Question 10

College Answer

The main points expected for a pass were:

• An appreciation that the aim of controlling gastric emptying is to present the food to the small bowel for absorption in a controlled manner e That there are both gastric neural and hormonal mechanisms e.g.
• Gastric volume and the hormone gastrin
• There are duodenal neural and hormonal mechanisms e.g. composition of the chyme, secretin and cholecystokinin and the influence of duodenal distension.

Extra points were given for mentioning the effect of sympathetic stimulation and pregnancy on gastric emptying

Common problems were not enough knowledge, naming hormones but not saying what their action was and including drugs.

Discussion

Gastric emptying:

• Fasting state:
  • Migrating motor complexes sweep the stomach at regular intervals
  • These are slow peristaltic waves that originate in the fundus
  • Their role is to keep the stomach empty of secretions and food debris
  • They are interrupted by eating
• Fed state: 
  • Receptive relaxation: shortly following swallowing, the proximal stomach relaxes by a vagally mediated reflex
  • Tonic contraction:  the body of the stomach contracts, propelling food slowly towards the antrum
  • "Lag" phase with solid food: 
    • The antrum contracts forcefully against a tightly closed pylorus
    • This triturate the food until it is well mixed with gastric secretions and its particles are less than 1-2mm in size
    • During this phase, there is minimal passage of food into the duodenum
    • Depending on the food, this takes 30-90 minutes
  • Linear emptying phase with solid food: 
    • small food particles are pushed through the pyloric sphincter at a stable linear rate
    • Time to empty half of the meal = 30 minutes for relatively nutrient-poor solids, 60-120 minutes for especially fatty solids
  • Exponential emptying with liquids:
    • Liquids have no lag phase and begin emptying rapidly and immediately
    • Emptying rate is exponential and determined by the antral-duodenal pressure gradient, which is usually 5mmHg but which can increase to over 40 mmHg with peristalsis
    • 15-20 minutes is the half-time of non-nutritive fluids, eg. water

Regulation of gastric emptying:

• Factors that affect gastric emptying:
  • Position (fastest when upright or right lateral)
  • Consistency of the food (solids empty slower)
  • The volume of liquid (larger volume empties faster)
  • Gastric pH (acidic pH decreases the rate of emptying)
  • Tonicity of contents (decreases gastric emptying)
  • Lipid content (fatty meals slow gastric emptying)
  • Caloric content (high-calorie meals slow gastric emptying)
• Hormonal influences on gastric emptying:

  Hormones which:
  Enhance gastric empyting	Slow gastric emptying
  Ghrelin Motilin	Cholecystokinin Secretin Gastric inhibitory polypeptide Glucagon Glucagon-like peptides 1 and 2

Minami, Howard, and Richard W. Mccallum. "The physiology and pathophysiology of gastric emptying in humans." Gastroenterology 86.6 (1984): 1592-1610.

Hellström, Per M., Per Grybäck, and Hans Jacobsson. "The physiology of gastric emptying." Best Practice & Research Clinical Anaesthesiology 20.3 (2006): 397-407.

Mittal, Ravinder K., and Raj K. Goyal. "Sphincter mechanisms at the lower end of the esophagus." GI Motility online (2006).

Goyal, Raj K., Yanmei Guo, and Hiroshi Mashimo. "Advances in the physiology of gastric emptying." Neurogastroenterology & Motility 31.4 (2019): e13546.