Anatomy of Large Bowel
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🧩 Sections of the Large Intestine
- Caecum: First part, in the right iliac fossa. Connects to the ileum via the ileocecal valve, regulating chyme entry.
- Appendix: Worm-like projection of the caecum; contains lymphoid tissue → potential immune role.
📌 Clinical: site of acute appendicitis.
- Ascending Colon: Runs upward on the right side → hepatic flexure (near liver).
- Transverse Colon: Runs across abdomen (right → left) → splenic flexure (near spleen). Highly mobile. 🌀
- Descending Colon: Travels down left abdomen → sigmoid colon.
- Sigmoid Colon: S-shaped loop storing faeces until defecation. 📌 Common site for diverticulitis.
- Rectum: Stores faeces before elimination; ends at anorectal junction.
- Anus: Terminal GI tract opening; controlled by internal sphincter (involuntary, smooth muscle) & external sphincter (voluntary, skeletal muscle).
⚙️ Physiology of the Large Intestine
The large intestine is essential for 💧 water & electrolyte absorption, 💨 bacterial fermentation, 💩 faeces formation, and 🛡️ immune protection.
🔑 Functions
- 💧 Absorption: Reabsorbs water & electrolytes (mainly Na⁺, Cl⁻). ~90% of water is absorbed → solidifies faeces.
- 🦠 Bacterial Fermentation: Microbiota ferment fibre → produce gases + SCFAs (acetate, butyrate, propionate).
👉 SCFAs = energy source for colonocytes + systemic metabolism.
- 💩 Formation & Storage: Indigestible residue compacted → faeces, stored in rectum until defecation.
- 🧴 Mucus Secretion: Goblet cells lubricate faeces & protect mucosa.
- 🛡️ Immune Function: Gut-associated lymphoid tissue (GALT) & microbiota defend against pathogens.
🚽 Defecation
Defecation = rectal distension + sphincter control + peristaltic waves → expulsion of faeces.
- 📈 Rectal Distension: Stretch receptors trigger defecation reflex.
- 🚪 Anal Sphincters:
– Internal sphincter (involuntary) relaxes
– External sphincter (voluntary) allows conscious control.
- 🌊 Peristalsis: Strong waves + relaxation of sphincters = faecal expulsion.
🩸 Blood Supply
- Arterial Supply:
– Superior mesenteric artery (SMA): caecum → proximal 2/3 of transverse colon.
– Inferior mesenteric artery (IMA): distal 1/3 transverse colon → rectum.
- Venous Drainage:
– Superior & inferior mesenteric veins → hepatic portal vein → liver for processing.
🧠 Nerve Supply
- Sympathetic (↓ motility): via superior & inferior mesenteric plexuses → reduces motility/secretion, contracts internal sphincter.
- Parasympathetic (↑ motility):
– Vagus nerve: proximal colon.
– Pelvic splanchnics (S2–S4): distal colon, rectum, anus.
📌 Key for defecation reflex.
⚠️ Clinical Relevance
- 🚫 Constipation: Slow transit → ↑ water absorption → hard stools.
- 💦 Diarrhoea: Rapid transit/secretion → poor water reabsorption.
- 🎗️ Colon Cancer: Common malignancy, esp. sigmoid & rectum → screened by colonoscopy.
- 🔥 IBD: Crohn’s disease & ulcerative colitis → chronic inflammation, diarrhoea, pain, bleeding.
- 📌 Diverticulitis: Inflamed pouches, esp. sigmoid colon → pain, fever, altered bowel habits.
📊 Quick Teaching Table
| Section |
Key Feature |
Blood Supply |
Clinical Note |
| Caecum |
Ileocecal valve + appendix |
SMA |
Appendicitis |
| Ascending |
Right-sided, hepatic flexure |
SMA |
Right-sided colon cancer → occult bleed |
| Transverse |
Mobile, longest part |
SMA (proximal 2/3), IMA (distal 1/3) |
Watershed zone (ischaemia risk) |
| Descending |
Left-sided, splenic flexure |
IMA |
Splenic flexure = watershed area (ischaemia) |
| Sigmoid |
S-shaped, faecal storage |
IMA |
Diverticulitis common |
| Rectum |
Faecal storage, anal sphincters |
IMA + internal iliac branches |
Haemorrhoids, cancer |
