Bile Composition, Synthesis, and Digestive Functions

Bile Composition and Physiological Significance

Bile (p. 224) is a watery mixture of organic and inorganic compounds that plays an essential role in fat digestion and absorption. It also serves as a medium for metabolic products. The daily secretion of bile is approximately 1000 ml.

Main Components of Bile

Bile contains no digestive enzymes. Its main components include:

• Bile salts (conjugated bile acids)
• Phospholipids (phosphatidylcholine)
• Cholesterol
• Proteins
• Electrolytes
• Water
• Bilirubin

Structure of Bile Acids

Bile acids consist of 24 carbons with two or three hydroxyl groups and a side chain terminating in a carboxyl group. They are amphipathic (possessing both polar and nonpolar sides), which provides their emulsification function.

Synthesis of Bile Acids and Salts

Bile acids are synthesized in the liver by hepatocytes. This is a multistep reaction that uses cholesterol to form cholic acid or chenodeoxycholic acid.

Bile acids are then conjugated to either glycine or taurine by an amide bond to form bile salts (e.g., glycocholic acid). This conjugation enhances their amphipathic nature, making them more effective detergents.

The Pathway of Bile

Hepatocytes provide the primary secretion of bile salts and other organic substances into the bile canaliculi. Epithelial cells then provide water and electrolytes. The pathway follows: Hepatocytes → Bile canaliculi → Bile ducts (or gallbladder) → Common bile duct → Duodenum.

Action of Intestinal Flora

Bacteria in the intestine may remove glycine or taurine, resulting in bile acids. Secondary bile acids, such as deoxycholic acid, are formed by removing the hydroxyl group.

Primary Functions of Bile

Emulsification of Fats

Bile salts decrease the surface tension of fat, allowing large fat droplets to break into smaller particles. This enhances the digestion of fats, as digestive enzymes can only act on the surface of the lipid droplet.

Micelle Formation

Micelles are small aggregates of bile salts and products of fat digestion. Amphipathic bile acids shield the hydrophobic side of fat digestion products, increasing their solubility in the aqueous environment. Micelles are then transported to the mucosal cells for the absorption of fats.

Enterohepatic Circulation

The body efficiently reabsorbs approximately 95% of bile salts for reuse. This mixture of bile salts and bile acids is primarily reabsorbed by the ileum via the Na⁺-bile salt cotransporter.

The process involves active transport out of mucosal cells into the portal blood, followed by uptake by hepatocytes via an isoform of the cotransporter. Less than 3% of bile salts are lost in the feces.

Bile Salt Deficiency and Cholelithiasis

When more cholesterol enters the bile than can be solubilized by bile salts and phospholipids, precipitation occurs, leading to the formation of gallstones (cholelithiasis).