Ch18: Abdomen III — Inferocolic Compartment & Retroperitoneal Space（结肠下区与腹膜后隙）

# Ch18: Abdomen III — Inferocolic Compartment, Retroperitoneal Space & Posterior Abdominal Wall

Preparatory Mindset

The inferocolic compartment (below the transverse colon) contains the small intestine (jejunum/ileum), cecum & appendix, and the colon. The retroperitoneal space holds the kidneys, ureters, suprarenal glands, abdominal aorta & IVC with their branches, and the posterior abdominal wall. The lecture's key content: SMA/IMA branches, jejunum vs ileum differences, appendix positions & McBurney point, the hepatic portal vein & portal-systemic anastomoses (a guaranteed exam item), and retroperitoneal structures.

Exam mindset: SMA/IMA branches, the appendix (positions, McBurney point, blood supply), and portal hypertension / portal-systemic anastomoses (concepts 40-53) are the highest-yield items. The 局解mc大全 tests mesenteric ischemia, AAA, and the SMA (19-year-old footballer with psoas tear — relevant to femoral nerve/psoas relations).

Core Concepts

1. Small intestine — jejunum vs ileum (MUST KNOW)

• Duodenojejunal flexure & ligament of Treitz: marks the junction; the ligament suspends the flexure (surgical landmark)
• Blood supply: SMA → jejunal & ileal arteries (15-18) → arterial arcades → vasa recta
• Clinical: Meckel's diverticulum (concept 35): remnant of the vitelline duct, 2 feet (60 cm) from the ileocecal junction, 2 inches long, in 2% of people, may contain gastric mucosa → bleeding/perforation ("rule of 2s"); volvulus (concept 38) — mesentery twists → ischemia; mesenteric ischemia (concept 41) — SMA occlusion (embolism) → severe pain out of proportion

2. Large intestine

- Location: right iliac fossa; base at the convergence of the three teniae coli - Surface projection: McBurney point — one-third of the line from the right ASIS to the umbilicus - Positions (variable): retrocecal (most common), pelvic, subcecal, pre/post-ileal, paracolic - Blood supply: appendicular artery (from ileocolic a., terminal branch of SMA) — an end artery → ischemia/perforation in appendicitis - Pain of appendicitis (concept 37): initial visceral (umbilical) pain (distension of the appendix — T10) → later somatic right iliac fossa pain (inflamed parietal peritoneum — iliohypogastric/ilioinguinal)

- SMA: ileocolic (→ appendicular), right colic, middle colic (transverse colon) - IMA: left colic, sigmoid arteries, superior rectal (supplies the distal half of the large intestine from the left transverse colon to the mid-rectum) - Marginal artery (of Drummond): complete arcade along the colon margin, connecting SMA & IMA territories — protects against occlusion

• Parts: cecum, appendix, ascending colon (right/hepatic flexure), transverse colon (left/splenic flexure), descending colon, sigmoid colon, rectum
• Features (distinguishing from small intestine): teniae coli, haustra, epiploic (omental) appendices
• Cecum: in the right iliac fossa; ileocecal valve prevents reflux of large-intestine contents into the ileum
• Appendix (MUST KNOW):
• Blood supply of the colon:
• Clinical: diverticulosis/diverticulitis (left colon, sigmoid — "left-sided appendicitis"); colorectal cancer (sigmoid most common); volvulus of sigmoid (elderly, "coffee-bean" sign)

3. Superior mesenteric artery (SMA) — MUST KNOW

• Arises from the abdominal aorta at L1, behind the neck of the pancreas
• Emerges below the pancreas, crosses the 3rd part of the duodenum (SMA syndrome site), enters the root of the mesentery → right iliac fossa
• Branches: inferior pancreaticoduodenal, jejunal & ileal (15-18), ileocolic (→ appendicular), right colic, middle colic
• SMA occlusion: acute mesenteric ischemia — severe abdominal pain out of proportion to examination; embolus (AF) vs thrombus; high mortality
• SMA syndrome: compression of the 3rd part of duodenum between SMA & aorta (weight loss, prolonged bedrest)

4. Inferior mesenteric artery (IMA)

• Arises at L3, runs left; branches: left colic, sigmoid arteries (3-4), superior rectal
• IMA occlusion (MCQ): often asymptomatic because the marginal artery & superior rectal anastomoses provide collaterals — the 局解mc大全 asks exactly this ("IMA often occludes without symptoms; its normal territory must be supplied by collateral circulation")

5. Hepatic portal vein & portal-systemic anastomoses (MUST KNOW — concept 46)

1. Esophageal: portal → left gastric → esophageal venous plexus → esophageal → azygos → SVC — esophageal varices (most dangerous — bleed massively) 2. Rectal: portal → inferior mesenteric → superior rectal → rectal venous plexus → inferior rectal & anal → internal iliac → IVC — hemorrhoids 3. Paraumbilical: portal → paraumbilical → periumbilical network → (thoracoepigastric/superior epigastric → SVC) & (superficial/inferior epigastric → IVC) — caput medusae 4. Retroperitoneal/vertebral: small tributaries of mesenteric veins ↔ retroperitoneal & vertebral venous plexuses

• Formation: splenic vein + superior mesenteric vein join behind the neck of the pancreas → hepatic portal vein
• Tributaries: SMV, splenic vein, inferior mesenteric vein (usually → splenic vein), right & left gastric veins, cystic vein, paraumbilical veins
• Characteristics of the portal system: both ends connect to capillaries (intestinal capillaries → hepatic sinusoids); tributaries have no valves → blood can flow in reverse in portal hypertension
• Portal-systemic anastomoses (4 sites — MUST KNOW):
• Clinical (concept 46): portal hypertension (cirrhosis) → esophageal varices (most dangerous — leading cause of death in portal HTN), hemorrhoids, caput medusae, splenomegaly (congestive); portacaval shunt (splenic vein → left renal vein)

6. Retroperitoneal space & posterior abdominal wall

• Kidneys (concept 49): retroperitoneal, T12-L3, right lower than left; hilum = renal artery, renal vein (anterior), ureter (posterior) — "AVU" (artery, vein, ureter from anterior to posterior: vein, artery, pelvis); right renal vein receives no tributaries (short); left renal vein receives the left suprarenal & left gonadal veins (and crosses anterior to the aorta)
• Nephrolithiasis (concept 50): stones at 3 narrowings of the ureter (PUJ, pelvic brim/crossing iliac vessels, vesicoureteric junction) → renal colic; CT is gold standard (see Imaging)
• Ureter (concept 57): crosses the pelvic brim anterior to the iliac vessels; in the female, runs behind the uterine artery ("water under the bridge") — uterine artery ligation during hysterectomy must protect the ureter
• Suprarenal (adrenal) glands: on the upper poles of the kidneys; right = drains to IVC (short vein); left = drains to left renal vein; "AMEN" (Artery: superior from inferior phrenic, Middle from aorta, EN = inferior from renal)
• Abdominal aorta: branches — celiac (T12/L1), SMA (L1), renal arteries (L1-2), gonadal (L2), IMA (L3), common iliac (L4 bifurcation); AA (concept 42): abdominal aortic aneurysm — usually below the renal arteries (infrarenal), most common in elderly men; rupture → sudden back/abdominal pain + shock; repair >5.5 cm
• IVC: formed by common iliac veins at L5; anterior to the vertebral column, right of the aorta; receives lumbar, right gonadal, renal, right suprarenal, hepatic veins
• Psoas major & femoral nerve (MCQ): psoas major arises from T12-L5, passes under the inguinal ligament; the femoral nerve (L2-4) emerges lateral to psoas, between psoas & iliacus; psoas tear/hematoma (19-year-old footballer) may compress the femoral nerve → weak hip flexion & knee extension — this exact scenario is in the MCQ bank

7. Posterior abdominal wall

• Muscles: psoas major, iliacus, quadratus lumborum, transversus abdominis (posterior part)
• Lumbar plexus (L1-L4): within psoas; branches: iliohypogastric (L1), ilioinguinal (L1), genitofemoral (L1-2), lateral femoral cutaneous (L2-3), femoral (L2-4), obturator (L2-4)
• Sympathetic trunk: along the vertebral column, with lumbar ganglia

High-Yield Points

• Jejunum vs ileum: jejunum = thick, few arcades, long vasa recta, prominent plicae; ileum = thin, many arcades, short vasa recta, Peyer's patches
• Meckel's diverticulum: "rule of 2s" — 2 feet from ileocecal junction, 2 inches long, 2% of people, 2 types of ectopic tissue (gastric & pancreatic)
• Appendix: retrocecal most common; McBurney point; appendicular artery (end artery); pain: umbilical → RIF
• Colon features: teniae coli, haustra, epiploic appendices
• SMA at L1 (crosses 3rd part of duodenum); IMA at L3
• Portal vein formation: SMV + splenic vein behind the neck of the pancreas; no valves in tributaries
• Portal-systemic anastomoses: esophageal varices (most dangerous), hemorrhoids, caput medusae, retroperitoneal
• Left renal vein: longer, receives left suprarenal & left gonadal veins, crosses the aorta
• Ureter in female: "water under the bridge" (behind the uterine artery)
• AAA: infrarenal, >5.5 cm → repair, rupture → shock
• Femoral nerve (L2-4) between psoas & iliacus — psoas injury → weak hip flexion/knee extension
• IMA occlusion often asymptomatic (collaterals: marginal artery)

Topic Summary

The inferocolic compartment holds the jejunum (few arcades/long vasa recta/thick) vs ileum (many arcades/short vasa recta/lymphoid), the colon (teniae coli, haustra, epiploic appendices; SMA: ileocolic/right colic/middle colic; IMA: left colic/sigmoid/superior rectal; marginal artery), and the appendix (retrocecal most common; McBurney point; appendicular end artery; visceral→somatic pain). The SMA (L1, crosses the 3rd duodenum) & IMA (L3) supply the gut; IMA occlusion is usually asymptomatic. The hepatic portal vein (SMV + splenic behind the pancreatic neck; valveless tributaries) connects to the systemic circulation at 4 anastomoses — esophageal varices being the most dangerous in portal hypertension. The retroperitoneum contains the kidneys (hilum AVU), ureters ("water under the bridge"), suprarenals, abdominal aorta (celiac/SMA/renal/IMA/common iliac; AAA infrarenal), IVC, psoas & the lumbar plexus (femoral nerve between psoas & iliacus).