Question 21

College Answer

This was another important, relevant and essential aspect of basic physiology, for which candidates tended to lack sufficient breath and depth of the required knowledge. For a good answer candidates were expected to mention liquids empty much faster (and in an exponential fashion) than solids (which have a linear pattern), rate of emptying depends on the pressure gradient generated by the antrum against pyloric resistance. Antral pump activity is of most importance and that it is influenced by signals from both the stomach (eg distension) and the
duodenum (volume, osmolarity, pH) and humoral factors (gastrin, cholecystikinin, secretin) and nervous stimulation (general parasympathetic nervous stimulation enhances gastric motility and sympathetic stimulation opposes it). 
Syllabus: Q2d
References: Guyton and Hall Textbook of Medical Physiology, Chp 63, Power and Kam, Principles
of Physiology for the Anaesthetists, pg 193

Discussion

Factors which control gastric emptying:

• In the fasting state:
  • Migrating motor complexes (MMCs) sweep the stomach at regular intervals (60-90min)
  • 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.
• In the fed state: 
  • Neural control is mainly vagal:
    • Vagal inhibition plays a role in  receptive relaxation: shortly following swallowing, the proximal stomach relaxes.
    • The gastric excitatory vagal circuit (GEVMC) is 
    • Catecholamine excess (including what you get with pain and stress) decreases gastric emptying, but sympathetic innervation does not have much of a role in gastric motility control (Goyal et al, 2019)
  • Myogenic control:
    • Tonic contraction:  the body of the stomach contracts in response to stretch, propelling food slowly towards the antrum
    • Antrum reflexively contracts against the closed pylorus, triturating the food bolus
    • Antro-duodenal pressure gradient generated by resting antral tone and antral contractions is the main determinant of gastric emptying, especially for liquids
  • Hormonal control:
    • Ghrelin and motilin enhance gastric emptying
    • Cholecystokinin, secretin, gastric inhibitory polypeptide and glucagon all inhibit gastric emptying
    • The main factors influencing the release of these hormones are the characteristics of the chyme entering the duodenum 
• Rate of gastric emptying depends on:
  • Consistency of the food (solids empty slower)
    • Solids empty with an initial "lag" phase, during which the food bolus is triturated
    • After this, gastric emptying of solids occurs at a zero-order linear rate
    • Time to empty half of the meal = 30 minutes for relatively nutrient-poor solids, 60-120 minutes for especially fatty solids
    • Liquid empty at an exponential rate
  • Position (fastest when upright or right lateral)
  • 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)

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.