Question 7(p.2)

College Answer

Candidates were expected to list and briefly define the role of the various substances produced and secreted by the stomach. These included the hormones gastrin and somatostatin, the enzymes pepsin, lipase and gelatinase, the electrolytes Na+, K+ and HCO3- , HCl and water, prostaglandins and mucus, and intrinsic factor. For example: HCl secreted by parietal cells to produce a very acidic environment pH 1-3.5. This optimizes proteolytic activity of pepsin, has a direct proteolytic role, aids ferric iron conversion to the more soluble ferrous ion, and is important for bactericidal activity and innate immunity. It also stimulates pancreatic and biliary secretions. Good answers divided the functions into digestive, hormonal, mucosal protection, immunity etc. Marks were not gained for mention of the secretions of other GIT organs. 

SyllabusQ12b
Reference: Guyton and Hall 11th edition p791-799

Discussion

You could probably just classify the main functions of exocrine gastric secretions into two main categories: digestive and protective.  But then it seems like when the college say "describe the functions of the gastric secretions", they meant all the secretions, including the endocrine or paracrine hormones. That's probably not how the candidates would have interpreted it, which is why 60% of them failed that question.

• Immune function:
  • pH acts to decontaminate bacteria in food. Martinsen et al (2019) had found numerous examples of increased risk of infection if this protective function were interrupted.
  • Proteolytic enzymes also act on microbial cell wall components and endotoxin
• Barrier functions
  • Gastric mucus acts as a barrier and neutralising agent against the gastric acid. This is a phylogenetically ancient function, probably developed in cartilaginous fish over 400 million years ago. 
• Macronutrient digestive function:
  • Gastric acid 
  • Proteolytic enzymes begin the digestion of the protein in the food bolus (pepsin is the main one, and it accounts for about 15% of the total protein breakdown according to Smith et al, 2010)
  • Gastric lipase hydrolyzes the ester bonds of triglycerides, contributing something like 30% of the total lipid catabolism in the gut (Gargouri et al, 1986).
• Micronutrient digestive function:
  • Pepsin also helps ferric iron (Fe³⁺) conversion to the more soluble ferrous (Fe²⁺) ion, according to some widely cited experiments from Jacobs & Miles, 1969.
  • Intrinsic factor binds to B12, creating a complex which can be absorbed in the terminal ileum
• Endocrine and paracrine functions
  • Gastrin, produced by G-cells, which stimulates gastric acid secretion
  • Histamine, produced by enterochromaffin cells, which also stimulates acid secretion
  • Somatostatin, produced by δ-cells, which inhibits gastric acid production
  • Ghrelin, a peptide hormone produced by epithelial cells of the stomach which influences satiety and appetite.

Gamble, James L., Monroe A. McIver, and With the Assistance of Pauline Marsh and Elizabeth Matteson. "The acid-base composition of gastric secretions." The Journal of experimental medicine 48.6 (1928): 837-847.

Strong, J. A., D. Cameron, and M. J. Riddell. "The electrolyte concentration of human gastric secretion." Quarterly Journal of Experimental Physiology and Cognate Medical Sciences: Translation and Integration 45.1 (1960): 1-11.

Kauffman Jr, G. L. "Gastric mucus and bicarbonate secretion in relation to mucosal protection." Journal of clinical gastroenterology 3.Suppl 2 (1981): 45-50.

Gargouri, Youssef, et al. "Importance of human gastric lipase for intestinal lipolysis: an in vitro study." Biochimica et Biophysica Acta (BBA)-Lipids and Lipid Metabolism 879.3 (1986): 419-423.

Jacobs, A., and P. M. Miles. "Role of gastric secretion in iron absorption." Gut 10.3 (1969): 226.

Schubert, Mitchell L. "Functional anatomy and physiology of gastric secretion." Current opinion in gastroenterology 31.6 (2015): 479-485.

Gribble, Fiona M., Frank Reimann, and Geoffrey P. Roberts. "Gastrointestinal hormones." Physiology of the gastrointestinal tract (2018): 31-70.

Ahmed, Monjur, and Sarah Ahmed. "Functional, diagnostic and therapeutic aspects of gastrointestinal hormones." Gastroenterology research 12.5 (2019): 233.

Martinsen, Tom C., Reidar Fossmark, and Helge L. Waldum. "The Phylogeny and biological function of gastric juice—microbiological consequences of removing gastric acid." International journal of molecular sciences 20.23 (2019): 6031.

Ong, Bill Y., Richard J. Palahniuk, and Maureen Cumming. "Gastric volume and pH in out-patients." Canadian Anaesthetists’ Society Journal 25.1 (1978): 36.