Gastrointestinal Physiology

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Gastrointestinal Physiology — Key Facts for NEET PG

• GI secretions: Saliva (~1.5 L/day), gastric juice (~2 L/day), pancreatic juice (~1.5 L/day), bile (~1 L/day)
• GI hormones: Gastrin (stomach), secretin (duodenum), CCK (duodenum), GIP (duodenum)
• Migrating motor complex (MMC): Cycle every 90–120 min during fasting — “housekeeper of gut”
• Parasympathetic: Increases GI activity; Sympathetic: Decreases GI activity
• ⚡ Exam tip: Secretin is released by duodenal S cells in response to acid → stimulates pancreatic HCO₃⁻ secretion

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Gastrointestinal Physiology — NEET PG Study Guide

GI Tract Organization

Layers (wall from inside out):

1. Mucosa: Epithelium + lamina propria + muscularis mucosae
2. Submucosa: Dense connective tissue, Meissner’s plexus
3. Muscularis externa: Circular + longitudinal muscles, Auerbach’s plexus
4. Serosa: Visceral peritoneum (adventitia where retroperitoneal)

Enteric Nervous System:

• Meissner’s plexus (submucosal): Controls glandular secretion
• Auerbach’s plexus (myenteric): Controls motility
• Operates independently (can function without CNS input)

⚡ Exam tip: Hirschsprung disease = absence of ganglion cells (aganglionic megacolon) — Meissner and Auerbach plexuses missing

Salivary Secretion

Salivary Glands: Parotid, submandibular, sublingual, minor glands

Composition:

• α-amylase (ptyalin): Digests starch
• Lingual lipase: Digests fats
• Mucin (mucus): Lubrication, protection
• Lysozyme: Antibacterial
• IgA: Immune protection
• Electrolytes: Na⁺, K⁺, Cl⁻, HCO₃⁻ (but hypotonic to plasma)

Control:

• Parasympathetic: Primary stimulus for salivation
  • Chorda tympani (CN VII) → submandibular, sublingual
  • Glossopharyngeal (CN IX) → parotid
• Sympathetic: Reduces salivation (thick, viscous secretion)
• Conditioned reflex: Sight, smell, thought of food

⚡ Exam tip: Bell’s palsy (CN VII lesion) → decreased salivation from submandibular and sublingual glands; taste may also be affected on anterior 2/3 of tongue

Gastric Secretion

Gastric Glands contain:

• Parietal (oxyntic) cells: HCl + intrinsic factor
• Chief (peptic) cells: Pepsinogen
• G cells: Gastrin
• D cells: Somatostatin
• Enterochromaffin-like (ECL) cells: Histamine

HCl Secretion Mechanism (parietal cells):

1. CO₂ + H₂O → H₂CO₃ (carbonic anhydrase)
2. H₂CO₃ → H⁺ + HCO₃⁻
3. H⁺ secreted into lumen via H⁺/K⁺-ATPase
4. HCO₃⁻ exchanged for Cl⁻ via basolateral Cl⁻/HCO₃⁻ exchanger
5. Cl⁻ secreted via Cl⁻ channels → lumen

Gastric Phases:

Phase	Trigger	Secretion
Cephalic	Sight, smell, taste of food	Moderate acid, enzyme
Gastric	Food in stomach (distension, peptides)	High acid, enzyme
Intestinal	Chyme in duodenum	Inhibits gastric secretion

Factors Regulating Gastric Acid:

• Stimulate: ACh, gastrin, histamine (H₂), vagal stimulation
• Inhibit: Somatostatin, prostaglandins, secretin, GIP, duodenal acid, low pH

⚡ Exam tip: H₂ blockers (cimetidine) block histamine receptors → ↓ acid; Proton pump inhibitors (omeprazole) block H⁺/K⁺-ATPase → ↓↓ acid

Pancreatic Secretion

Composition:

• Enzymes (acinar cells):
  • Proteases: Trypsinogen, chymotrypsinogen, procarboxypeptidase
  • Amylase: Digests carbohydrates
  • Lipase: Digests fats (with colipase)
  • Nucleases: Digests nucleic acids
• Bicarbonate (duct cells): Neutralizes gastric acid in duodenum

Pancreatic Duct Cells:

• HCO₃⁻ secretion via Cl⁻/HCO₃⁻ exchanger (CFTR)
• High pH (~8) pancreatic juice

Control of Pancreatic Secretion:

Stimulus	Mechanism
Secretin	↑ HCO₃⁻ secretion (aqueous component)
CCK	↑ enzyme secretion
ACh	↑ enzyme secretion
Vagal stimulation	↑ enzyme secretion

⚡ Exam tip: Acute pancreatitis = premature activation of trypsinogen → trypsin → autodigestion of pancreas

Bile Secretion

Bile Composition:

• Bile acids/salts: Emulsify fats
• Bilirubin: Waste product from hemoglobin breakdown
• Cholesterol: Excreted in bile
• Phospholipids: Lecithin
• Bile: ~500–600 mL/day

Bile Acids:

• Primary: Cholic acid, chenodeoxycholic acid (from cholesterol)
• Secondary: Deoxycholic acid, lithocholic acid (modified by bacteria)
• Enterohepatic circulation: Bile acids recycled (95% reabsorbed, 5% lost)

Gallbladder:

• Concentrates and stores bile
• Releases bile in response to CCK
• Absorbs water and electrolytes

Intestinal Absorption

Carbohydrate Absorption:

• Monosaccharides: Glucose, galactose (active transport via SGLT1 with Na⁺)
• Fructose (facilitated diffusion via GLUT5)

Protein Absorption:

• Amino acids and small peptides
• Na⁺-dependent cotransport
• Some peptides absorbed intact (insulin-like)

Fat Absorption:

• Bile salts emulsify fats → micelles
• Micelles transport fatty acids, monoglycerides, cholesterol to brush border
• Reassembled into chylomicrons → lacteals → thoracic duct

Fat-Soluble Vitamins: A, D, E, K (absorbed with fat)

Water and Electrolytes:

• Na⁺: Absorbed throughout SI (coupled with glucose, amino acids, ions)
• Cl⁻: Absorbed in exchange for HCO₃⁻
• Iron: Absorbed in duodenum (requires Fe²⁺; vitamin C enhances absorption)
• Calcium: Absorbed in duodenum and proximal jejunum (vitamin D enhances)

⚡ Exam tip: Thiamine (B1) deficiency → Wernicke-Korsakoff (CNS) + beriberi (CV); Folate deficiency → megaloblastic anemia, neural tube defects

GI Motility

Mouth and Esophagus:

• Deglutition: Voluntary → pharyngeal (involuntary) → esophageal
• UES (cricopharyngeus): Prevents air swallowing
• LES (lower esophageal sphincter): Prevents reflux
• Primary peristalsis: Swallowing-induced
• Secondary peristalsis: Distension-induced

Stomach Motility:

• Pacemaker: Interstitial cells of Cajal (ICC) — slow waves at 3/min (human)
• Types: Peristalsis (antrum), retropulsion, receptive relaxation
• Gastric emptying: 4–5 hours for solid meal; controlled by duodenum

Intestinal Motility:

• Segmentation: Rhythmic contraction of circular muscle (mixing)
• Peristalsis: Propulsion (moving contents forward)
• Migrating Motor Complex (MMC): Between meals; clears residual contents
• Mass movements: Large scale propulsion (colon); triggered by gastrocolic reflex

⚡ Exam tip: Achalasia cardia = failure of LES relaxation + aperistalsis (CN X dysfunction); Hirschsprung = aganglionic segment

GI Hormones

Hormone	Source	Stimulus	Main Action
Gastrin	G cells (stomach)	Peptides, distension	↑ HCl, growth of gastric mucosa
Secretin	S cells (duodenum)	Acid in duodenum	↑ pancreatic HCO₃⁻
CCK	I cells (duodenum)	Fats, peptides	↑ pancreatic enzymes, gallbladder contraction
GIP	K cells (duodenum)	Glucose, fats	Insulin release (incretin effect), ↓ gastric acid
Motilin	M cells (duodenum)	Fasting	Stimulates MMC, hunger signal
Somatostatin	D cells (stomach)	Acid	Inhibits gastrin and HCl

⚡ Exam tip: Zollinger-Ellison syndrome = gastrinoma → excessive gastric acid → peptic ulcers; VIPoma = watery diarrhea, hypokalemia, achlorhydria

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Gastrointestinal Physiology — Comprehensive NEET PG Notes

Detailed Stomach Physiology

Gastric Secretion Phases:

Cephalic Phase (20–30% of total):

• Conditioned reflex via vagus nerve
• ACh released at nerve endings → stimulates parietal cells directly + stimulates G cells → gastrin → HCl

Gastric Phase (60–70% of total):

• Distension → vago-vagal reflexes
• Peptides/amino acids → direct stimulation of G cells
• Longest phase

Intestinal Phase (5–10% of total):

• Initially stimulates gastric secretion (enterogastric reflex)
• Later inhibits gastric secretion (enterogastric inhibition)
• Acid in duodenum → secretin → inhibits gastrin release

Parietal Cell Regulation:

• Stimuli: ACh (M3 receptors), gastrin (CCK-B receptors), histamine (H2 receptors)
• All converge on increasing [Ca²⁺]i and cAMP
• H₂ blockers: Cimetidine, ranitidine
• PPI: Omeprazole, esomeprazole (covalent inhibition)

Intrinsic Factor:

• Secreted by parietal cells
• Binds vitamin B12 in stomach
• Required for B12 absorption in terminal ileum (cubilin receptor)
• Deficiency → pernicious anemia (megaloblastic, neurological features)

⚡ Exam tip: Autoimmune destruction of parietal cells → achlorhydria + intrinsic factor deficiency → B12 deficiency → subacute combined degeneration of spinal cord

Detailed Pancreatic Physiology

Pancreatic Acinar Cells:

• Synthesize and secrete digestive enzymes
• Enzymes stored as zymogen granules
• Secretion triggered by CCK, ACh
• Pancreatic proteases: Trypsin is the master enzyme
  • Trypsinogen → enterokinase (duodenal) → trypsin → activates other proteases
  • Acute pancreatitis: Trypsinogen activated prematurely → autodigestion

Enterokinase (duodenal brush border):

• Secretin-stimulated cholinergic mechanisms
• Membrane-bound enzyme
• Cleaves trypsinogen → trypsin

Pancreatic Ductal Secretion:

• CFTR mutations → cystic fibrosis → thick secretions → pancreatic insufficiency
• Pancreatic juice bicarbonate concentration can reach 150 mEq/L
• Neutralizes gastric acid (pH ~8)

Detailed Bile Physiology

Bile Salt Function:

1. Emulsify large fat globules into smaller droplets
2. Form micelles for fat transport to brush border
3. Activate pancreatic lipase
4. Solubilize cholesterol

Enterohepatic Circulation:

• Bile salts reabsorbed from terminal ileum
• Portal vein → liver → resecreted into bile
• Loss of bile salts: Diarrhea, steatorrhea
• Primary bile acids: Synthesized from cholesterol in liver
• Secondary bile acids: Modified by colonic bacteria

Gallstone Formation:

• Cholesterol stones (80%): Supersaturation of bile with cholesterol
• Pigment stones (20%): Calcium bilirubinate (hemolytic conditions)
• Risk factors: Obesity, female, 40+, fertile, rapid weight loss

Detailed Intestinal Absorption

Carbohydrate Digestion:

• Salivary amylase → starch → maltose, maltotriose, α-dextrins
• Pancreatic amylase → further breakdown
• Brush border enzymes → monosaccharides
• Lactase deficiency → lactose intolerance (bacterial fermentation → gas, diarrhea)

Protein Digestion:

• Pepsin (stomach) → proteases, peptones
• Pancreatic proteases → oligopeptides, amino acids
• Brush border peptidases → amino acids, dipeptides, tripeptides
• Most absorbed as amino acids or small peptides

Fat Digestion:

• Emulsification: Bile salts (amphipathic molecules)
• Micelle formation: Bile salts + fat digestion products
• Absorption: Monoglycerides, fatty acids diffuse into enterocytes
• Chylomicron formation: Reassembled with apolipoprotein B in enterocytes
• Transport: Lacteals → thoracic duct → systemic circulation

Iron Absorption:

• Duodenum and proximal jejunum
• Fe³⁺ → reduced to Fe²⁺ by ascorbic acid (vitamin C)
• Fe²⁺ absorbed via DMT1 transporter
• Ferroportin exports iron from enterocytes
• Hepcidin regulates ferroportin (↓ in iron deficiency)

⚡ Exam tip: Hemochromatosis → iron accumulation → liver cirrhosis, diabetes, bronze skin; In thalassemia, iron absorption is increased → secondary iron overload

GI Immune System

GALT (Gut-Associated Lymphoid Tissue):

• Peyer’s patches (ileum) — M cells sample antigens
• Tonsils, appendix
• Solitary lymphoid follicles

Secretory IgA:

• Dimeric IgA produced by plasma cells in lamina propria
• Transported across epithelium by poly-Ig receptor (pIgR)
• Secretory piece protects IgA from proteolysis
• Function: Prevents pathogen attachment, neutralizes toxins

Control of GI Function

Enteric Nervous System:

• Can function independently
• Neurotransmitters: ACh (excitatory), VIP, NO (inhibitory)
• Programs: Peristalsis reflex, secretion reflex, inhibitory reflexes

Vagal Afferents:

• Chemoceptors and mechanoreceptors in GI wall
• Transmit to nucleus tractus solitarius (NTS)
• Subconscious control

Gastroileal Reflex: Stomach distension → accelerates ileal emptying

Gastrocolic Reflex: Stomach distension → mass movement in colon → urge to defecate

Defecation Reflex:

• Fecal distension of rectum → internal anal sphincter relaxes (involuntary)
• External anal sphincter contracted (voluntary control)
• If socially acceptable → external sphincter relaxes → defecation

GI Clinical Correlations

Peptic Ulcer Disease:

• H. pylori (60–70%):Produces urease → NH₃ → damages mucosa
• NSAIDs: Inhibit COX → ↓ prostaglandins → ↓ mucus/HCO₃⁻ secretion
• Acid hypersecretion: Zollinger-Ellison, antral G cell hyperfunction

Malabsorption Syndromes:

Syndrome	Defect	Features
Celiac disease	Anti-gluten antibodies	Villous atrophy, steatorrhea
Lactose intolerance	Lactase deficiency	Bloating, diarrhea after dairy
Tropical sprue	Unknown	Folate deficiency, B12 deficiency
Whipple disease	Tropheryma whipplei	PAS-positive macrophages

Short bowel syndrome: Reduced absorptive area → diarrhea, malnutrition

Practice Questions for NEET PG

1. Describe the phases of gastric secretion and their regulation.
2. Explain the mechanism of HCl secretion by parietal cells.
3. What is the role of CCK and secretin in pancreatic secretion?
4. Describe fat absorption and chylomicron formation.
5. Compare the enteric nervous system with the autonomic nervous system in GI control.
6. A patient with gallstones develops jaundice. Explain the mechanism.
7. Describe the mechanism of the defecation reflex.

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