Gastrointestinal Anatomy · Surgery

Large Intestine Parts

The large intestine is 1.5 m long and runs from the ileocaecal valve to the anal canal. Its ten segments have distinct anatomy, and knowing them enables accurate localisation of pathology.

✦ The Mnemonic

"Can Anyone Help Stop All Rectal Problems Resolving Anally"

Caecum · Appendix · Ascending · Hepatic flexure · Transverse · Splenic flexure · Descending · Sigmoid · Rectum · Anal canal

Ca Caecum Blind-ending sac; ileocaecal valve prevents reflux; most mobile part — can volvulate
Ap Appendix Appendix from posteromedial caecum; base at McBurney's point; lymphoid tissue
AC Ascending Colon Retroperitoneal; hepatic flexure at level of 9th rib
HF Hepatic Flexure Right colic flexure; anterior to right kidney; supplied by SMA
TC Transverse Colon Most mobile; has mesentery; watershed between SMA and IMA
SF Splenic Flexure Left colic flexure; supported by phrenocolic ligament; supplied by IMA
DC Descending Colon Retroperitoneal; IMA territory
SC Sigmoid Colon Mobile; mesentery; most common site of diverticular disease and volvulus
Re Rectum 12 cm; no mesentery; three Houston's valves; extraperitoneal below peritoneal reflection
An Anal Canal 4 cm; dentate line divides squamous from columnar; internal + external sphincters

📚 Clinical Breakdown

Distinguishing features of colon vs small bowel: colon has haustra (sacculations from taeniae coli), taeniae coli (three longitudinal muscle bands), and appendices epiploicae (fat tags). Small bowel has circular folds (valvulae conniventes) that cross the entire lumen.

The dentate line in the anal canal is the embryological boundary between endoderm (above) and ectoderm (below). Above: columnar epithelium, autonomic innervation (no pain), superior rectal venous drainage. Below: squamous epithelium, somatic innervation (pain), inferior rectal venous drainage. This distinction explains why internal haemorrhoids are painless and external haemorrhoids are painful.

Colorectal cancer predominantly affects the rectum and sigmoid (60%). The left colon (descending and sigmoid) presents with altered bowel habit and rectal bleeding; the right colon (ascending, caecum) presents insidiously with iron-deficiency anaemia and weight loss due to blood loss into the large lumen.

Most common volvulus site Sigmoid colon (then caecum)
Dentate line above Columnar, autonomic — painless haemorrhoids
Dentate line below Squamous, somatic — painful haemorrhoids
CRC most common site Rectum + sigmoid

⭐ Clinical Pearl

McBurney's point: the surface marking of the appendix base — one-third of the way from the ASIS to the umbilicus on the right side. Maximal tenderness here, along with Rovsing's sign (palpation of the left iliac fossa produces pain in the right), supports the diagnosis of appendicitis.

← Small Intestine Parts