Unit 18 Question Bank

Real question — TMU 2021–22 final, Section III Q5.

Superior mesenteric artery — L1, midgut

Supplies from the descending part of the duodenum to the distal third of the transverse colon.

• Inferior pancreaticoduodenal artery — its first branch
• From the left side: jejunal and ileal arteries (12–15), forming arcades and vasa recta
• From the right side:
  • Middle colic artery — transverse colon
  • Right colic artery — ascending colon
  • Ileocolic artery — terminal ileum, caecum, and the appendicular artery

Inferior mesenteric artery — L3, hindgut

Supplies from the distal third of the transverse colon to the upper rectum.

• Left colic artery — descending colon
• Sigmoid arteries (2–3) — sigmoid colon
• Superior rectal artery — terminal branch, upper rectum

The link between them

All the colic branches anastomose along the mesenteric border of the colon to form the marginal artery of Drummond, connecting the two territories. It is weakest at the splenic flexure — the watershed where ischaemic colitis occurs.

Real question — TMU 2021–22 final, Section III Q7.

Formation

Formed behind the neck of the pancreas, at about the level of L2, by the union of the superior mesenteric vein and the splenic vein.

Tributaries

1. Superior mesenteric vein — midgut
2. Splenic vein — spleen, pancreas, and much of the stomach
3. Inferior mesenteric vein — hindgut; usually joins the splenic vein
4. Left gastric vein
5. Right gastric vein
6. Cystic vein — from the gallbladder
7. Paraumbilical veins

Course and termination

Ascends in the free edge of the lesser omentum (hepatoduodenal ligament), lying posterior to the common bile duct (right) and the proper hepatic artery (left). At the porta hepatis it divides into right and left branches.

Functional points

• It carries about 75% of the blood supply to the liver, deoxygenated but nutrient-rich
• It begins and ends in capillaries — a true portal system
• It has no valves, so obstruction reverses flow

Portosystemic anastomoses

In portal hypertension, flow reverses into: the lower oesophagus (left gastric ↔ oesophageal veins → varices), the anal canal (superior ↔ middle and inferior rectal veins → haemorrhoids), the periumbilical region (paraumbilical ↔ superficial epigastric veins → caput medusae), and the retroperitoneum.

Why they matter

The hepatic portal vein has no valves. When flow through the liver is obstructed — most often by cirrhosis — pressure rises and blood reverses into the sites where portal and systemic veins communicate. Normally trivial, these become the clinical picture of portal hypertension.

Why oesophageal varices are the dangerous one

They lie immediately beneath a thin mucosa in a tube that food passes through under pressure. Rupture causes massive haematemesis, and mortality from a first variceal bleed remains high. Haemorrhoids from the same cause are uncomfortable but rarely dangerous, and caput medusae is a sign rather than a threat.

Other features of portal hypertension

Splenomegaly from congestion of the splenic vein, and ascites from raised portal capillary pressure with hypoalbuminaemia.

Two arterial territories

• Superior mesenteric artery (L1) — midgut: caecum, appendix, ascending colon and the proximal two-thirds of the transverse colon, via the ileocolic, right colic and middle colic arteries
• Inferior mesenteric artery (L3) — hindgut: distal third of the transverse colon, descending and sigmoid colon and upper rectum, via the left colic, sigmoid and superior rectal arteries

The marginal artery of Drummond

The terminal branches of all the colic arteries anastomose end to end along the mesenteric border of the colon, forming one continuous vessel from the ileocaecal junction to the rectosigmoid. Short vasa recta pass from it to the bowel wall.

The watershed

The anastomosis is weakest where the two territories meet, at the splenic flexure. In a period of low perfusion — cardiac arrest, major haemorrhage, aortic surgery — this segment is the first to become ischaemic.

Ischaemic colitis

Presents as left-sided abdominal pain with bloody diarrhoea, classically in an elderly patient after a hypotensive episode. The site of disease is predicted entirely by the anatomy of the anastomosis.

Rectal supply — worth adding

The rectum has a triple supply: superior rectal (from the IMA), middle rectal (from the internal iliac) and inferior rectal (from the internal pudendal) arteries — which is why rectal ischaemia is rare.

Origin and course

Arises from the front of the abdominal aorta at L1, just below the celiac trunk, passes behind the neck of the pancreas and in front of the third part of the duodenum, then runs downward and to the right between the layers of the mesentery.

Territory

The whole midgut: from the descending part of the duodenum to the distal third of the transverse colon.

Branches

• Inferior pancreaticoduodenal artery — first branch, anastomosing with the superior pancreaticoduodenal from the gastroduodenal
• Left side: 12–15 jejunal and ileal arteries. In the jejunum, one or two arcades with long vasa recta; in the ileum, four or five tiers of arcades with short vasa recta — a difference surgeons use to identify the level of bowel
• Right side: middle colic, right colic and ileocolic (→ appendicular) arteries

Occlusion

• An embolus — often from atrial fibrillation — infarcts almost the entire small intestine and right colon
• The presentation is deceptive: severe central abdominal pain with a soft abdomen and few signs, because the peritoneum is not yet inflamed
• Guarding and rigidity appear only once the bowel is dead, by which time mortality is very high
• The SMA territory has few collaterals, unlike the stomach — which is why occlusion is catastrophic here and rarely so in the foregut