Viva O1(i)

What secretions are produced by the gastrointestinal tract, and what are their volumes?
Secretions Daily volume
Saliva 500-1500ml
Gastric secretions 1000-2000ml
Bile 1000ml
Pancreatic secretions 1000ml
Small bowel secretions      3000ml
Total 7000-8000 ml

(from Camilleri, 2004)

What is saliva?
  • A "mucoserous exocrine secretion"
  • The product of minor (10%) and major (90%) salivary glands
  • Chemical properties and constituents:
    • Hypotonic
    • Sodium-poor
    • Multiple macromolecular (i.e. protein) components
    • Hydrated mucin glycoproteins
    • Immunoglobulins
    • Amylase
  • Flow rate: 500-1500ml/day
  • Of this, 90% is "stimulated" flow

Humphrey & Williamson (2001)

What is the role of saliva?
  • Lubrication and protection (barrier function, mainly mucins)
  • Buffering action and clearance (mainly bicarbonate)
  • Maintenance of tooth integrity  (remineralisation of enamel by calcium and phosphate)
  • Antibacterial activity (immunoglobulins, especially IgA)
  • Taste and swallowing (lubricating the food bolus, acting as a chemically inert carrier fluid to dissolve molecules for taste sensation)
  • Digestion (amylase in saliva begins to act on carbohydrates)

Humphrey & Williamson (2001)

What is the clinical relevance of knowing the volume of saliva secretion?
  • Under normal circumstances
    • Intubated patients produce very little oral secretions
    • This contributes to poor oral hygiene, as the antibacterial effects of saliva are lost
    • This affects the microbiome of the posterior pharynx
  • If oral secretions are being produced:
    • The intubated patient will be unable to swallow effectively
    • The oral secretions produced by such patients will pool in the oropharynx
    • This pool will not be accessible to routine oral hygiene measures
    • Bacterial load of these fluids will increase
    • They will eventually make their way past the low pressure endotracheal cuff and cause VAP
What are gastric secretions, and which cells or glands produce them?
  • Multiple products of multiple cell types:
    • Goblet cells secrete mainly mucus
    • Chief cells secrete mainly pepsinogen and gastric lipase. Pepsinogen is converted into active pepsin in the low pH environment of the stomach.
    • Parietal cells produce mainly hydrochloric acid and intrinsic factor
What is the composition of gastric secretions?
  • Water (95%)
  • Hydrochloric acid (pH 1.5-3.5)
  • Mucus, rich in bicarbonate (which forms an "unstirred layer"); contains 20g/L of a viscous mucin polymer
  • Electrolyte composition:
    Electrolyte concentration    (mmol/L)
    Chloride 130-180
    Sodium 10-60
    Potassium 12-20
    Calcium 1.00-1.30

(Kaufmann, 1981)

What is the function of gastric secretions?
  • Immune function: pH acts to decontaminate bacteria in food. 
  • Barrier functions: Gastric mucus acts as a barrier and neutralising agent against the gastric acid. 
  • Macronutrient digestive function:
    • Gastric acid 
    • Proteolytic enzymes
    • Gastric lipase
  • Micronutrient digestive function:
    • Pepsin also helps ferric iron (Fe3+) conversion to the more soluble ferrous (Fe2+
    • Intrinsic factor binds to B12
What is the volume of gastric secretions?
  • Most textbooks quote 1000-2000ml/day or 100ml/hr
  • Normal fasted gastric aspirate volume is 70-150ml
What is the clinical relevance of this?
  • The rate of gastric secretion is related to the residual gastric volume
  • This has implications for measurement of enteral feed tolerance and for safety of intubation
What factors increase or decrease the rate of gastric secretion?
  • Factors that increase gastric acid secretion: 
    • Parasympathetic nervous system (via the vagus nerve)
    • Gastrin, during the cephalic and gastric phases of digestion
    • Histamine; during the cephalic and gastric phases of digestion)
    • Mechanical stretch (by vago-vagal and local smooth muscle stretch reflexes)
    • Peptides, caffeine, alcohol - sensed by mucosal chemoreceptors of the antral G cells, which then release gastrin
  • Factors that decrease gastric acid secretion
    • Somatostatin, if the luminal pH decreases to below 1.5
    • Small intestine hormones
      • Vasoactive intestinal polypeptide (VIP)
      • Secretin
      • Cholecystokinin
    • Prostaglandins PGE2 and PGI2
  • Mucus secretion by goblet cells:
    • Mainly regulated by mechanotransduction triggers, such as the distension of the stomach wall with food. 
What is the mechanism of gastric acid formation?
  • Takes place in gastric parietal cells:
    • The apical surface of the parietal cell has numerous small secretory canaliculi which increase the apical surface area of the cells
    • Resting parietal cells have numerous vesicles the membranes of which have acid-secretory transmembrane proteins
    • With stimulation, these vesicles fuse with the apical canaliculi to rapidly increase the secretory capacity of these cells
  • Basal membrane ion transport in parietal cells
    • CO2 and water are able to diffuse into parietal cells passively
    • Carbonic anhydrase converts the CO2 and H2O into HCO3- and H+
    • HCO3-  is then exchanged for chloride at the basal membrane
  • Apical membrane ion transport in parietal cells
    • H+ generated by carbonic anhydrase is exported by an ATP-powered H+/K+exchange pump, otherwise known as the "proton pump" - this is the molecular drug target of PPIs 
    • The potassium used in this exchange is returned to the gut lumen by KCNQ1/KCNE potassium channels in the apical membrane of the parietal cells.
    • Chloride is exported through apical chloride channels (ClC-2)
  • Net effect of parietal cell activity
    • In the gut lumen, chloride concentration increases from 120 mmol/L up to 180 mmol/L (thus pH decreases)
    • In the portal vein, CO2 concentration decreases: gastric carbonic anhydrase activity consumes CO2, which is therefore lower in the portal venous blood than in the arterial blood delivered to the stomach, i.e. the actively secreting stomach has a negative respiratory quotient
    • This increases the pH of portal venous blood during the cephalic and gastric phases of digestion, a phenomenon known as the "alkaline tide"


Coate, Katie C., Steven A. Kliewer, and David J. Mangelsdorf. "SnapShot: Hormones of the gastrointestinal tract." Cell 159.6 (2014): 1478-1478.

Thomas, Adrian. "Gastrointestinal secretions and vomiting." Anaesthesia & Intensive Care Medicine 7.2 (2006): 54-56.

Welcome, Menizibeya Osain. "Gastrointestinal Exocrine (Lumencrine) Secretions. The Reception Theory as the Basis for Developing the First Antisecretory Pharmacotherapy Drugs." Gastrointestinal Physiology. Springer, Cham, 2018. 773-870.

Camilleri, Michael. "Chronic diarrhea: a review on pathophysiology and management for the clinical gastroenterologist." Clinical Gastroenterology and Hepatology 2.3 (2004): 198-206.

Debongnie, J. C., and S. F. Phillips. "Capacity of the human colon to absorb fluid." Gastroenterology 74.4 (1978): 698-703.

Humphrey, Sue P., and Russell T. Williamson. "A review of saliva: normal composition, flow, and function." The Journal of prosthetic dentistry 85.2 (2001): 162-169.

de Almeida, P. Del Vigna, et al. "Saliva composition and functions: a comprehensive review." J contemp dent pract 9.3 (2008): 72-80.

Dennesen, Paul, et al. "Inadequate salivary flow and poor oral mucosal status in intubated intensive care unit patients." Critical care medicine 31.3 (2003): 781-786.

Ekström, J. "Autonomic control of salivary secretion." Proceedings of the Finnish Dental Society. Suomen Hammaslaakariseuran toimituksia 85.4-5 (1989): 323-31.

Meyers, R. Lee, and Roy C. Orlando. "In vivo bicarbonate secretion by human esophagus." Gastroenterology 103.4 (1992): 1174-1178.

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.

Waldegger, Siegfried. "Heartburn: cardiac potassium channels involved in parietal cell acid secretion." Pflügers Archiv 446.2 (2003): 143-147.

Engevik, Amy C., Izumi Kaji, and James R. Goldenring. "The physiology of the gastric parietal cell." Physiological reviews 100.2 (2020): 573-602.

Malinowska, Danuta H., et al. "Characterization, Regulation and Localization of the Gastric Cl-Channel Associated with Gastric Acid Secretion." Mechanisms and Consequences of Proton Transport. Springer, Boston, MA, 2002. 233-245.

Niv, Yaron, and Gerald M. Fraser. "The alkaline tide phenomenon." Journal of clinical gastroenterology 35.1 (2002): 5-8.

Ayazi, Shahin, et al. "Measurement of gastric pH in ambulatory esophageal pH monitoring." Surgical endoscopy 23.9 (2009): 1968-1973.

Odes, H. S., et al. "Cholinergic regulation of guinea pig duodenal bicarbonate secretion." American Journal of Physiology-Gastrointestinal and Liver Physiology 265.2 (1993): G270-G276.

Chandra, Rashmi, and Rodger A. Liddle. "Neural and hormonal regulation of pancreatic secretion." Current opinion in gastroenterology 25.5 (2009): 441.

Owyang, Chung, and John A. Williams. "Pancreatic secretion." Yamada's Textbook of Gastroenterology (2015): 450-473.

Domschke, S., et al. "Inhibition by somatostatin of secretin-stimulated pancreatic secretion in man: a study with pure pancreatic juice." Scandinavian journal of gastroenterology 12.1 (1977): 59-63.

Niebergall-Roth, Elke, and Manfred V. Singer. "Enteropancreatic reflexes mediating the pancreatic enzyme response to nutrients." Autonomic Neuroscience 125.1-2 (2006): 62-69.

Nathanson, Michael H., and James L. Boyer. "Mechanisms and regulation of bile secretion." Hepatology 14.3 (1991): 551-566.