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Foundation Sciences · Physiology

Gastrointestinal Physiology

⏱️ 45 mins read 📖 Physiology 🎯 MLA Relevance: High

Gastrointestinal (GI) physiology describes the mechanical and chemical processes used to break down food, absorb nutrients, and eliminate waste. It involves highly coordinated motility, secretion of digestive enzymes and bile, and specialized transport mechanisms across the intestinal epithelium. The system is regulated by the enteric nervous system and a variety of gut hormones including gastrin, secretin, and cholecystokinin.

📌 Learning Objectives

  • Describe the four main processes of the gastrointestinal system: motility, secretion, digestion, and absorption.
  • Explain the roles of key digestive enzymes and hormones in different parts of the GI tract.
  • Identify the major anatomical structures of the GI tract and their physiological functions.
  • Apply knowledge of GI physiology to understand common clinical presentations.
  • Explain the regulation of GI function by the enteric nervous system and hormonal mechanisms.
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Curriculum Mapped
UK MLA Curriculum

📋 Overview

The GI tract functions across four processes: motility, secretion, digestion, and absorption. Digestion begins in the mouth (amylase) and continues in the stomach, where HCl and pepsin begin protein breakdown. The stomach's parietal cells secrete HCl (stimulated by Gastrin, ACh, and Histamine) and intrinsic factor (essential for Vitamin B12 absorption). The small intestine (duodenum, jejunum, ileum) is the primary site for nutrient absorption, aided by its massive surface area (plicae circulares, villi, microvilli). The pancreas contributes alkaline juice and enzymes (lipase, protease, amylase), while the liver produces bile, stored in the gallbladder, to emulsify fats. Absorption of water and electrolytes occurs mainly in the small intestine and colon. Motility is regulated by the Enteric Nervous System (ENS), comprising the Myenteric (Auerbach’s) and Submucosal (Meissner’s) plexuses. Hormones like Cholecystokinin (CCK) stimulate gallbladder contraction and pancreatic secretion while inhibiting gastric emptying. Secretin stimulates bicarbonate release from the pancreas to neutralize gastric acid in the duodenum.

🔬 Basic Science

Chemical digestion involves hydrolysis. Carbohydrates are broken into monosaccharides by amylases and brush border disaccharidases (lactase, sucrase). Proteins are broken into amino acids or di/tri-peptides by gastric pepsin and pancreatic trypsin/chymotrypsin. Lipids require emulsification by bile salts to form micelles, allowing pancreatic lipase to act. These nutrients are then absorbed via specific transporters (e.g., SGLT-1 for glucose and galactose). The gastric barrier is protected by prostaglandins which stimulate mucus and bicarbonate secretion; NSAIDs inhibit this, risking peptic ulcers. The 'Cephalic phase' of digestion is triggered by the sight/smell of food, mediated via the Vagus nerve. The 'Gastric phase' and 'Intestinal phase' follow as food reaches those areas. Liver physiology is central to the GI system, involving the metabolism of carbohydrates, storage of glycogen/vitamins, detoxification, and synthesis of albumin/clotting factors.

🏥 Clinical Relevance

Dysfunction leads to malabsorption (e.g., Celiac disease, Crohn's), motility issues (e.g., Achalasia, Gastroparesis), or acid-related disease (e.g., GERD, Peptic Ulcers). Terminal ileal resection (Crohn's surgery) leads to B12 deficiency and bile acid malabsorption (causing diarrhea). Jaundice occurs when bilirubin metabolism (conjugation in the liver) or excretion (bile ducts) is disrupted. Pancreatitis involves premature activation of enzymes, leading to auto-digestion. Acute GI bleed is a surgical emergency.

🧪 Investigations

Endoscopy (OGD/Colonoscopy) allows direct visualization and biopsy. Breath tests (e.g., Urea breath test for H. pylori, Hydrogen breath test for lactose intolerance). Liver Function Tests (LFTs) assess synthetic function (Albumin, PT) and cholestasis/damage (Bilirubin, ALP, ALT). Stool tests (Faecal calprotectin) screen for inflammation.

💊 Management

Gastric acid suppression uses Proton Pump Inhibitors (PPIs) or H2-receptor antagonists. Malabsorption is managed by dietary exclusion (gluten-free in Celiac) or replacement (B12 injections). Inflammatory Bowel Disease (IBD) requires immunosuppression. Acute management of GI bleeding involves resuscitation and endoscopic hemostasis.

Revision Resources – expand the sections below for high-yield notes, exam pearls, key facts and further reading.

🎯 MLA High-Yield Notes & Quick Revision ⌄
Remember the sites of absorption: 'I Feel Bad' (Iron - Duodenum, Folate - Jejunum, B12 - Ileum). Understand the role of the Vagus nerve in stimulating digestion. Note how PPIs work (irreversible inhibition of the H+/K+ ATPase pump).
Abdominal pain Dysphagia Nausea and vomiting Diarrhoea Constipation Jaundice Malabsorption syndromes Gastro-oesophageal reflux disease (GORD) Peptic ulcer disease
  • GI tract functions: motility, secretion, digestion, absorption.
  • Stomach: HCl (parietal cells), pepsin (chief cells), intrinsic factor (parietal cells).
  • Small intestine: primary site for nutrient absorption; large surface area.
  • Pancreas: alkaline fluid, amylase, lipase, proteases.
  • Liver/Gallbladder: bile production/storage for fat emulsification.
  • ENS: Myenteric (motility), Submucosal (secretion/blood flow).
⭐ Exam Pearls ⌄
⭐ High Yield
Parietal cells secrete HCl and intrinsic factor (for B12 absorption).
The small intestine is the primary site for nutrient absorption due to its large surface area.
Pancreatic enzymes (amylase, lipase, proteases) are crucial for digestion in the small intestine.
Bile, produced by the liver and stored in the gallbladder, emulsifies fats.
The Enteric Nervous System (ENS) independently regulates GI motility and secretion.
Gastrin stimulates HCl secretion; Secretin stimulates bicarbonate release; CCK stimulates gallbladder contraction and pancreatic enzyme release.
💡 Clinical Pearl
Pernicious Anaemia: Results from a lack of intrinsic factor, leading to Vitamin B12 malabsorption.
Peptic Ulcer Disease: Often caused by an imbalance between aggressive factors (HCl, pepsin) and protective factors in the stomach/duodenum.
Coeliac Disease: Immune reaction to gluten causing villous atrophy in the small intestine, impairing nutrient absorption.
Pancreatitis: Inflammation of the pancreas leading to premature activation of digestive enzymes, causing autodigestion.
⚠️ Exam Tip — Common Mistakes
Confusing the roles of different GI hormones (e.g., gastrin vs. secretin vs. CCK).
Underestimating the importance of the enteric nervous system as a 'second brain'.
Misunderstanding the difference between digestion (breakdown) and absorption (uptake).
Forgetting the specific enzymes produced by the pancreas and their substrates.
Not appreciating the massive surface area adaptations in the small intestine for absorption.
🔑 Key Facts ⌄
Parietal cells produce HCl and Intrinsic Factor.
Chief cells produce Pepsinogen.
Iron is absorbed in the duodenum; Folate in the jejunum; B12 in the terminal ileum (iron-duo, folate-jejun, B12-ileum).
The Migrating Motor Complex (MMC) clears debris during the fasting state.
Bile salts undergo enterohepatic circulation (reabsorbed in the ileum).
G-cells in the stomach antrum secrete Gastrin in response to protein and distension.
🔗 Related Topics ⌄
🔬 Endocrine Physiology Foundation Sciences → 🔬 Renal Physiology Foundation Sciences → 🔬 Acid-Base Balance Foundation Sciences → 🔬 Neurophysiology Foundation Sciences → 📚 Pharmacology of the GI system
📚 References ⌄
  1. TeachMePhysiology - The Gastrointestinal System
  2. NICE CKS: Dyspepsia and GORD
  3. GMC MLA Content Map - Gastrointestinal system
  4. Sleisenger and Fordtran's Gastrointestinal and Liver Disease

Further Resources

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