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Anatomy & Physiology of the Gastrointestinal (GI) Tract

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1. Overview

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2. Layers of the GI Wall

Enteric Nervous System (ENS) Plexuses

• Myenteric plexus (Auerbach's) — between circular and longitudinal muscle layers → controls motility
• Submucosal plexus (Meissner's) — in the submucosa → controls secretion and absorption
• A deep muscular plexus is also embedded within the circular muscle layer

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3. Organs of the GI Tract

A. Mouth & Oropharynx

• Function: Mastication (chewing), initial carbohydrate digestion
• Salivary glands (parotid, submandibular, sublingual) secrete salivary amylase (ptyalin) → breaks starch → maltose
• Mucus lubricates the food bolus
• Deglutition (swallowing) has 3 phases: voluntary (oral), involuntary pharyngeal, esophageal

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B. Esophagus

• Length: ~25 cm; runs from cricopharyngeus to gastroesophageal junction
• Layers: Mucosa (stratified squamous epithelium), submucosa, muscularis (upper 1/3 skeletal, lower 2/3 smooth), adventitia (no serosa)
• Sphincters:
  • Upper esophageal sphincter (UES) — cricopharyngeus muscle, prevents reflux into pharynx
  • Lower esophageal sphincter (LES) — prevents gastric reflux; resting tone ~15–25 mmHg
• Function: Propels food via primary peristalsis (triggered by swallowing); secondary peristalsis clears residual food

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C. Stomach

Anatomy

• Rugae = mucosal folds that allow distension (capacity ~1–1.5 L)
• Gastric pits open into gastric glands

Gastric Secretion

• Principal secretory products: HCl (from parietal cells) and pepsinogen (from chief cells)
• Functions of gastric acid:
  • Protein digestion (activates pepsinogen → pepsin)
  • Absorption of Fe²⁺, Ca²⁺, Mg²⁺, vitamin B₁₂
  • Bactericidal activity

Phases of Gastric Acid Secretion

• Basal acid secretion: follows a circadian rhythm — highest at night, lowest in the morning; driven by vagal and histaminergic tone
• Inhibition: secretin (acid in duodenum), CCK, GIP → enterogastric reflex

Cell Types in Gastric Glands

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D. Small Intestine

Divisions

Mucosal Specializations (Increase Surface Area ~600×)

• Plicae circulares (valves of Kerckring) — circular folds of mucosa
• Villi — finger-like projections; each has lacteals (lymph) and capillaries
• Microvilli (brush border) — on enterocytes; contain digestive enzymes

Secretions

• Secretin (S-cells) — released by acid in duodenum → stimulates pancreatic bicarbonate
• CCK (I-cells) — released by fats/proteins → stimulates pancreatic enzymes + gallbladder contraction
• Brush border enzymes: lactase, sucrase, maltase, enterokinase (activates trypsinogen)

Absorption Summary

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E. Large Intestine (Colon)

Divisions

• Cecum + Appendix (vermiform)
• Ascending colon → Transverse colon → Descending colon → Sigmoid colon
• Rectum (~15 cm) → Anal canal (~4 cm)

Anatomical Features

• Haustra — sacculations due to incomplete longitudinal muscle bands (taeniae coli)
• Epiploic appendages — fatty appendages on outer surface
• No villi — mucosa has crypts (Lieberkühn) only; lined by columnar cells + goblet cells

Physiology

• Water & electrolyte absorption: absorbs ~1.5 L/day of water; Na⁺ absorbed, K⁺ secreted
• Bacterial fermentation: gut microbiota ferments undigested carbohydrates → short-chain fatty acids (SCFAs: butyrate, propionate, acetate) → energy source for colonocytes
• Motility: haustral churning (slow mixing), mass movements (3–4×/day, especially after meals — gastrocolic reflex)
• Defecation reflex: rectal distension → internal anal sphincter relaxes (involuntary) → external anal sphincter (voluntary) relaxes

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F. Rectum & Anal Canal

• Rectum: storage reservoir; highly distensible
• Internal anal sphincter: smooth muscle, involuntary
• External anal sphincter: skeletal muscle, voluntary (pudendal nerve)
• Anorectal angle: maintained by puborectalis muscle; important for continence

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4. Accessory Digestive Organs

A. Liver

• Largest internal organ (~1.5 kg); right + left lobes; functional unit = hepatic lobule (portal triad: portal vein + hepatic artery + bile duct)
• Functions:
  • Bile synthesis (500–1000 mL/day) → emulsification of fats
  • Metabolism of carbohydrates, proteins, lipids
  • Detoxification, drug metabolism (CYP450)
  • Protein synthesis (albumin, clotting factors II, VII, IX, X)
  • Storage of glycogen, vitamins A, D, B₁₂, iron

B. Gallbladder

• Stores and concentrates bile (~10× concentration)
• Bile: bile salts + bilirubin + cholesterol + phospholipids
• Released in response to CCK
• Enterohepatic circulation: bile salts reabsorbed at terminal ileum → portal vein → liver → re-secreted

C. Pancreas

• Exocrine (99% of mass): acini secrete digestive enzymes
  • Proteases: trypsinogen, chymotrypsinogen, elastase (secreted as zymogens)
  • Lipase, amylase, phospholipase
  • Stimuli: CCK → enzymes; Secretin → bicarbonate-rich fluid (neutralizes duodenal acid)
• Endocrine (Islets of Langerhans):
  • α-cells: glucagon
  • β-cells: insulin
  • δ-cells: somatostatin

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5. GI Motility Overview

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6. GI Hormones Summary

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7. GI Blood Supply

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8. Nerve Supply

• Parasympathetic: Vagus nerve (CN X) — foregut + midgut; Pelvic splanchnic nerves (S2–S4) — hindgut → stimulates motility + secretion
• Sympathetic: Splanchnic nerves (T5–L2) → celiac, superior & inferior mesenteric ganglia → inhibits motility, causes vasoconstriction
• ENS (enteric nervous system): "second brain" — can function independently; ~500 million neurons; uses ACh, NO, VIP, serotonin (5-HT3/4 important targets for drugs)

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Key Takeaways

1. The GI wall has 4 layers; the ENS (Auerbach's + Meissner's plexuses) governs autonomous gut function
2. Gastric acid secretion has 3 phases: cephalic, gastric, intestinal; parietal cells secrete HCl via H⁺/K⁺-ATPase
3. The small intestine is the primary site for digestion and absorption; surface area is amplified ~600× by folds, villi, microvilli
4. The colon mainly absorbs water, electrolytes, and produces SCFAs via bacterial fermentation
5. GI hormones (gastrin, secretin, CCK, GIP, motilin) coordinate digestion in a highly regulated sequence
6. The liver, gallbladder, and pancreas are essential accessory organs; bile and pancreatic enzymes are critical for fat and protein digestion