- Bile synthesis and secretion
- Nutrient metabolism
- Synthesis of plasma proteins
- Secretion and modification of hormones
- Storage of essential molecules
- Removal of aged blood cells
- Muscular sac
- Stores and concentrates bile (which is synthesized in the liver)
- 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.
Regulated by secretin and CCK
- 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.
- 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.
- Cholesterol precursor = hydrophobic portion, the cholesterol precursor,
- Composed mainly of non-polar hydrocarbons, which interact with the lipid droplets.
- Polar hydroxyl and carboxyl groups = hydrophilic portion, polar hydroxyl and carboxyl groups
- Exposed to the surrounded aqueous solution.
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.