Makindo Medical Notes

Related Subjects: |Anatomy of the Ear |Anatomy of the Oesophagus |Anatomy of the Diaphragm |Anatomy of Large Bowel |Anatomy of Small Bowel |Anatomy of the Biliary system |Anatomy of the Eye |Anatomy of the Larynx |Anatomy of the Ear |Anatomy of Male Genitalia

🧩 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