Page 2 of 9
PY4.1-12 | Gastro-intestinal Physiology — Part 1
CLINICAL SCENARIO
A 45-year-old auto-rickshaw driver arrives at the Government Rajaji Hospital OPD in Puducherry complaining of burning pain in the upper abdomen that worsens 2–3 hours after meals but improves after eating. He has been self-medicating with antacids for six months. His wife adds that he frequently wakes up at night with a sour taste in his mouth.
His symptoms — peptic ulcer disease complicated by gastro-oesophageal reflux — affect an estimated 6–10% of the Indian population. Understanding why the stomach secretes acid, what regulates that secretion, and how that regulation fails in disease states is the foundation of rational pharmacotherapy.
Everything in this module traces back to a single question: How does the gut know what to do, and what happens when it forgets?
WHY THIS MATTERS
Why GI Physiology matters for your clinical practice:
- In India, gastrointestinal diseases account for approximately 20% of all outpatient visits — more than cardiac or respiratory diseases combined in primary care.
- Every prescription you write for a GI drug (antacids, PPIs, pro-kinetics, laxatives, anti-diarrhoeals, antibiotics for H. pylori) demands a precise understanding of the physiology you are targeting.
- PY4.1–PY4.12 form the physiological bedrock for medicine postings in Year 2, where you will examine real patients with abdominal complaints.
- PY4.12 directly prepares you for your first clinical skill: performing an abdominal examination in the Physiology practical.
RECALL
Before we begin, recall what you already know:
From NCERT Biology (Class 11–12):
• The alimentary canal is a muscular tube running mouth → pharynx → oesophagus → stomach → small intestine → large intestine → rectum → anus.
• Digestion = mechanical breakdown + chemical (enzymatic) breakdown.
• Enzymes are biological catalysts that work on specific substrates.
From PY1 (General Physiology): You studied autonomic nervous system control — the enteric nervous system is the ANS component embedded in the GIT wall.
From Anatomy (AN) classes: You should have the 3D map of abdominal organs — stomach in the left hypochondrium, liver in the right, pancreas retroperitoneal. We will add the function to the structure you already know.
Functional Anatomy of the Digestive System (PY4.1)
The digestive system has two categories of organs: those forming the alimentary canal (aliment- = "to nourish") and the accessory organs.
Figure: Functional Anatomy of the Digestive System (PY4.1)
The alimentary canal is a continuous muscular tube roughly 7.6 metres long:
• Mouth → Pharynx → Oesophagus → Stomach → Small intestine (duodenum, jejunum, ileum) → Large intestine (caecum, colon, rectum) → Anal canal
Accessory organs — salivary glands, liver, gallbladder, pancreas — secrete substances into the canal but are not part of the tube itself.
Wall layers (outermost to innermost): Every part of the alimentary canal shares the same 4-layer organisation:
1. Serosa — outer covering (visceral peritoneum)
2. Muscularis externa — outer longitudinal + inner circular smooth muscle (creates movements)
3. Submucosa — connective tissue with blood vessels, Meissner's plexus (submucosal nerve plexus)
4. Mucosa — innermost layer containing Auerbach's plexus (myenteric plexus) between the two muscle layers
The Enteric Nervous System (ENS): Often called the "second brain," the ENS is a network of 500 million neurons embedded in the gut wall. It can function independently of the brain and spinal cord — that is why your gut continues to move even under general anaesthesia. The ENS works through Meissner's (sensory/secretomotor) and Auerbach's (motor, controls peristalsis) plexuses.
GI Hormones — The Chemical Messengers of the Gut (PY4.2)
The Big Four GI Hormones
| Hormone | Source Cells | Location | Stimulus | Main Actions |
|---|---|---|---|---|
| Gastrin | G cells | Gastric antrum | Protein in stomach, vagal stimulation | Increases gastric acid and pepsinogen secretion, gastric motility |
| CCK | I cells | Duodenum, jejunum | Fatty acids, amino acids in duodenum | Gallbladder contraction, pancreatic enzyme secretion, delays gastric emptying |
| Secretin | S cells | Duodenum | Acid (H+) in duodenum | Pancreatic bicarbonate secretion, inhibits gastric acid |
| GIP | K cells | Duodenum, jejunum | Glucose, fat | Stimulates insulin release (incretin effect), inhibits gastric acid |
The Big Four GI Hormones
| Hormone | Source Cells | Location | Stimulus | Main Actions |
|---|---|---|---|---|
| Gastrin | G cells | Gastric antrum | Protein in stomach, vagal stimulation | Increases gastric acid and pepsinogen secretion, gastric motility |
| CCK | I cells | Duodenum, jejunum | Fatty acids, amino acids in duodenum | Gallbladder contraction, pancreatic enzyme secretion, delays gastric emptying |
| Secretin | S cells | Duodenum | Acid (H+) in duodenum | Pancreatic bicarbonate secretion, inhibits gastric acid |
| GIP | K cells | Duodenum, jejunum | Glucose, fat | Stimulates insulin release (incretin effect), inhibits gastric acid |
The gut is the largest endocrine organ in the body. Gastrointestinal hormones are peptides secreted by specialised enteroendocrine cells scattered in the mucosa. They regulate secretion, motility, and appetite.
Figure: GI Hormones — The Chemical Messengers of the Gut (PY4.2)
The Big Four GI Hormones (know these cold):
| Hormone | Secreted by | Stimulus | Main Actions |
|---|---|---|---|
| Gastrin | G cells of gastric antrum | Protein in stomach, vagal stimulation | ↑ Gastric acid, ↑ pepsinogen, ↑ gastric motility |
| Secretin | S cells of duodenum | Acid (low pH) in duodenum | ↑ Pancreatic HCO₃⁻, ↓ gastric acid |
| Cholecystokinin (CCK) | I cells of duodenum/jejunum | Fat and protein in duodenum | ↑ Pancreatic enzymes, ↑ bile release, ↓ gastric emptying |
| GIP (Gastric Inhibitory Peptide) | K cells of duodenum | Fat and carbohydrate | Inhibits gastric acid, stimulates insulin release |
Other important hormones:
• Motilin — stimulates migrating myoelectric complex (MMC), the "housekeeper" contractions that sweep the gut clean between meals (every 90 min). Erythromycin is a motilin agonist used as a prokinetic drug.
• Ghrelin — the "hunger hormone" from the fundus, peaks before meals, stimulates appetite.
• Vasoactive Intestinal Peptide (VIP) — relaxes smooth muscle, stimulates intestinal secretion. VIPoma = watery diarrhoea syndrome.
• Somatostatin — the universal inhibitor: inhibits gastrin, secretin, CCK, insulin, glucagon. Used clinically as octreotide for variceal bleeding.
Regulation pattern: Most GI hormones follow a negative feedback loop — the end-product of their action inhibits further hormone release. Example: Gastrin → acid → low pH → inhibits gastrin release (via somatostatin).
SELF-CHECK — : Anatomy & GI Hormones
Which nerve plexus is responsible for controlling peristaltic movements of the gut?
A. Meissner's plexus (submucosal)
B. Auerbach's plexus (myenteric)
C. Celiac plexus
D. Hypogastric plexus
Reveal Answer
Answer: B. Auerbach's plexus (myenteric)
A patient develops profuse watery diarrhoea and hypokalemia. Imaging shows a pancreatic tumour. The most likely hormone secreted in excess is:
A. Gastrin
B. CCK
C. VIP
D. Motilin
Reveal Answer
Answer: C. VIP
Saliva — Composition, Secretion, Functions, Regulation (PY4.3)
Saliva is produced by three paired glands: parotid (serous, largest — produces amylase-rich watery saliva), submandibular (mixed, contributes ~70% of total volume), and sublingual (mucous).
Figure: Saliva — Composition, Secretion, Functions, Regulation (PY4.3)
Total daily production: 1–1.5 litres.
Composition:
• Water (99.5%)
• Salivary amylase (ptyalin) — digests starch → maltose
• Mucin — lubricates bolus
• Lysozyme, IgA, lactoferrin — antimicrobial defence
• Kallikrein — cleaves kinin, causes vasodilation of salivary gland blood vessels
• Lingual lipase — begins fat digestion in the mouth
Mechanism of secretion: Salivary acini (grape-like clusters) produce the primary secretion (isotonic, plasma-like). As it flows through the striated ducts, Na⁺ is reabsorbed and K⁺ is secreted → final saliva is hypotonic (unlike all other GI secretions, which are isotonic or hypertonic).
Functions of saliva:
1. Lubrication and bolus formation
2. Initiation of starch and fat digestion
3. Solvent action (allows tasting)
4. Antimicrobial protection (IgA, lysozyme)
5. Buffering — bicarbonate maintains oral pH ~6.8
6. Promotes dental health — mineralisation of teeth
Regulation: Saliva is under exclusively neural control (no hormonal control — unlike other GI secretions).
• Unconditioned reflex: Food in mouth → mechano/chemoreceptors → salivatory nuclei (pons and medulla) → secretion.
• Conditioned reflex: Pavlov's dog experiment — sight/smell/thought of food triggers salivation.
• Parasympathetic (chorda tympani → submandibular; glossopharyngeal → parotid): stimulates copious, watery, enzyme-rich saliva.
• Sympathetic (superior cervical ganglion): stimulates small volume, viscous, mucus-rich saliva — "dry mouth with fear."