Question 8

College Answer

Candidates were required to provide a description of gastric emptying (40% marks). Although the question showed  the allocation of marks, many candidates did not provide sufficient detail for this section. This required some description of what gastric emptying is (the co-ordinated emptying of chyme from the stomach into the duodenum).

Better answers provided detail regarding the process of gastric emptying in the fed and fasted state and differentiated between liquids, solids, carbohydrate, protein and fats. Factors regulating emptying included an outline of peristaltic waves, the basal electrical rhythm and its modulation, the migratory motor complex (MMC) and its modulation, neural input, stretch and hormonal control.

Many candidates erred by answering the question "the regulation of gastric secretions" rather than the question (the regulation of gastric emptying). Although they scored well for hormonal control, they missed out on marks for the other factors relevant to the regulation of gastric emptying.

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.