Gastrointestinal tract anatomy

Stomach

• Parts: cardia, fundus and body (parietal cells making acid and intrinsic factor, chief cells), antrum (G cells making gastrin), pylorus at about L1 (transpyloric plane).
• Stomach bed (behind the lesser sac): pancreas, left kidney and adrenal, spleen, splenic artery, transverse mesocolon.
• Arteries: left gastric (coeliac; lesser curve ulcer bleed), right gastric, right gastroepiploic (from the gastroduodenal), left gastroepiploic and short gastrics (splenic). Rich anastomoses on both curves protect it from ischaemia. A lesser curve ulcer (commonest site near the incisura) bleeds from the left gastric artery; a posterior wall ulcer perforates into the lesser sac and may erode the splenic artery.
• Gastric MALT lymphoma is driven by *H. pylori* and may regress after eradication.
• Endoscopic ultrasound shows five layers: bright mucosal interface, dark deep mucosa, bright submucosa, dark muscularis propria (4th layer, origin of GIST), bright serosa.
• Benign ulcer on barium: crater projects outside the gastric outline, thin lucent line across its neck (Hampton line), smooth folds radiating to the edge.
• Organoaxial volvulus (with a paraoesophageal hernia): greater curve flips above the lesser curve.
• Hypertrophic pyloric stenosis on ultrasound: muscle wall over 3 mm thick, channel elongated.
• Ménétrier disease: giant folds in fundus and body, antrum spared, protein loss.

Duodenum

• C-shaped loop round the head of the pancreas, from pylorus (L1) to the duodenojejunal flexure (L2), held by the ligament of Treitz (divides upper from lower GI bleeding).
• First 2–2.5 cm (duodenal cap) is intraperitoneal: an anterior perforation gives gas under the diaphragm. The rest is retroperitoneal: a rupture after a handlebar or steering-wheel blow (D2–D3 junction, crushed against the spine) shows gas outlining the right kidney and psoas, and an intramural haematoma gives a coiled-spring pattern.
• A posterior wall ulcer of D1 erodes the gastroduodenal artery behind it, causing major haematemesis; an anterior D1 ulcer perforates, giving free gas. The bile duct opens into D2 at the major papilla.
• D3 crosses between the aorta behind and the SMA and SMV in front. Loss of mesenteric fat after rapid weight loss narrows this angle (SMA syndrome); the same angle compresses the left renal vein (nutcracker).
• Neonate: the normal DJ flexure lies left of the left pedicle at the level of the duodenal bulb. A flexure on the right, a corkscrew duodenum, an SMV lying left of the SMA (normally right) or a clockwise whirlpool of mesentery round the SMA means malrotation with midgut volvulus. At surgery for malrotation, Ladd bands run from the malpositioned caecum across D2 to the posterior abdominal wall and can obstruct the duodenum even without volvulus. Duodenal atresia: double bubble, no distal gas (Down syndrome).

Small bowel

• Small bowel is dilated above 3 cm; its folds cross the whole width of the lumen ("stack of coins").
• Coeliac disease: fewer jejunal folds, more ileal folds (jejunisation). Crohn disease: transmural fibrosis narrows the terminal ileum to a string sign, loops separated. Intramural haemorrhage (over-anticoagulation): uniformly thickened, dense jejunal wall.
• Whipple disease: thick jejunal folds, partial villous atrophy with many PAS-positive macrophages (*Tropheryma whipplei*).
• Meckel diverticulum: persistence of the vitellointestinal duct; antimesenteric border of the ileum about 60 cm from the ileocaecal valve; ectopic gastric mucosa takes up technetium-99m pertechnetate. Intussusception in infants is usually ileocolic: target or concentric-ring mass on ultrasound.

Large bowel

• Recognised by haustra (incomplete folds produced by the taeniae coli), peripheral position and appendices epiploicae. Torsion of an appendage gives a small fat-density oval with a bright rim and a central dot.
• Dilated limits: colon 6 cm, caecum 9 cm (3–6–9 rule). By the law of Laplace the caecum, with the largest radius, has the highest wall tension and perforates first.
• A competent ileocaecal valve with a distal colonic obstruction makes a closed loop: colon dilates, small bowel does not.
• Sigmoid volvulus: elderly constipated or institutionalised patient; long sigmoid mesocolon on a narrow root; inverted-U coffee bean loop rising from the left iliac fossa towards the right upper quadrant. First treatment is endoscopic (flexible sigmoidoscopic) decompression with a flatus tube, unless there is peritonitis or gangrene. Caecal volvulus: caecum with a persistent mesentery twists into the left upper quadrant.
• Toxic megacolon in ulcerative colitis shows first in the transverse colon (above 6 cm on a supine film, where gas rises).
• Acute mesenteric ischaemia: SMA thrombosis (atherosclerosis) or embolism leaves dark purple small bowel; the IMA territory (descending and sigmoid colon) is spared unless the IMA is also occluded.
• Ischaemia: thumbprinting from submucosal oedema and blood; watershed at the splenic flexure (SMA–IMA) and the rectosigmoid (Sudeck point). The marginal artery keeps the left colon alive after IMA ligation.
• Ulcerative colitis: continuous from the rectum, loss of haustra. Hirschsprung disease: the narrow distal segment lacks ganglion cells in the myenteric and submucosal plexuses (also lost in Chagas megacolon), with a funnel transition to dilated colon. Annular carcinoma: short shouldered "apple-core" stricture. Sigmoid diverticulitis can form a colovesical fistula (gas in the bladder). Necrotising enterocolitis: pneumatosis in a preterm baby.

Colonic mobilisation, rectum and anal canal

• The ascending and descending colon are secondarily retroperitoneal. Their mesentery fused with the posterior wall, leaving the nearly bloodless fusion fascia of Toldt; the surgeon incises the peritoneum lateral to the colon (white line) and lifts it medially in this plane. Going too deep enters Gerota fascia and endangers the ureter and gonadal vessels.
• Rectosigmoid junction: at about S3, where the taeniae spread out into a continuous longitudinal muscle coat and the sigmoid mesocolon ends; the rectum has no haustra, appendices epiploicae or mesentery.
• Embryology: the urorectal septum divides the cloaca into the rectum and anal canal behind and the urogenital sinus in front.
• Continence: the puborectalis sling holds the anorectal angle at about 90°; in defecation puborectalis relaxes, straightening the angle.

Ileocaecal valve and appendix

• The appendix arises from the posteromedial caecum where the three taeniae converge, about 2 cm below the ileocaecal valve.
• Positions: retrocaecal commonest (psoas sign, flank pain), then pelvic (bladder and rectal symptoms, obturator sign). In late pregnancy the enlarging uterus pushes it upwards.
• The appendicular artery (from the ileocolic) is an end artery, so gangrene and perforation come early; lymph goes to ileocolic nodes along the superior mesenteric chain. A child's short greater omentum cannot wall it off.
• Ultrasound of appendicitis: blind-ending, non-compressible tube with outer diameter over 6 mm. A laminated calcified density in the right iliac fossa is an appendicolith.
• A gridiron (McBurney) incision splits the muscles in the line of their fibres, but can injure the iliohypogastric nerve (or ilioinguinal), weakening the conjoint tendon and posterior wall and leading to a later direct inguinal hernia.

Remember Folds that cross the whole lumen mean small bowel; folds that do not mean colon.

Exam trap The duodenal cap is intraperitoneal; the second and third parts are retroperitoneal, so their rupture shows no gas under the diaphragm.

Exam trap The caecum perforates first in a distal colonic obstruction even though the tumour is in the sigmoid.

How FCPS asks about this

• Bleeding posterior duodenal ulcer: gastroduodenal artery; lesser curve ulcer bleeds from the left gastric artery.
• Coffee bean sign from the left iliac fossa: sigmoid volvulus; endoscopic decompression first.
• Corkscrew duodenum, DJ flexure on the right: malrotation with volvulus; Ladd bands at surgery.
• Meckel diverticulum: persistent vitellointestinal duct, 60 cm from the ileocaecal valve.
• Inferior rectal artery: branch of the internal pudendal artery.
• Where sigmoid becomes rectum: taeniae spread into a complete coat.
• Short shouldered "apple-core" stricture: annular adenocarcinoma.