Control of Gastric Emptying

Describe the control of gastrointestinal motility, including sphincter function.

Gastric emptying is a neurally and hormonally mediated process which aims to present food to the small bowel in a controlled manner. Different drugs, hormones, and physiological states can either encourage or inhibit gastric emptying.

Determinants of Gastric Emptying

Rate of gastric emptying is a function of:

  • Antral pressure
    Main determinant as pyloric resistance tends to be low, and is affected by:
    • Stomach
    • Duodenum
    • Systemic factors
    • Drugs
  • Pyloric resistance

Stomach

  • Gastric distension
    Vagal excitation from gastric stretch causes release of gastrin, increasing peristaltic frequency.
  • Composition of chyme:
    • Liquids empty faster than solids
      • Liquids have a half-time of ~20 minutes, and empty in an exponential fashion
      • Solids have a half-time of ~ 2 hours, with a dwell time of ~30 minutes, and empty in a linear fashion
    • Protein independently stimulates gastrin release

Duodenum

The duodenum has hormonal mechanisms which have a negative feedback on gastric emptying. These include:

  • Duodenal distension
  • Hypoosmolar and hyperosmolar chyme
  • Acidic chyme
    In reponse to acid the duodenum releases secretin and somatostatin:
    • Secretin directly inhibits gastric smooth muscle
    • Somatostatin inhibits gastrin release
  • Fat and protein
    Fat and protein breakdown products stimulate release of cholecystokinin, which inhibits gastrin.
    • Carbohydrate-rich meals empty faster than protein, which empty faster than fat.

Systemic

  • Motilin released by the small bowel enhances the strength of the migrating motor complex, a peristaltic wave of contraction through the whole GIT which occurs every 60-90 minutes
  • Sympathetic input from the coeliac plexus inhibits gastric emptying
  • Pregnancy has a number of effecs on gastric emptying:
    • Progesterone relaxes smooth muscle and inhibits gastric smooth muscle response to ACh and gastrin, as well as creating incompetence of the LoS leading to GORD
    • Gastrin production increases
      Some gastrin is produced by the placenta.
    • Gastric acid production is increased during the the third trimester
  • Parasympathetic input enhances gastric motility

Effect of drugs

Drugs which increase gastric emptying include:

  • Metoclopramide
  • Erythromycin

Drugs which inhibit gastric emptying include:

  • Opioids
  • Alcohol
  • Anticholinergic agents

References

  1. CICM July/September 2007
  2. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
Last updated 2017-10-07

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