Module 14A: Bile and Digestion Notes

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Topic: GUT SECRETIONS — How is bile (from the liver) involved in digestion?
Lecturer: Dr. Julia Choate

Substance	Volume (mL)	pH
Saliva	$1000$	$7.0-8.0$
Stomach	$1500$	$1.0-3.5$
Pancreas	$1000$	$8.0-8.3$
Bile	$1000$	$7.8$
Small Intestine	$1800$	$7.5-8.0$
Large Intestine	$200$	$7.5-8.0$
Total	$6700$	-
Needs to be neutralised by alkaline secretions from pancreas and liver (bile).

Page 3: Pathway for bile secretion into the gut lumen

Bile salts (cholesterol derivatives) — help to emulsify fats in the small intestine.
Cholesterol — precursor of bile salts.
Lecithin (phospholipid) — combines with bile salts to emulsify fats.
Electrolytes and H2O (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻).
HCO₃⁻ (bicarbonate) — neutralizes acidic chyme in the duodenum.
Bile pigments — produced by breakdown of haemoglobin or myoglobin.

Average intake of fats/lipids = $70-100\ \mathrm{g/day}$ (mainly triglycerides).
Chemical digestion: triglycerides → fatty acids and monoglycerides via pancreatic lipase.
Emulsion: mixture of two liquids that are normally immiscible.
Need shaking/contractions and an emulsifying agent (bile).

Bile salts and lecithin help to emulsify fats in the small intestine.
Process requires: small intestine contractions and the presence of emulsifying agents (bile salts + lecithin).
Emulsion droplets typically shown as ~ $0.5-1\ \mathrm{mm}$ in size.
Surface charges: droplets are negatively charged (−ve) due to bile salts (emulsifying action).
The combination of bile salts and lecithin acts as the emulsifying agents enabling fat dispersion.

Pancreatic lipase (and colipase) break down triglycerides into fatty acids and monoglycerides.
Process outline:
- Emulsion droplet contains triglyceride.
- Lipase acts at the triglyceride, producing fatty acids and monoglycerides.
- Bile salts facilitate diffusion and micelle formation.
- Micelles transport fatty acids and monoglycerides to the intestinal epithelial cells.
Key structures:
- Emulsion droplet → Triglyceride → Lipase → Fatty acids + monoglycerides.
- Micelle formation with bile salts and bile pigments (in some diagrams).
Micelles: ~ $4-7\ \mathrm{nm}$ in diameter.
Outcome: Free molecules of fatty acids and monoglycerides in the lumen ready for absorption at the intestinal epithelium.

Acidity of chyme stimulates secretin.
Secretin triggers alkaline secretions from bile ductal cells (and pancreatic ducts) to neutralise chyme.
Fats stimulate cholecystokinin (CCK) release.
CCK actions include regulation of biliary and pancreatic function: gall bladder contraction, etc.

CCK has multiple actions in response to fats in the duodenal lumen:
1) Gall bladder contraction.
2) Hepatopancreatic sphincter (Sphincter of Oddi) relaxation.
3) Pancreas (pancreatic lipase) activity increases.
4) Stomach (inhibits churning).
5) Pyloric sphincter (constricts).
Mechanism: Fats in the duodenal lumen stimulate CCK secretion from enteroendocrine cells in the duodenal mucosa.
Actions of CCK are listed above and coordinate fat digestion and regulation of bile/pancreatic secretions.

Bile salts and lecithin act as emulsifying agents for triglycerides.
Pancreatic lipase and colipase are required for the chemical digestion of triglycerides.
Entry of bile into the duodenum is regulated by cholecystokinin (CCK).
Secretin leads to alkaline secretions that neutralise acidic chyme from the stomach.