterol
• Phospholipids (lecithin)
• Cholesterol
• Bilirubin (heme breakdown pigment)
• Electrolytes, proteins

Storage & Release

• Stored/concentrated in gallbladder (5–10x concentration via water absorption).
• Fatty meal → CCK release → gallbladder contraction + sphincter of Oddi relaxation → bile into duodenum.

Digestive Functions

1. Emulsification: Bile salts form micelles → break fat globules into droplets (↑ SA for lipase).
2. Absorption: Mixed micelles solubilize fatty acids, monoglycerides, fat-soluble vitamins (A,D,E,K) for enterocyte uptake.
3. Cholesterol excretion: Primary route for cholesterol elimination.
4. Antimicrobial: Detergent action limits small intestinal bacterial overgrowth.

Enterohepatic Circulation

95% of bile salts reabsorbed in terminal ileum → portal blood → liver → resecreted. Cycles 6–10x/meal. Pool size ~3–5 g.

Clinical Relevance

• Gallstones: Cholesterol supersaturation.
• Obstructive jaundice: Bile duct blockage → bilirubinemia, pale stools, dark urine.
• Bile acid malabsorption: Diarrhea (ileal resection, Crohn's).