Gastric Secretions

Describe the composition, volumes and regulation of gastrointestinal secretions

The GIT produces a a number of substances which can be classified by region and function:

  • Saliva
    • H2O (98%)
    • Digestive proteins
      • Amylase
      • Lipase
      • Mucin
      • Haptocorrin
        Binds Vitamin B12.
    • Immunological proteins
      • Lysozyme
      • Lactoferrin
      • IgA
  • Gastric
    • Digestive
      • HCl
      • Gastrin
      • Pepsin
      • Intrinsic Factor
    • Mucosal Protection
      • Mucous
      • HCO3-
  • Small Bowel
    • Digestive
      • Pancreatic
        • Lipase
        • Amylase
        • Trypsinogen
      • Endocrine
        • Secretin
        • Somatostatin

Control of Secretions

Secretion occurs in three phases:

  • Cephalic
    Thought/sight/taste/smell of food, resulting in vagal-mediated stimulus to release gastrin. Accounts for ~30% of production.
  • Gastric
    Stretch of the stomach stimulates HCl secretion and gastrin release. Accounts for ~50% of production.
  • Intestinal
    A drop in pH of the proximal duodenum releases secretin to stimulate the exocrine pancreas.

Salivary Secretions

Approximately 1L of saliva is produced by the parotid, submandibular, and sublingual glands each day.

Saliva has four main functions:

  • Lubrication
    • Mucin
  • Digestion
    • Amylase
    • Lipase
      Particularly important in neonates who produce little pancreatic lipase.
  • Neutralisation of acid
    For protection prior to vomiting.
  • Antibacterial

Gastric Secretions

The stomach produces ~2L of secretions per day:

  • Acid secretion
    Parietal cells contain an H+-K+ exchange pump.
    • H+ is produced by carbonic anhydrase on CO2 and water, with 'waste' HCO3- removed from the cell in exchange for Cl-.
      • High levels of acid production result in large amounts of bicarbonate being secreted into blood
        • This creates an alkaline tide as portal venous pH increases dramatically
        • Respiratory quotient of the stomach may become negative due to consumption of CO2
    • This pump is activated in response to increased levels of intracellular Ca2+ from stimulation by:
      • ACh
      • Histamine (H2)
      • Gastrin
    • Inhibited by:
      • Low gastric pH
      • Somatostatin
  • Gastric
    Gastin is a peptide family secreted from antral G cells.

    • Secretion is stimulated by:
      • Neural (vagal) stimulation in the cephalic phase of digestion
        Main mechanism.
      • Protein and amino acids in the stomach
      • Drugs
        • Alcohol
        • Caffeine
    • Secretion is inhibited by:
      • Low pH
      • Secretin
      • Glucagon
    • Gastrin has a number of pro-digestive effects:
      • Stimulates gastric acid secretion
      • Stimulates pancreatic secretion
      • Stimulates biliary secretion
      • Increases gastric and intestinal motility
  • Pepsinogens
    Chief cells secrete pepsinogen I and is released by ACh or β stimulation. Pepsinogen is cleaved to pepsin in the gastric lumen, and breaks down protein.

  • Intrinsic Factor
    Parietal cells produce intrinsic factor, which forms a complex with B12 which facilitates its later absorption in the terminal ileum.

  • Mucous
    Neck cells produce mucopolysaccharide, glycoprotein, and HCO3- in response to stimulus by prostaglandins, which protects mucosa and lubricates food.

  • Pancreatic Secretions
    Exocrine pancreatic secretions are produced by the acinar and ductal cells, at the rate of 1.5L per day.

    • Release is stimulated by:
      • CCK
      • Secretin
      • ACh
        Via vagal stimulation.
    • Consist of:
      • HCO3-
        To alkalinise gastric contents.
        • Pancreatic bicarbonate production lowers venous pH, and neutralises the alkaline tide of the stomach.
      • Water
      • Enzymes
        • Trypsinogen
          Proteolysis.
        • Amylase
          Hydrolysis of glycogen, starch, and complex carbohydrate.
        • Lipase
          Hydrolysis of dietary triglycerides.

Endocrine Function

  • Cholecystokinin (CCK) is a peptide family secreted by pancreatic acinar cells. Cholecystokinine:
    • Regulates satiety
    • Regulates leptin release from fat
    • Stimulates secretions from the gallbladder and duodenum
  • Secretin stimulates pancreatic release. Secretin is:
    • Released by the proximal duodenum in response to low pH
  • Motilin stimulates the migrating motor complex. Motilin is:
    • Released cyclically from M cells in the small bowel

References

  1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
Last updated 2017-10-07

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