Gastric Secretions

Describe the composition, volumes and regulation of gastrointestinal secretions

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

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.

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

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

Gastrin
Gastrin 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 which is released by ACh or β stimulation. Pepsinogen is cleaved by hydrochloric acid in the stomach to become pepsin. Pepsin 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: activated to trypsin by duodenal enteropeptidases.
      Proteolysis.
    - Amylase
      Hydrolysis of glycogen, starch, and complex carbohydrate.
    - Lipase
      Hydrolysis of dietary triglycerides.

Cholecystokinin (CCK) is a peptide family secreted by intestinal enteroendocrine cells (I cells) in the mucosa of the duodenum and jejunum. Cholecystokinin:
- 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

Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.

Last updated 2020-09-01