LIVER

Digestive function

• Bile synthesis and secretion

Other functions

• Nutrient metabolism
• Synthesis of plasma proteins
• Secretion and modification of hormones
• Storage of essential molecules
• Removal of aged blood cells
• Detoxification

GALLBLADDER

• Muscular sac
• Stores and concentrates bile (which is synthesized in the liver)

BILE

• Cholesterol-derived and alkaline
• Secreted by the liver
• Stored in the gallbladder.
• Released into the digestive tract postprandially (following ingestion of a meal) upon sphincter of Oddi opening.

Bile Secretion

Regulated by secretin and CCK

Secretin

• Secreted from the duodenum in response to acidic chime in the duodenum
• Acts on the liver to stimulates bile secretion.
• Bile (which contains bicarbonate) neutralizes the acidic chyme.

CCK

• Secreted from duodenum in response to fatty acids present in the chyme
• Acts on the gallbladder – produces gallbladder contraction
• Acts on sphincter of Oddi to promote its relaxation
• Thus, stimulates bile flow into the duodenum – the bile salts (another major component of bile) can emulsify fats (like a detergent) for their digestion and subsequent absorption in the small intestine.

Note: bile also contains cholesterol, lecithin (phospholipids), bile pigments, and trace metals.

Bile Salt Recycling in Enterohepatic Circulation

• Bile salts pass down the length of the small intestine to the ileum
• Reabsorbed into circulation at the ileum (they the enter recycling pathway: enterohepatic circulation).
• Travel through the hepatic portal vein back to the liver where they are recycled and re-secreted into newly formed bile.

Note: The hepatic portal vein drains nutrient-rich blood from the small intestine to the liver for metabolic processing.

• Small amount of bile salts continues through the rest of the digestive tract; approximately 5% of bile salts are eliminated in feces (along with other bile components).

Note: The liver synthesizes more bile salt from cholesterol to account for its loss.

Bile Salt Structure

Bile Salts = Amphipathic molecules, meaning they have hydrophobic and hydrophilic sides.

1. Cholesterol precursor = hydrophobic portion, the cholesterol precursor,

• Composed mainly of non-polar hydrocarbons, which interact with the lipid droplets.

2. Polar hydroxyl and carboxyl groups = hydrophilic portion, polar hydroxyl and carboxyl groups

• Exposed to the surrounded aqueous solution.

Fat Emulsification

Bile salt’s amphipathic nature aids in fat digestion.

• Bile salts and phospholipids (another amphipathic molecule and emulsifying agent) increase the surface area of large fat globules
• Aid in their breakdown into smaller emulsification droplets and prevent their reaggregation.
  Lipid droplets comprise triglycerides.
  Lipase, with the help of colipase, digest triglycerides into their simpler components:

Monoglyceride (glycerol), and

[Two] fatty acids.

which are absorbed by the small intestine.

• Bile salts arrange the monoglyceride, fatty acids, and phospholipids to form spherically-arranged micelles.
• Promoted by amphipathic nature of fatty acids and phospholipids promotes this spherical formation
• Micelles continuously form and breakdown.

Micelles = Holding stations for digested fats

• Continuously exchange lipids with the surrounding solution.
• Form to keep otherwise insoluble fats in small, soluble aggregates.
• Break down to replenish digested fat products that are absorbed.