Large Intestine
The large intestine (caecum + colon + rectum + anal canal) spans ~1.5 m from the ileocaecal valve to the anus. It receives ~1.5 L of watery ileal effluent daily and reduces it to ~150 mL of formed stool — recovering ~90% of incoming water. Its defining biological feature is the gut microbiome (~10¹³ organisms, ~1 kg by weight), which ferments indigestible dietary fibre to short-chain fatty acids (butyrate, propionate, acetate) fuelling colonocytes, regulating immune tone via FOXP3+ Tregs, and signalling to the liver and brain. Dysbiosis underlies IBD, CRC, metabolic disease, and neuropsychiatric pathology.
Overview
The large intestine is the terminal segment of the gastrointestinal tract. Unlike the small intestine, its mucosa lacks villi — the absorptive surface consists of flat colonocytes lining straight crypts (crypts of Lieberkühn). Yet it recovers most of the water in ileal effluent, synthesises microbial vitamins (K2, B7, B9), and houses the densest concentration of immune cells in the body.
The colon's defining characteristic is its microbiome: ~10¹³ bacteria (~1 kg by weight), predominantly anaerobes (Firmicutes ~60%, Bacteroidetes ~25%). These organisms transform indigestible dietary fibre and resistant starch into short-chain fatty acids (SCFAs) — butyrate, propionate, and acetate — that serve as the primary colonocyte fuel, modulate systemic inflammation via HDAC inhibition and GPR signalling, regulate epigenetics, and produce signals that reach the liver, brain, and immune system. The large intestine is simultaneously a metabolic factory, an immune organ, and a neuroendocrine interface.
Gross Anatomy
| Segment | Length | Key features |
|---|---|---|
| Caecum | 6–8 cm | Blind pouch receiving ileal contents at ileocaecal valve; appendix (lymphoid organ) attaches posteromedially |
| Ascending colon | 15 cm | Right side; retroperitoneal; hepatic flexure at right |
| Transverse colon | 50 cm | Mobile, intraperitoneal; transverse mesocolon; splenic flexure at left |
| Descending colon | 25 cm | Left side; retroperitoneal |
| Sigmoid colon | 35–40 cm | S-shaped; mobile; commonest site of diverticular disease and volvulus |
| Rectum | 12–15 cm | Follows sacral curve; rectal ampulla; partially extraperitoneal |
| Anal canal | 3–4 cm | Dentate line separates squamous from columnar epithelium; internal + external anal sphincters |
Distinctive wall features: Taeniae coli (three longitudinal smooth muscle bands, shorter than the colon → haustral sacculations), haustra (segmental pouches for slow mixing), and appendices epiploicae (peritoneal fat tags). The haustral pattern is the hallmark of the colon on plain imaging.
Microscopic Structure
The colonic mucosa has no villi. Straight crypts of Lieberkühn (~0.5 mm deep) are lined by a characteristic multi-cellular population:
Surface colonocytes
Absorptive cells with shorter microvilli than enterocytes. Express ENaC (aldosterone-regulated, distal colon), AQP3/AQP8, and NHE3/DRA (electroneutral Na⁺/Cl⁻ co-absorption, proximal colon). Principal water and electrolyte absorbing cells.
Goblet cells
~30% of crypt cells in distal colon. Secrete MUC2 mucin producing a thick bilayer (inner adherent layer — virtually sterile; outer loose layer — inhabited by bacteria). The mucus layer is the primary host-microbiome interface.
Enteroendocrine L-cells
Secrete GLP-1, GLP-2, PYY, and serotonin. GLP-1 drives the incretin effect and appetite suppression — GLP-1 receptor agonists (semaglutide) pharmacologically replicate this. PYY produces the ileal brake (slows transit).
LGR5+ stem cells
At the crypt base. Wnt/Notch/EGF-driven self-renewal generates all colonocyte lineages. LGR5 is also a marker of colorectal cancer stem cells — the seed of CRC relapse and metastatic potential.
Function — SCFA Production and Water Absorption
The two most fundamental colonic functions — microbial fermentation and water recovery — are deeply coupled: SCFAs are absorbed with Na⁺ and water, and their production creates osmotic conditions supporting fluid reabsorption.
Dietary fibre / resistant starch (undigested)
│
▼
Gut microbiome fermentation (~10¹³ bacteria)
┌─────────────────────────────────────────────────────┐
│ Firmicutes (Roseburia, F. prausnitzii, Ruminococcus) │
│ → Butyrate │
│ ├─ Primary colonocyte fuel (~70% of energy) │
│ ├─ HDAC inhibitor → epigenetic anti-inflammatory│
│ ├─ GPR109a → induces colonic FOXP3+ Tregs │
│ └─ TP53 activation → pro-apoptotic in CRC cells │
│ │
│ Bacteroidetes (Bacteroides, Prevotella) │
│ → Propionate → portal vein → hepatic │
│ gluconeogenesis (propionyl-CoA → succinyl-CoA) │
│ │
│ Most bacteria → Acetate → systemic circulation │
│ → crosses BBB → hypothalamic appetite regulation │
└─────────────────────────────────────────────────────┘
│
▼
Colonocyte SCFA + Na⁺ uptake + water (AQP3/AQP8)
→ 1.5 L ileal effluent → ~150 mL formed stool
Electrogenic Na⁺ absorption (distal colon): aldosterone-regulated ENaC drives Na⁺ uptake → water follows via aquaporins and paracellular routes. Secretory diarrhoea (cholera): cholera toxin → irreversible Gs activation → cAMP → PKA → CFTR → massive Cl⁻ secretion → water follows → litres of watery stool per day.
Immune Function
The colonic lamina propria houses the densest concentration of immune cells in the body. The microbiome–immune relationship is bidirectional and calibrates both local and systemic immune tone:
| Cell / Structure | Function |
|---|---|
| IgA plasma cells | Dimeric IgA + secretory component (pIgR transcytosis) → sIgA in lumen; neutralises pathogens without inducing inflammation; most abundant antibody in the gut |
| Tolerogenic macrophages | IL-10-producing; sample luminal bacteria via transepithelial dendrites; resist NF-κB activation; phenotypically altered in IBD |
| Colonic Tregs (FOXP3+) | Induced by butyrate via GPR109a and by commensal Clostridia; suppress excessive responses to microbial antigens; depleted in IBD and CRC |
| Isolated lymphoid follicles | Colonic equivalent of Peyer's patches; M-cell antigen sampling; IgA class switching; germinal centre formation |
| γδ intraepithelial lymphocytes | Stress surveillance of epithelium; cytotoxic toward infected or transformed colonocytes |
Dysbiosis cascade: ↑Proteobacteria + ↓Firmicutes → ↑luminal LPS → TLR4/NF-κB → pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) → mucosal barrier disruption → systemic low-grade inflammation → insulin resistance, metabolic syndrome, IBD, and possibly neuropsychiatric disease.
Gut–Brain Axis and ENS
The colon's enteric nervous system (ENS) contains ~100 million neurons — comparable to the spinal cord — organised as myenteric (Auerbach's) and submucosal (Meissner's) plexuses with interstitial cells of Cajal (ICCs) as pacemakers. Three motor patterns dominate:
Haustral contractions
Rhythmic segmental contractions (3–12/min) mixing colonic contents without net propulsion. Slow transit allows maximal water and SCFA absorption. Prolonged transit time → harder, drier stool (constipation).
Mass movements (HAPCs)
High-amplitude propagating contractions, 1–3 per day, triggered by the gastrocolic reflex (meal → colonic motor response). 5-HT4 receptor-mediated. Propel content from ascending to sigmoid colon in one sweep.
Gut–brain axis: SCFA-derived signals (especially acetate crossing the blood–brain barrier) and tryptophan metabolites (serotonin, indole derivatives from Lactobacillus/Clostridium) signal via the vagus nerve and circulation to the CNS. Gut dysbiosis is associated with depression, anxiety, Parkinson's disease (colonic α-synuclein propagation before CNS disease), and autism spectrum disorder in epidemiological studies.
Pathology
Colorectal Cancer (CRC)
Third most common cancer globally; second most common cancer death. The Vogelstein adenoma-carcinoma sequence: APC mutation (Wnt initiation) → KRAS mutation (growth) → SMAD4/TGF-β loss (progression) → TP53 mutation (carcinoma). MSI pathway (~15%): MMR gene deficiency → hypermutation → neoantigens → immunogenic tumours → responds to pembrolizumab. Lynch syndrome (MLH1/MSH2/MSH6/PMS2 germline mutations) carries 50–80% lifetime CRC risk. Prevention: aspirin reduces risk ~30–40%; colonoscopy removes precursor adenomas.
Ulcerative Colitis (UC)
Chronic relapsing mucosal inflammation from rectum proximally in continuous pattern. Histology: crypt abscesses, crypt distortion, goblet cell depletion — no granulomata (distinguishes from Crohn's). Complications: toxic megacolon (colon >6 cm + systemic toxicity → perforation risk), primary sclerosing cholangitis (4% of UC), CRC risk ~0.5–1%/year after 8–10 years of pan-colitis. Treatment ladder: 5-ASA → steroids → azathioprine/6-MP → anti-TNF (infliximab) / vedolizumab / JAK inhibitors (upadacitinib) → proctocolectomy (curative).
Clostridioides difficile Infection (CDI)
Post-antibiotic disruption of microbiome → C. difficile spores germinate → Toxin A (enterotoxin) + Toxin B (cytotoxin) → NF-κB/MAPK → pseudomembranous colitis (yellow-white fibrin plaques). Risk factors: clindamycin, fluoroquinolones, cephalosporins, PPIs, age >65. Treatment: fidaxomicin (preferred) or oral vancomycin; faecal microbiota transplantation (FMT, ~90% efficacy) for recurrent CDI.
Diverticular Disease
False diverticula (mucosal herniation where blood vessels penetrate muscularis propria) in sigmoid colon. Associated with low-fibre diet → small hard stools → ↑intraluminal pressure. Diverticulitis: faecalith obstruction → bacterial overgrowth → micro-perforation → pericolic abscess, fistula, peritonitis. Diverticular bleeding: most common cause of lower GI haemorrhage; venous, self-limiting in 75%.
Hirschsprung Disease (Congenital Aganglionic Megacolon)
Failure of neural crest cell migration into distal colon → absent ENS ganglia (Auerbach's and Meissner's plexus) → functional intestinal obstruction → proximal megacolon. RET, EDNRB, EDN3 gene mutations. Presents neonatally (failure to pass meconium within 48 h). Diagnosis: suction rectal biopsy (absent ganglion cells, hypertrophied nerve trunks, ↑AChE). Treatment: surgical resection of aganglionic segment with pull-through procedure.
Cross-Atlas Connections
References
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