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Complete Anatomy of the Gastrointestinal (GI) Tract — MBBS 1st Year

Source: Gray's Anatomy for Students; Color Atlas of Human Anatomy Vol. 2 (Internal Organs)

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Overview

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1. Esophagus

Overview

• Begins: Lower border of cricoid cartilage — vertebral level C6
• Ends: Cardiac opening of stomach — vertebral level T11
• Passes through: neck → posterior mediastinum → esophageal hiatus in diaphragm (T10)

Course & Relations

• Lies anterior to vertebral column, posterior to trachea (in neck/superior mediastinum)
• In the superior mediastinum: crossed by the arch of the aorta (left) and azygos vein (right)
• In the posterior mediastinum: lies posterior to the left atrium (clinically important — esophageal compression can affect cardiac imaging)
• As it reaches the diaphragm, it deflects anteriorly and to the left

Constrictions (4 sites — clinically important for lodged objects and corrosive injuries)

1. Pharyngoesophageal junction (cricopharyngeus) — level C6
2. Crossed by the arch of the aorta — T4/5
3. Compressed by the left main bronchus — T4/5
4. Passage through the esophageal hiatus — T10

Blood Supply

• Cervical: Inferior thyroid arteries
• Thoracic: Esophageal branches of thoracic aorta + bronchial arteries
• Abdominal: Left gastric artery (branch of celiac trunk)

Venous Drainage

• → Azygos/hemiazygos veins (systemic) + left gastric vein (portal)
• Portosystemic anastomosis at the lower esophagus → site of esophageal varices in portal hypertension

Innervation

• Parasympathetic: Vagus nerve (CN X) — forms the esophageal plexus which reconstitutes as anterior (left) and posterior (right) vagal trunks below the diaphragm
• Sympathetic: T5–T10 via greater splanchnic nerve

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2. Peritoneum & Mesenteries

• Parietal peritoneum: lines the abdominal wall; innervated by spinal nerves T7–L1 (sensitive to well-localized pain)
• Visceral peritoneum: covers the organs; innervated by autonomic nerves (pain is poorly localized, referred)
• Peritoneal cavity: potential space between parietal and visceral layers; closed in men, semi-open in women (via uterine tubes)

Classification of Organs

Key Peritoneal Folds

• Greater omentum: hangs from greater curvature of stomach; acts as "policeman of abdomen" (limits infection spread)
• Lesser omentum: connects lesser curvature of stomach + duodenum to liver (hepatogastric + hepatoduodenal ligaments)
• Mesentery proper: fan-shaped fold suspending jejunum and ileum; root runs from duodenojejunal flexure (L2 left) to ileocecal junction (right iliac fossa)
• Transverse mesocolon: suspends transverse colon
• Sigmoid mesocolon: suspends sigmoid colon
• Epiploic foramen (Foramen of Winslow): communication between greater and lesser peritoneal sacs

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3. Stomach

Overview

Parts (4)

Curvatures

• Greater curvature (left/inferior): attachment of gastrosplenic ligament and greater omentum
• Lesser curvature (right/superior): attachment of lesser omentum
• Cardiac notch (angle of His): angle between esophagus and fundus
• Angular incisure (incisura angularis): bend on lesser curvature marking pyloric antrum

Blood Supply — all from celiac trunk

Venous Drainage → Hepatic Portal System

• Right and left gastric veins → portal vein
• Short gastric + left gastro-omental veins → splenic vein
• Right gastro-omental vein → superior mesenteric vein

Lymphatics

Innervation

• Parasympathetic (secretomotor + motor): Anterior vagal trunk (left vagus) + posterior vagal trunk (right vagus)
• Sympathetic (inhibitory + vasomotor): T6–T9 via greater splanchnic nerve → celiac plexus
• Pain from stomach is referred to the epigastrium

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4. Small Intestine (~6–7 m)

4A. Duodenum

• Above major papilla (foregut): Superior pancreaticoduodenal artery (from gastroduodenal artery — celiac trunk)
• Below major papilla (midgut): Inferior pancreaticoduodenal artery (from superior mesenteric artery)

4B. Jejunum

• Makes up proximal 2/5 of the jejuno-ileal loop (~2.5 m)
• Located mostly in the left upper quadrant
• Features: Wider diameter, thicker walls, more prominent plicae circulares (valvulae conniventes), longer vasa recta, fewer arterial arcades, deeper red color
• Blood supply: Jejunal branches of the superior mesenteric artery (SMA)

4C. Ileum

• Makes up distal 3/5 (~3.5 m)
• Located mostly in the right lower quadrant
• Features: Narrower, thinner walls, fewer and less prominent plicae circulares, shorter vasa recta, more arterial arcades, more mesenteric fat, Peyer's patches (lymphoid follicles — especially prominent in distal ileum)
• Ends at ileocecal junction in right iliac fossa
• Blood supply: Ileal branches of SMA

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5. Large Intestine (~1.5 m)

• Taeniae coli: 3 longitudinal bands of muscle (converge at appendix and merge into continuous layer at rectum)
• Haustra: sacculations between taeniae
• Omental (epiploic) appendices: fatty peritoneal tags attached to colon

5A. Cecum

• First part of large intestine, in right iliac fossa
• Intraperitoneal but usually has no mesentery
• Receives the ileum via the ileocecal valve (ileocecal fold)
• The three taeniae coli converge here → point to the appendix base

5B. Vermiform Appendix

• Blind-ended tube, 2–20 cm long (average 9 cm)
• Base: at the confluence of the three taeniae coli, at the posteromedial wall of cecum, McBurney's point (junction of lateral 1/3 and medial 2/3 of line from ASIS to umbilicus) — site of tenderness in appendicitis
• Position varies: retrocecal (most common ~65%), pelvic, pre-ileal, post-ileal
• Blood supply: Appendicular artery (branch of ileocolic artery from SMA), runs in mesoappendix
• Clinical: Appendicitis pain referred to umbilicus initially (T10 dermatome), then localizes to RIF

5C. Colon

5D. Rectum

• Begins at S2–S3 (rectosigmoid junction), 12–15 cm long
• Has 3 lateral flexures externally and 3 transverse (Houston's) folds internally
• No taeniae coli (smooth external longitudinal muscle layer)
• Peritoneal coverage: upper 1/3 covered anteriorly and laterally; middle 1/3 only anteriorly; lower 1/3 extraperitoneal
• Relations: Posterior = sacrum/coccyx; Anterior in men = bladder, seminal vesicles, prostate; Anterior in women = uterus, vagina

5E. Anal Canal (~4 cm)

• Begins at the anorectal junction (anorectal ring, pelvic diaphragm level), ends at the anus
• Pectinate (dentate) line (at level of anal valves): most important anatomical landmark

• Internal anal sphincter: smooth muscle (involuntary), thickening of circular layer of muscularis
• External anal sphincter: skeletal muscle (voluntary), supplied by pudendal nerve (S2, S3, S4) + perineal branch of S4

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6. Blood Supply Summary

Three Unpaired Branches of the Abdominal Aorta

Venous Drainage — Hepatic Portal System

• Inferior mesenteric vein → splenic vein
• Formed behind the neck of the pancreas at vertebral level L2

Portosystemic (Portocaval) Anastomoses — exam favorite!

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7. Lymphatic Drainage

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8. Innervation

• Foregut (stomach, duodenum) → epigastrium
• Midgut (small intestine, appendix) → periumbilical/umbilical region
• Hindgut (large intestine) → hypogastrium

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9. Histology of the GI Wall (General Layers)

1. Mucosa — epithelium + lamina propria + muscularis mucosae
2. Submucosa — dense CT, Meissner's (submucosal) plexus, blood vessels
3. Muscularis externa — inner circular + outer longitudinal smooth muscle; Auerbach's (myenteric) plexus between them
4. Adventitia / Serosa — CT covering; serosa where covered by peritoneum

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10. Developmental Anatomy (Gut Rotation)

• The gut tube develops from endoderm (epithelium) + splanchnic mesoderm (muscle, CT, peritoneum)
• Foregut: supplied by celiac artery; derivatives: esophagus–liver–pancreas
• Midgut: herniates into umbilical cord at week 6, rotates 270° counterclockwise around SMA axis, returns to abdomen at week 10
• Hindgut: descends into pelvis; cloaca is divided into urogenital sinus (anterior) and anorectal canal (posterior) by the urorectal septum
• Clinical: Malrotation and midgut volvulus can cause acute bowel ischemia — surgical emergency

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Quick Revision Summary Table

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