Large intestine
Intestinum crassum
Also known as: large bowel, colon
The large intestine is the final segment of the gut, about 1.5 m long, running from the ileocaecal junction to the anus. It absorbs water and electrolytes, hosts the fermenting microbiota, and stores and expels faeces.
Overview
The large intestine has four parts: the caecum with the vermiform appendix; the colon, divided into ascending, transverse, descending and sigmoid segments; the rectum; and the anal canal. It forms a frame around the coils of the small intestine. The ascending and descending colon are fixed retroperitoneally. The transverse and sigmoid colon swing freely on mesocolons.
Three surface features identify the caecum and colon. The taeniae coli are three bands formed by the outer longitudinal muscle. The haustra are sacculations that form because the taeniae are shorter than the bowel wall. The omental (epiploic) appendices are small fat-filled peritoneal tags. The rectum and anal canal have none of these features. Inside, there are no villi and no plicae circulares. The mucosa is flat and pitted with straight crypts full of goblet cells, which make lubricating mucus.
Each day about 1.5 L of ileal effluent enters the colon. Absorption of Na⁺ (largely through aldosterone-sensitive ENaC channels distally), Cl⁻ and water reduces this to faeces of about 100–200 g, roughly three-quarters water. The colon can increase its absorption several-fold before diarrhoea results. It secretes K⁺ and HCO₃⁻, which is why diarrhoea causes hypokalaemia and acidosis.
The colon carries the densest microbial community in the body, on the order of 10¹³ bacteria dominated by Bacteroidetes and Firmicutes. These bacteria ferment undigested fibre into short-chain fatty acids. Butyrate is the main fuel for colonocytes, and acetate and propionate are absorbed and used elsewhere. The bacteria also synthesise vitamin K₂ and biotin, deconjugate bile acids and turn bilirubin into urobilinogen and stercobilin, which gives faeces their brown colour.
- Location
Frames the abdomen. The caecum lies in the right iliac fossa, the ascending colon on the right flank, the transverse colon across the upper abdomen, the descending colon on the left flank and the sigmoid colon in the left iliac fossa and pelvis. The rectum and anal canal lie in the pelvis and perineum.
Parts & subdivisions
- Blind pouch below the ileocaecal junction, with the worm-like appendix attached.
- Retroperitoneal; ends at the right colic (hepatic) flexure.
- Intraperitoneal on the transverse mesocolon; ends at the left colic (splenic) flexure.
- Retroperitoneal; descends on the left flank.
- Mobile S-shaped loop on the sigmoid mesocolon.
- Storage and continence apparatus.
- The three features that identify the colon.
Anatomy
- Innervation
Midgut part: vagus (parasympathetic) and lesser and least splanchnic nerves (T10–T12, sympathetic) via the superior mesenteric plexus. Hindgut part: pelvic splanchnic nerves (S2–S4) (parasympathetic) and lumbar splanchnic nerves (L1–L2) via the inferior mesenteric and hypogastric plexuses. Visceral pain is referred to the umbilical region (midgut) or the hypogastrium (hindgut).
- Arterial supply
Midgut part (caecum to the proximal two-thirds of the transverse colon): SMA via the ileocolic, right colic and middle colic arteries. Hindgut part: IMA via the left colic, sigmoid and superior rectal arteries. The marginal artery (of Drummond) links all the colic arteries along the mesenteric border. Watershed zones lie at the splenic flexure (Griffiths point) and the rectosigmoid junction (Sudeck point). The rectum and anal canal also receive the middle and inferior rectal arteries.
- Venous drainage
Superior and inferior mesenteric veins → hepatic portal vein. The lower rectum and anal canal also drain to systemic veins (internal iliac).
- Lymphatic drainage
Epicolic → paracolic → intermediate (along the colic arteries) → superior or inferior mesenteric nodes → para-aortic nodes and cisterna chyli. Spread of cancer follows the arterial pedicles, which is the basis of oncological resections.
- Histology
Flat mucosa with straight crypts rich in goblet cells, absorptive colonocytes and lymphoid follicles. The outer longitudinal muscle is gathered into taeniae except in the rectum. Serosa covers the intraperitoneal parts and adventitia the retroperitoneal surfaces.
- Development
The caecum, appendix, ascending colon and proximal two-thirds of the transverse colon come from the midgut. The rest, down to the pectinate line, comes from the hindgut. The caecal bud appears in week 6 and is the last part to return to the abdomen, descending from below the liver into the right iliac fossa.
Function
- Absorbs water (about 1.3–1.4 L/day) and Na⁺ and Cl⁻; secretes K⁺, HCO₃⁻ and mucus.
- Microbial fermentation of fibre into short-chain fatty acids; synthesis of vitamin K₂ and biotin.
- Forms, stores and propels faeces (haustral churning, mass movements).
- Defecation and continence (rectum and anal canal).
Clinical significance
Hirschsprung disease (congenital aganglionic megacolon)
Neural crest cells fail to migrate all the way, so a segment of distal bowel (always including the rectum) has no ganglion cells in either the myenteric or the submucosal plexus. That segment stays tonically contracted, and the bowel upstream dilates. Most infants fail to pass meconium within 48 hours, and they develop distension, bilious vomiting and the risk of enterocolitis. It is associated with RET mutations and Down syndrome. Anorectal manometry shows no rectoanal inhibitory reflex. Diagnosis is by rectal suction biopsy (no ganglion cells, thickened nerve trunks, increased acetylcholinesterase staining). Treatment is a pull-through resection.
Diverticular disease
Acquired false (pulsion) diverticula form where the mucosa and submucosa herniate through the muscle at points where the vasa recta penetrate. They are most common in the sigmoid colon and are linked to a low-fibre diet and high segmental pressures. Complications are diverticulitis (left iliac fossa pain and fever), abscess, perforation, fistula (colovesical fistula causes pneumaturia) and painless bleeding.
Ischaemic colitis
Low flow affects the watershed areas, the splenic flexure and the rectosigmoid. It occurs in elderly patients with vascular disease, after aortic surgery that ligates the IMA, or in shock. Patients have crampy left-sided pain followed by bloody diarrhoea, and imaging shows "thumbprinting".
Mnemonics
Identifying colon: THE colon
Taeniae coli · Haustra · Epiploic (omental) appendices. None of them are found on the small bowel or the rectum.
Test yourself
Which organ is intraperitoneal (NOT retroperitoneal)?
- Descending colon
- Head of the pancreas
- Transverse colon
- Second part of the duodenum
Answer: Transverse colon
The transverse colon hangs on the transverse mesocolon and is intraperitoneal. The other three are secondarily retroperitoneal (SAD PUCKER).
The superior mesenteric artery supplies the gut from:
- The lower oesophagus to the major duodenal papilla
- The major duodenal papilla to the proximal two-thirds of the transverse colon
- The distal third of the transverse colon to the pectinate line
- The jejunum to the sigmoid colon
Answer: The major duodenal papilla to the proximal two-thirds of the transverse colon
The SMA (L1) supplies the midgut, from just distal to the major papilla to the proximal two-thirds of the transverse colon. The coeliac trunk supplies the foregut, and the IMA supplies the hindgut down to the pectinate line.
Hirschsprung disease results from:
- Failure of neural crest cells to colonise the distal bowel, leaving it aganglionic
- Hypertrophy of the internal anal sphincter
- Autoimmune destruction of interstitial cells of Cajal
- Failure of recanalisation of the rectum
Answer: Failure of neural crest cells to colonise the distal bowel, leaving it aganglionic
Neural crest cells fail to migrate all the way, so the distal colon and rectum lack ganglion cells in both the myenteric and submucosal plexuses. That segment cannot relax, causing functional obstruction with delayed passage of meconium. It is associated with RET mutations and Down syndrome.
Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.