GI System Anatomy - Comprehensive Overview

---

1. GENERAL ORGANIZATION

• Upper GI tract: mouth, pharynx, esophagus, stomach, duodenum (to ligament of Treitz)
• Lower GI tract: jejunum, ileum, cecum, colon, rectum, anal canal

---

2. LAYERS OF THE GI WALL

• Myenteric (Auerbach's) plexus: Between longitudinal and circular muscle layers - controls gut motility
• Submucosal (Meissner's) plexus: In the submucosa - controls secretion and mucosal blood flow

---

3. ESOPHAGUS

• Begins at the pharyngoesophageal junction (level of thyroid cartilage C6) = Upper Esophageal Sphincter (UES)
• Passes posterior to the trachea and heart, through the diaphragmatic hiatus (at T10)
• Ends at the esophagogastric junction (EGJ), 1-2 cm below the diaphragmatic hiatus, left of T10 vertebral body
• Adult length: ~25 cm
• Three anatomic narrowings (sites of foreign body impaction and strictures):
  1. UES (cricopharyngeus) - ~15 cm from incisors
  2. Aortic arch / left mainstem bronchus - ~23 cm
  3. Lower esophageal sphincter (LES) / diaphragmatic hiatus - ~40 cm

• Lined by non-keratinized stratified squamous epithelium
• No serosal layer (clinically important - anastomotic leaks spread freely to the mediastinum)
• Upper 1/3: striated muscle; middle 1/3: mixed; lower 1/3: smooth muscle

• Yamada's Textbook of Gastroenterology, p. 120

---

4. STOMACH

• J-shaped dilation; volume ~30 mL (neonate) to 1.5-2 L (adult)
• EGJ lies left of T10, 1-2 cm below diaphragmatic hiatus
• Gastroduodenal junction (pylorus) lies at L1, right of midline

• Lesser omentum (gastrohepatic ligament) from lesser curvature to liver
• Greater omentum hangs from fundus/greater curvature: to transverse colon (gastrocolic ligament), spleen (gastrosplenic ligament), diaphragm (gastrophrenic ligament)
• Posterior stomach = anterior wall of omental bursa (lesser sac)
• Posterior relations: pancreas, transverse colon, diaphragm, spleen, left kidney, left adrenal

• Parietal cells (oxyntic cells): Large, eosinophilic, with extensive intracellular canalicular system; secrete HCl and intrinsic factor
• Chief cells (zymogen cells): Basophilic (rich in RER); secrete pepsinogen I & II
• ECL cells (argyrophilic/enterochromaffin-like): In oxyntic glands; contain histamine - stimulate parietal cells via H2 receptors
• D cells (closed endocrine cells): Secrete somatostatin via long processes (paracrine)

• G cells (open endocrine cells): Secrete gastrin; stimulated by gastric distension, vagal stimulation, amino acids
• Mucous cells predominate
• Sleisenger and Fordtran's Gastrointestinal and Liver Disease, p. 830-834

---

5. SMALL INTESTINE

• Mucosa: Epithelium + lamina propria + muscularis mucosae
  • Villi: Finger-like projections; each contains a central lacteal (lymphatic), capillary plexus, and central arteriole/venule
  • Crypts of Lieberkühn: Base of villi; contain stem cells, Paneth cells, goblet cells
  • Microvilli (brush border): Increase surface area ~30x; coated by glycocalyx
  • Plicae circulares (valvulae conniventes): Circular folds from D2 onward - increase surface area 3x
• Submucosa: Contains Brunner's glands (duodenum only) - secrete alkaline mucus
• Muscle layers: Outer longitudinal + inner circular; myenteric plexus between them
• Serosa: Covers mobile (mesenteric) small bowel completely

• Superior Mesenteric Artery (SMA) supplies the entire small intestine
• Vasa recta are the terminal intestinal arterial branches (no anastomosis within the wall - clinically vulnerable to ischemia)
• Jejunum: Long, straight vasa recta; fewer arterial arcades
• Ileum: Short vasa recta with greater arborization; more arcades (→ better collateral, less at risk per unit)
• Venous drainage via Superior Mesenteric Vein (SMV) → portal vein
• Mulholland and Greenfield's Surgery, p. 2362-2370

---

6. LARGE INTESTINE

Cecum

• Saccular beginning; diameter ~7.5 cm, length ~10 cm
• Intraperitoneal, no mesentery; thin wall (highest risk of perforation in obstruction)
• Ileocecal valve: Thickened, nipple-shaped invagination of circular muscle; terminal ileum enters medial border
• Cecal dilation >12 cm on plain film = risk of ischemic necrosis/perforation

Appendix

• Arises ~3 cm below ileocecal valve; 8-10 cm long, blind-ending tube
• Positions: retrocecal (65%) > pelvic (31%) > subcecal (2.3%) > preileal (1%) > retroileal (0.4%)
• Located by following the anterior taenia coli to its convergence at the appendix base

Ascending Colon

• From ileocecal junction to hepatic flexure; ~15 cm long
• Posterior surface retroperitoneal (fixed by fascia of Toldt)
• Mobilized by incising the white line of Toldt; beware of proximity to 2nd part of duodenum

Transverse Colon

• Hepatic to splenic flexure; ~45 cm; completely intraperitoneal
• Suspended by transverse mesocolon; greater omentum attached superiorly

Descending Colon

• Splenic flexure to sigmoid; retroperitoneal; ~25 cm

Sigmoid Colon

• Redundant, mobile loop; most common site of diverticular disease
• Transitions to rectum at rectosigmoid junction (~15 cm from anal verge)

Rectum

• 15-20 cm long; begins at rectosigmoid junction, ends at anus
• Divided into thirds by peritoneal reflections:
  • Upper 1/3: Covered by peritoneum anteriorly and laterally (>10 cm above dentate line)
  • Middle 1/3: Peritoneum anteriorly only (5-10 cm above dentate line)
  • Lower 1/3: Completely extraperitoneal (1-5 cm above dentate line)
• Three valves of Houston: Lateral folds - proximal and distal fold right, middle folds left
• No taeniae coli, no haustra, no epiploic appendices
• Mesorectum: Visceral mesentery of rectum; contains lymphovascular supply - key in Total Mesorectal Excision (TME)
• Anterior relations: rectovesicular pouch (male), pouch of Douglas (female) - lowest point of peritoneal cavity; site of "drop metastases" - Blumer shelf
• Sabiston Textbook of Surgery, p. 2058-2063

---

7. BLOOD SUPPLY - OVERVIEW

Arterial supply (three celiac axis territories):

• Griffiths' point: Splenic flexure (junction SMA/IMA territories)
• Sudeck's point: Rectosigmoid junction (superior rectal artery terminal branches)

Venous drainage:

• SMV + splenic vein = portal vein (behind neck of pancreas)
• IMV drains into splenic vein
• Portal-systemic anastomoses (clinically important in portal hypertension):
  • Esophageal veins (left gastric ↔ azygos) → esophageal varices
  • Rectal veins (superior rectal/IMV ↔ middle & inferior rectal/systemic) → hemorrhoids
  • Paraumbilical veins → caput medusae
  • Retroperitoneal veins of Retzius

---

8. LYMPHATIC DRAINAGE

• Small intestine: Lacteals → mesenteric lymph nodes → superior mesenteric nodes → cisterna chyli → thoracic duct
• Stomach: Along gastric arteries to celiac lymph nodes
• Colon: Epicolic → paracolic → intermediate (along named vessels) → principal (at SMA/IMA origin)
• Rectum: Upper rectum → inferior mesenteric nodes; lower rectum → internal iliac nodes; anal canal below dentate line → inguinal nodes
• Fat-soluble nutrients absorbed as chylomicrons bypass the portal system and enter via thoracic duct directly into systemic circulation

---

9. INNERVATION

Extrinsic (autonomic)

Intrinsic (Enteric Nervous System)

• Myenteric (Auerbach's) plexus: Between longitudinal and circular muscle - controls peristalsis
• Submucosal (Meissner's) plexus: In submucosa - controls secretion/absorption and mucosal blood flow
• ENS contains ~500 million neurons (more than the spinal cord) and functions semi-independently

---

10. PERITONEAL RELATIONSHIPS SUMMARY

---

11. ACCESSORY ORGANS (Overview)

Liver

• Largest organ; ~1500 g; lies in RUQ under right hemidiaphragm
• Couinaud segments (I-VIII) based on portal and hepatic vein distribution - each is a functionally independent unit with its own portal triad and hepatic vein branch
• Falciform ligament divides anatomic left and right lobes; contains ligamentum teres (obliterated umbilical vein)
• Functional division: Right and left lobes divided by Cantlie's line (Rex-Cantlie line) = from gallbladder fossa to IVC - this corresponds to the right and left branches of the portal vein
• Porta hepatis: Entry of portal vein, hepatic artery, exit of bile duct - all within hepatoduodenal ligament (right free edge of lesser omentum)
• Blood supply: Dual - portal vein (~75%) + hepatic artery (~25%)

Gallbladder & Biliary Tree

• Gallbladder: Pear-shaped, 7-10 cm long; capacity ~50 mL; sits in a fossa on the visceral surface of the liver (segment IV/V junction)
• Triangle of Calot (hepatocystic triangle): Bounded by cystic duct (lateral), common hepatic duct (medial), and liver edge (superior) - contains the cystic artery (typically from right hepatic artery) - must be identified in cholecystectomy
• Bile duct anatomy: Right and left hepatic ducts → common hepatic duct → (joined by cystic duct) → common bile duct (CBD) → passes posterior to duodenum → joins pancreatic duct → enters duodenum at ampulla of Vater (papilla of Vater, in D2) - guarded by the sphincter of Oddi

Pancreas

• Retroperitoneal organ; 12-20 cm long; lies at the level of L1-L2
• Parts: head (nestled in C-loop of duodenum), uncinate process (posterior to SMA/SMV), neck (anterior to portal/SMV confluence), body, tail (reaches splenic hilum)
• Main duct (Wirsung): Runs length of gland, joins CBD at ampulla of Vater
• Accessory duct (Santorini): Drains upper head, opens at minor papilla (~2 cm proximal to major papilla)
• Annular pancreas: Failure of ventral pancreatic bud rotation → ring around duodenum → duodenal obstruction
• Blood supply: Pancreaticoduodenal arteries (from gastroduodenal and SMA) for the head; splenic artery branches for body and tail

---

Quick Revision Table - Key Measurements

---