LO 2 — Anatomical Relationships

Serous Sac Anatomy

Peritoneal cavity normally contains only ___

A small amount of serous fluid (for lubrication during organ movement) — no air

Free air rises to ___ in upright patient; fluid collects in ___ (F) or ___ (M)

Superior (under the diaphragm — seen on upright CXR); rectovaginal pouch of Douglas (female); rectovesical pouch (male)

Pneumoperitoneum on CXR always indicates ___

Organ perforation — no air should normally be in the peritoneal cavity (e.g., perforated peptic ulcer, perforated diverticulitis)

Visceral peritoneum differs from parietal: visceral = ___, parietal = ___

Visceral = wraps around organs (inner layer); parietal = lines the abdominal/pelvic walls and diaphragm (outer layer)

Intraperitoneal vs Retroperitoneal — Classify Each Organ

Organ / Structure	IP or Retro?	Organ / Structure	IP or Retro?
Stomach	Intraperitoneal (mobile; connected by lesser and greater omenta)	Duodenum D2–D4	Retroperitoneal (fixed; only D1 proximal bulb is intraperitoneal)
Pancreas head/body/neck	Retroperitoneal (fixed to posterior abdominal wall)	Pancreas tail	Intraperitoneal — sits in the splenorenal ligament; only mobile part of pancreas
Jejunum & ileum	Intraperitoneal (mobile; suspended by mesentery of small intestine)	Ascending colon	Retroperitoneal (fixed)
Transverse colon	Intraperitoneal (on transverse mesocolon)	Descending colon	Retroperitoneal (fixed)
Sigmoid colon	Intraperitoneal (on sigmoid mesocolon)	Kidneys & ureters	Retroperitoneal (both fixed behind peritoneum)

Greater & Lesser Sacs

Lesser sac (omental bursa) is located ___

Posterior to the stomach and lesser omentum (posterior to the liver); a potential space where fluid can collect

Greater and lesser sacs communicate via ___

Omental foramen (foramen of Winslow / epiploic foramen)

Borders of omental foramen — anterior: ___; posterior: ___

Anterior: hepatoduodenal ligament (free edge of lesser omentum); Posterior: inferior vena cava (IVC)

Acute pancreatitis fluid / pancreatic pseudocyst collect in ___

Lesser sac (omental bursa) — posterior to the stomach; seen on CT as fluid collection posterior to stomach

Superior border of omental foramen = ___; inferior = ___

Superior: caudate lobe of the liver; Inferior: 1st part of duodenum (D1)

Greater & Lesser Omenta

Greater omentum attaches to ___, nicknamed ___

Greater curvature of the stomach; "Policeman of the Abdomen" (migrates to sites of inflammation to wall off infection)

Lesser omentum = ___ ligament + ___ ligament

Hepatogastric ligament (liver → stomach) + hepatoduodenal ligament (liver → D1)

Hepatoduodenal ligament contents (portal triad: 3 structures)

Hepatic artery proper (left), portal vein (posterior), common bile duct (right) — "VHD" or "Bill is Right"

Hepatogastric ligament connects ___ to ___, contains ___

Liver to the lesser curvature of the stomach; left and right gastric arteries (no major NVB)

Pringle maneuver = compress ___ to temporarily occlude ___

Hepatoduodenal ligament (between thumb in epiploic foramen and index finger anteriorly); portal vein + hepatic artery → controls hepatic inflow hemorrhage

LOs 1 & 2 — GI Tract Segments & Clinical Relationships

Esophagus — Regional Blood Supply

Region	Arterial Supply	Venous Drainage
Cervical	Inferior thyroid artery (from thyrocervical trunk)	Inferior thyroid vein
Thoracic	Multiple branches off the descending thoracic aorta	Azygous vein system (hemiazygos on left)
Abdominal	Inferior phrenic arteries + esophageal branch of the left gastric artery	Left gastric vein (coronary vein), short gastric veins, inferior phrenic veins → drain into portal system

Esophageal Varices & Achalasia

Abdominal esophageal veins drain into ___ system → in portal HTN, diverted to ___ system

Portal system (left gastric/short gastric veins); azygous/systemic system → esophageal varices form

Esophageal varices risk = ___ → clinical presentation: ___

Rupture; massive hematemesis (bright red blood vomiting) — potentially fatal upper GI bleed

Achalasia = failure of ___ to relax → dysphagia to ___

Lower esophageal sphincter (LES); both solids AND liquids from the onset (key distinguisher from esophageal stricture)

Achalasia barium swallow sign = ___

Bird's beak sign — dilated esophagus with smooth distal tapered stenosis at the LES

Achalasia in Chagas disease: T. cruzi destroys ___

Myenteric (Auerbach's) plexus neurons at the LES → secondary achalasia (same pathology as primary achalasia)

Stomach Anatomy (HY per Dr. Uren)

Stomach regions in order: ___

Cardia → Fundus → Body → Pylorus (pyloric antrum → pyloric canal → pyloric sphincter)

Lesser curvature blood supply: ___ arteries (within ___ omentum)

Left and right gastric arteries; lesser omentum (hepatogastric ligament)

Greater curvature blood supply: ___ arteries (within ___ omentum)

Left and right gastroomental (gastroepiploic) arteries; greater omentum

GDA runs ___ to D1; erosion by posterior ulcer → ___

Posterior; massive arterial hemorrhage — most common cause of life-threatening upper GI bleed from peptic ulcer disease

Short gastric arteries arise from ___ and supply the ___

Splenic artery; fundus of the stomach

Duodenum — 4 Parts

Part	Peritoneal Status	Key Anatomical Fact / Clinical Relevance
D1 (superior)	Intraperitoneal (only IP part of duodenum); site of duodenal peptic ulcers	Bile duct + gastroduodenal artery (GDA) run posteriorly; perforation → intraperitoneal free air
D2 (descending)	Retroperitoneal; directly anterior to the right kidney	Ampulla of Vater (junction of common bile duct + main pancreatic duct) opens on its posteromedial wall
D3 (horizontal)	Retroperitoneal; crosses the midline (L3 vertebra)	Crossed anteriorly by the SMA → SMA syndrome when aorto-mesenteric angle narrows → compresses D3
D4 (ascending)	Retroperitoneal	Held up by the suspensory ligament of Treitz (duodenojejunal junction) — surgical landmark dividing upper GI from lower GI

Jejunum vs Ileum — Key Differences

Jejunum: few, simple arcades; ileum: ___

Many, complex arcades (multiple tiers → greater vascular redundancy)

Jejunum: long vasa recta; ileum: ___

Short vasa recta (vessels reach the intestinal wall over a shorter distance)

Peyer's patches found in ___ (absent in ___)

Ileum (aggregated lymphoid follicles in submucosa; immunological surveillance); absent in jejunum

Jejunum: plicae circulares tall & packed; ileum: ___

Low, sparse plicae circulares (fewer circular folds; thinner wall; paler color)

LO 1 — Vasculature, Lymphatics & Portal System

Foregut / Midgut / Hindgut — Master Table

Division	Extent (start → end)	Artery	Aortic Level
Foregut	Distal esophagus → 2nd part of duodenum (+ liver, gallbladder, spleen, pancreas)	Celiac trunk	T12/L1
Midgut	3rd part of duodenum → proximal 2/3 of transverse colon (+ jejunum, ileum, cecum, appendix, ascending colon)	SMA (Superior Mesenteric Artery)	L1
Hindgut	Distal 1/3 of transverse colon → superior rectum (+ descending colon, sigmoid colon)	IMA (Inferior Mesenteric Artery)	L3

Celiac Trunk Branches

Celiac trunk 3 daughters: ___, ___, ___

Left gastric artery, splenic artery, common hepatic artery

Left gastric artery: supplies ___ + gives off ___ to distal esophagus

Left lesser curvature of stomach; esophageal branches (supply the abdominal esophagus; anastomoses with right gastric)

Splenic artery: supplies spleen + gives off ___ (fundus) + ___ (left greater curvature)

Short gastric arteries (to fundus); left gastroomental artery / left gastroepiploic artery (to left greater curvature)

GDA branches: ___ (right greater curvature) + ___ (pancreas/duodenum)

Right gastroomental artery (right gastroepiploic — anastomoses with left gastroomental along greater curvature); superior pancreaticoduodenal arteries

Hepatic artery proper → ___ hepatic (gives off cystic artery → gallbladder) + ___ hepatic

Right hepatic artery (gives cystic artery → gallbladder); left hepatic artery

SMA Branches (Midgut Supply)

Branch	Supplies
Inferior pancreaticoduodenal arteries	Pancreas and duodenum; anastomose with superior pancreaticoduodenal arteries (from GDA/celiac)
Middle colic artery	First 2/3 of transverse colon; anastomoses with left colic artery (IMA) at splenic flexure
Right colic artery	Ascending colon
Ileocolic artery	Cecum + terminal ileum; gives off the appendicular artery (ligated during appendectomy)
Jejunal / ileal arteries	Jejunum (few simple arcades, long vasa recta) and ileum (many complex arcades, short vasa recta)

IMA Branches (Hindgut Supply)

Branch	Supplies
Left colic artery	Descending colon; anastomoses with middle colic at splenic flexure watershed
Sigmoid arteries	Sigmoid colon (multiple branches)
Superior rectal artery	Superior rectum; anastomoses with inferior rectal arteries (internal iliac/systemic) → portosystemic anastomosis (anorectal varices)

Hepatic Portal System & Lymphatics

Portal vein formed by ___ + ___ + ___

Splenic vein + Superior mesenteric vein (SMV) + Inferior mesenteric vein (IMV)

3 portosystemic anastomoses in portal HTN: ___, ___, ___

(1) Esophageal varices — left gastric ↔ azygous; (2) Caput medusae — paraumbilical ↔ superficial abdominal wall veins; (3) Anorectal varices — superior rectal ↔ middle/inferior rectal veins

Cisterna chyli located at ___, gives rise to ___ duct → empties at ___

L1 (anterior to the aorta); thoracic duct; left venous angle (junction of left subclavian + left internal jugular veins)

Virchow's node = enlarged ___ lymph node; signals ___

Left supraclavicular lymph node; intra-abdominal malignancy (esp. gastric cancer) spreading via the thoracic duct → Troisier's sign

LOs 1 & 3 — Innervation, Referred Pain, Meckel's

Enteric Nervous System — Two Plexuses

Plexus	Location in Wall	Function
Myenteric (Auerbach's)	Between the circular and longitudinal smooth muscle layers	Controls smooth muscle contraction → peristalsis and GI motility (Myenteric = Muscle)
Submucosal (Meissner's)	In the submucosa (between mucosa and circular muscle layer)	Controls glandular secretions and mucosal blood flow (Submucosal = Secretions)

Hirschsprung Disease

Hirschsprung = congenital absence of ___ in ___ of colon/rectum

Ganglion cells (both myenteric + submucosal plexus neurons absent); distal colon and rectum (aganglionic segment)

Barium enema shows ___ between narrow ___ and dilated proximal colon (___)

Transition zone (the key diagnostic finding); narrow aganglionic segment; proximal dilated colon (megacolon)

Caused by failure of ___ cell migration; gene mutation = ___

Neural crest cell migration failure (fail to colonize distal gut during development); RET proto-oncogene mutation (also associated with MEN2A)

Autonomic Innervation Table (Dr. Uren: Must Memorize!)

⚠ Pelvic splanchnic nerves (S2–S4) = PSNS — NOT sympathetic! This is a classic trap.

GI Division	SNS Nerve	PSNS Nerve
Foregut	Greater splanchnic nerves (T5–T9)	Vagus nerve (CN X)
Midgut	Lesser splanchnic nerves (T10–T11)	Vagus nerve (CN X)
Hindgut	Lumbar splanchnic nerves (L1–L2)	Pelvic splanchnic nerves (S2–S4) — PARASYMPATHETIC, NOT sympathetic (classic trap!)

Referred Pain — Abdominal Patterns

Pain Location	GI Origin (organ + embryological division)	Spinal Levels
Epigastric	Foregut structures: stomach, duodenum, liver, gallbladder, pancreas (foregut visceral afferents)	T5–T9 (greater splanchnic nerves)
Periumbilical	Midgut structures: small intestine, appendix, right colon (midgut visceral afferents via lesser splanchnic)	T10–T11 (lesser splanchnic nerves) — classic appendicitis early pain
Hypogastric / suprapubic	Hindgut structures: left colon, sigmoid colon, rectum (hindgut visceral afferents via lumbar splanchnic)	L1–L2 (lumbar splanchnic nerves)

Meckel's Diverticulum — Rule of 2's

Meckel's = true diverticulum; embryological origin: persistence of ___ duct

Vitelline (omphalomesenteric) duct — normally obliterates by week 7 of development

Rule of 2's: 2% population, 2 inches long, ___ feet from ileocecal valve, first ___ years of life

2 feet proximal to the ileocecal valve; first 2 years of life (though can present at any age)

Most common ectopic tissue = ___; causes ___ → ulceration → bleeding

Ectopic gastric mucosa (most common); acid secretion → peptic ulceration of adjacent ileal mucosa → painless rectal bleeding

Meckel's scan: radiotracer = ___; taken up by ___; hot spot in ___

⁹⁹ᵐTc-pertechnetate (technetium); ectopic gastric mucosa (not pancreatic); RLQ (where terminal ileum sits)

Classic presentation in children: ___; mimics appendicitis because both are ___

Painless rectal bleeding (hematochezia) — most common; mimics appendicitis because both are midgut (RLQ location, periumbilical visceral pain)

Meckel's vs appendicitis: Meckel's → ___; appendicitis → fever, rebound tenderness at ___

Meckel's → painless rectal bleeding (+ Meckel's scan hot spot in RLQ); appendicitis → fever, elevated WBC, rebound tenderness at McBurney's point

Practice Questions — GI08 USMLE-Style

Q1. During cholecystectomy, the surgeon compresses the hepatoduodenal ligament between thumb and finger to stop liver bleeding (Pringle maneuver). Which 3 structures are occluded?

A. Portal vein, hepatic artery, common bile duct
B. Portal vein, IVC, cystic duct
C. Hepatic veins, hepatic artery, cystic duct
D. SMA, portal vein, cystic duct
E. Hepatic artery, celiac trunk, common bile duct

✅ A — Portal vein, hepatic artery proper, and common bile duct (portal triad). The hepatoduodenal ligament is the free edge of the lesser omentum and forms the anterior border of the omental foramen. All three structures in it are temporarily occluded to control hepatic inflow.

Q2. Which list contains ONLY retroperitoneal structures?

A. Stomach, duodenum D1, sigmoid colon, rectum
B. Suprarenal glands, aorta, duodenum D2–D4, pancreas body/tail
C. Suprarenal glands, kidneys, duodenum D2–D4, ascending colon
D. Liver, gallbladder, pancreas head, cecum
E. Spleen, transverse colon, sigmoid colon, appendix

✅ C — SAD PUCKER: Suprarenal glands, Aorta/IVC, Duodenum D2–D4, Pancreas (except tail), Ureters, Colon (ascending & descending), Kidneys, Esophagus, Rectum. Pancreas tail is intraperitoneal (splenorenal ligament). Stomach, spleen, transverse colon, sigmoid colon, liver, and gallbladder are intraperitoneal.

Q3. During trauma laparotomy, a surgeon enters the lesser sac. Through which structure did they most likely pass?

A. Through the greater omentum directly
B. Via the epiploic foramen (foramen of Winslow), posterior to the hepatoduodenal ligament
C. Through the left gastroepiploic vessels
D. Via the falciform ligament
E. Through the transverse mesocolon

✅ B — The epiploic foramen (foramen of Winslow) is the natural communication between the greater and lesser peritoneal sacs. Bounded anteriorly by the hepatoduodenal ligament, posteriorly by IVC, superiorly by caudate lobe of liver, inferiorly by D1.

Q4. Endoscopy shows a dilated esophagus with bird-beak narrowing at the GEJ. Manometry confirms failure of the LES to relax. What cell type is lost?

A. Interstitial cells of Cajal
B. Myenteric plexus inhibitory nitrergic neurons
C. Submucosal plexus secretomotor neurons
D. Chief cells of the gastric fundus
E. Parietal cells of gastric body

✅ B — Achalasia = loss of inhibitory neurons (NO/VIP) in the myenteric (Auerbach's) plexus at the LES → tonic contraction → failure of relaxation → progressive dysphagia to BOTH solids and liquids from onset. Chagas disease (T. cruzi) destroys these neurons → secondary achalasia.

Q5. A 72-year-old woman with atrial fibrillation develops sudden abdominal pain and bloody diarrhea after colonoscopy shows ischemic changes at the splenic flexure. Why is this location vulnerable?

A. Splenic flexure has a long mesentery prone to volvulus
B. Watershed area between SMA and IMA territories with limited collateral supply
C. Splenic flexure receives blood exclusively from IMA with no collaterals
D. Retroperitoneal; compressed by left kidney
E. Site of transition between taeniae coli and continuous longitudinal muscle

✅ B — The splenic flexure is a watershed zone between SMA territory (middle colic artery) and IMA territory (left colic artery). The marginal artery of Drummond is often tenuous here → most vulnerable during low-flow states, embolism, or AAA repair.

Q6. A patient with cirrhosis vomits bright red blood. Endoscopy reveals varices at the GEJ. These varices arise from dilation of which portosystemic anastomosis?

A. Superior rectal vein ↔ middle/inferior rectal veins
B. Paraumbilical veins ↔ superficial epigastric veins
C. Left gastric (coronary) vein ↔ azygos/esophageal veins
D. Splenic vein ↔ left renal vein
E. SMV ↔ inferior epigastric veins

✅ C — Esophageal varices form where the left gastric (coronary) vein (portal) meets the azygos/esophageal system (systemic). Most dangerous portosystemic shunt — rupture → massive hematemesis. Anorectal varices = superior rectal ↔ inferior/middle rectal; caput medusae = paraumbilical ↔ superficial abdominal.

Q7. A 3-day-old infant fails to pass meconium. Barium enema shows megacolon proximal to a narrow rectosigmoid segment. Rectal biopsy: absent ganglion cells. What is the underlying mechanism?

A. Neural crest cell migration failure → tonically constricted aganglionic segment
B. Hypertrophy of myenteric plexus → excessive peristalsis
C. Loss of pacemaker cells (ICC) in sigmoid colon
D. Overactivation of Meissner's plexus → hypersecretion
E. Absence of internal anal sphincter smooth muscle

✅ A — Hirschsprung disease: neural crest cells fail to migrate to the distal colon → absent myenteric AND submucosal plexus ganglion cells → aganglionic segment stays tonically contracted → functional obstruction → proximal colon dilates (megacolon). RET proto-oncogene mutation.

Q8. A 30-year-old man has painless rectal bleeding. Colonoscopy finds a 2 cm outpouching 2 feet proximal to the ileocecal valve with ectopic gastric and pancreatic tissue on biopsy. Where would a Meckel's scan show a hot spot?

A. Right upper quadrant
B. Left upper quadrant
C. Left lower quadrant
D. Right lower quadrant
E. Periumbilical region

✅ D — ⁹⁹ᵐTc-pertechnetate is taken up by ectopic gastric mucosa → hot spot in the RLQ where the terminal ileum lies (2 feet from ileocecal valve). Painless rectal bleeding in a child = Meckel's diverticulum until proven otherwise (Rule of 2's).

Abdominal Viscera I: GI Tract — Active Recall