Liver and Gallbladder Physiology

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.
  1. 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.

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