Gastrointestinal Secretions

Overview – Gastrointestinal Secretions

Gastrointestinal secretions play vital roles in digestion, protection, lubrication, immune defence, and regulation of acid–base balance. These secretions, derived from multiple organs along the GI tract, include water, enzymes, mucus, ions, and hormones. For medical students, understanding the source, composition, and control of these secretions is essential to interpret physiological functions and clinical disorders such as peptic ulcers, malabsorption, and diarrhoea.

Functions of GI Secretions

Proteins

  • Digestive enzymes – e.g. amylase, pepsin
  • Lubrication & protection – mucin
  • Immune defence – salivary immunoglobulins
  • Hormonal signalling – peptide hormones

Ions

  • pH regulation – via bicarbonate and hydrogen ions
  • Osmolality control – maintains luminal fluid balance (~300 mOsm)
  • Electrochemical gradients – facilitate nutrient and ion transport

Water

  • Solvent for enzymatic reactions
  • Transport medium for nutrients and ions
  • Maintains content consistency for digestion and excretion

Secretions by Site (Approx. Volumes)

SiteVolume (mL)Constituents
Mouth1500Saliva (amylase, mucin, immunoglobulins)
Stomach2500HCl, pepsinogen, intrinsic factor, mucus
Liver500Bile (bicarbonate, bile salts, cholesterol)
Pancreas1500Bicarbonate, amylase, lipase, proteases, nucleases
Intestines1000Intestinal juice, mucus, bicarbonate (Brunner’s glands)
Colon~0Mucus
Total + Oral Intake9000 mLIncludes ~2000 mL of drinking water
gastrointestinal Secretions Volumes & Constituents
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Key Secretory Structures

Salivary Glands

  • Parotid: α-amylase, antimicrobial agents
  • Submandibular: Mucin + α-amylase
  • Sublingual: Mucin-rich secretion

Stomach

  • Goblet Cells: Thick mucus with bicarbonate (protective)
  • Mucous Neck Cells: Thin acidic mucus
  • Parietal Cells:
    • HCl
    • Intrinsic factor (for vitamin B12 absorption)
  • Chief Cells:
    • Pepsinogen → activated to pepsin by HCl
  • Enteroendocrine Cells:
gastrointestinal secretions - Stomach
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Duodenum – Brunner’s Glands

  • Located proximal to hepatopancreatic sphincter
  • Secrete alkaline mucus to buffer acidic chyme

Liver

  • Produces bile: bile salts, phospholipids, emulsifiers
gastrointestinal secretions - Liver
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Pancreas (Exocrine)

  • Bicarbonate: neutralises gastric acid
  • Amylase: starch → maltose + glucose
  • Lipase: fats → fatty acids + monoglycerides
  • Proteases: protein breakdown
  • Nucleases: DNA/RNA → nucleotides
gastrointestinal secretions - Pancreas
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Intestine & Colon – Crypts of Lieberkühn

  • Paneth Cells: Defensins & lysozyme (antimicrobial)
  • Stem Cells: Regenerate gut epithelium
  • Goblet Cells: Secrete mucus
  • Enterocytes: Secrete alkaline intestinal fluid
gastrointestinal secretions - Crypts of Lieberkuhn
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Mechanisms of Secretion

Protein Secretion (e.g. mucin)

Exportable Protein Secretion
Source: https://journals.physiology.org/doi/full/10.1152/physrev.00010.2005

Ion Transport

  • Na/K ATPase: Sets up electrochemical gradient
  • Na/Cl/K Cotransporters and ion channels regulate secretion
Na:K ATPase
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Water Transport

  • Follows ion gradients via osmosis
  • Mediated by aquaporins
Aquaporins
OpenStax College, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

Specialised Examples

Fluid Secretion (e.g. saliva, pancreatic juice)

  1. Na/Cl/K cotransport → Cl- enters acinar cells
  2. Cl- secreted into lumen, Na+ follows
  3. Water follows ions via osmosis → through aquaporins
  4. Ions are partially reabsorbed downstream; water is not
Fluid Secretion of Salivary Glands & Pancreatic Acini
Note: Na/Cl/K-Cotransporter + Na/K-ATPase + Luminal Ion Channels
https://www.researchgate.net/figure/A-model-depicting-the-mechanism-of-acinar-cells-fluid-and-electrolyte-secretion_fig2_221799618

Bicarbonate Secretion (Pancreatic Ducts, Brunner’s Glands, Bile Ducts)

  1. CO₂ enters cells and reacts with H₂O (carbonic anhydrase)
  2. Forms H₂CO₃ → dissociates into H⁺ and HCO₃⁻
  3. H⁺ expelled via Na⁺/H⁺ antiporter
  4. HCO₃⁻ secreted into lumen via ion channels
HCO3 Secretion
Note: Na/H-Antiporter + Carbonic-Anhydrase + Na/K-ATPase + Ion Channels

Acid Secretion (Parietal Cells)

  • Stimulated by: Acetylcholine, Gastrin, Histamine
  • Mechanism:
    • Carbonic anhydrase generates H⁺
    • H⁺ secreted into lumen by H⁺/K⁺ ATPase
    • Cl⁻ enters via HCO₃⁻/Cl⁻ antiporter
  • Inhibited by:
    • Somatostatin (endocrine/paracrine)
    • Pharmacological agents:
      • Omeprazole: PPI – inhibits H⁺/K⁺ ATPase
      • Ranitidine: H2 receptor blocker – inhibits histamine signalling
      • Pentagastrin: Gastrin analogue – stimulates acid
      • Caffeine: Inhibits phosphodiesterase → ↑cAMP
H+ Secretion
Boumphreyfr, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
H+ Secretion 3
Creative Commons 3.0: https://commons.m.wikimedia.org/wiki/File:Control-of-stomach-acid-sec.png#mw-jump-to-license

Summary – Gastrointestinal Secretions

Gastrointestinal secretions support digestion, lubrication, pH balance, and immune defence. They arise from salivary glands, the stomach, liver, pancreas, and intestinal crypts, involving both fluid and biochemical products. Secretion mechanisms include protein trafficking, ion pumping, and osmotically driven water transport. For a broader context, see our Gastrointestinal Overview page.

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