This document provides an overview of the anatomy and clinical considerations of the gastrointestinal system. It discusses the anatomy and histology of the organs that make up the GI tract, including the esophagus, stomach, small intestine, and large intestine. It also reviews the vasculature, including the celiac trunk and superior and inferior mesenteric arteries. Key clinical entities are highlighted, such as hiatal hernias, ulcers, intestinal obstructions, and cancers. The document provides a comprehensive review of the gastrointestinal system.

1. USMLE® Step I
Review Anatomy of
Gastrointestinal System
TanatTabtieang, MD
Fellowship of the Royal College of Radiologists ofThailand
Lecturer, Department of Anatomy
Faculty of Medicine, Chulalongkorn University
Bangkok,Thailand
Tanat.T@chula.ac.th

2. Gastrointestinal System
• Abdominal wall and peritoneal cavity
• Upper GI tract (Esophagus, Stomach, Duodenum)
• Lower GI tract (Jejunum, Ileum, Colon, Rectum, Anal canal)
• Vascular supplies (Celiac trunk, SMA, IMA)
• Hepatobiliary system (Liver, Gallbladder, Biliary tree, Pancreas)
• Portal venous system and portosystemic anastomosis

3. Abdominal wall
Below arcuate line
Ext.Obl + Int.Obl +Transversus → Ant. rectus sheath
No post. rectus sheath
Above arcuate line
Ext.Obl + Int.Obl →Ant. rectus sheath
Int.Obl +Transversus → Post. Rectus sheath
Erector spinae
Psoas
Latissimus dorsi
Quadratus lumborum

4. Peritoneal relations
• Retroperitoneal organs
– 2nd, 3rd, and 4th parts of duodenum
– Ascending colon
– Descending colon
– Rectum
– Pancreas
– Kidneys, ureters
– Adrenal glands
– Aorta and IVC

5. Peritoneal cavity
• Greater sac
• Lesser sac / Omental bursa

6. Ligament Connection Structures contained Notes
Falciform Liver to anterior
abdominal wall
Ligamentum teres
(remnant of fetal umbilical
vein)
Hepatoduodenal Liver to 1st part of
duodenum
Portal triad Part of lesser omentum
May be compressed between thumb and
index finger placed in epiploic foramen to
control bleeding
Connect greater and lesser sacs
Gastrohepatic Liver to lesser curvature
of stomach
Gastric arteries Part of lesser omentum
Separates greater and lesser sacs
Gastrocolic Greater curvature and
transverse colon
Gastroepiploic arteries Part of greater omentum
Gastrosplenic Greater curvature and
spleen
Short gastric artery Part of greater omentum
Separates greater and lesser sacs
Splenorenal Spleen to posterior
abdominal wall
Splenic artery and vein

7. Inguinal canal
• Boundaries
• Contents
– Ilioinguinal nerve (L1)
– ♀: Round ligament of uterus
– ♂: Spermatic cord
• Testicular artery
• Pampiniform venous plexus
• Vas deferens
• Autonomic nerves &
Lymphatic
Anterior wall External oblique aponeurosis & Internal oblique muscle
Posterior wall Conjoint tendon (medial) &
Transversalis fascia (lateral)
Roof Internal oblique muscle &Transversus abdominis muscle
Floor Inguinal ligament & Lacunar ligament

8. Hernias
• Indirect inguinal hernia
– Through deep inguinal ring
– Lateral to inferior epigastric artery
• Direct inguinal hernia
– Through inguinal (Hasselbach’s) triangle
– Medial to inferior epigastric artery
• Femoral hernia
– Below inguinal ligament
– Through femoral canal, lateral to pubic tubercle

9. Primitive gut tube
Foregut Midgut Hindgut
Artery Celiac Superior mesenteric Inferior mesenteric
Parasympathetic
innervation
Vagus nerves Vagus nerves Pelvic splanchnic nerves
Sympathetic
innervation
• Preganglionics:
thoracic splanchnic
nerves,T5−T9
• Postganglionic cell
bodies: celiac
ganglion
• Preganglionics:
thoracic splanchnic
nerves,T9−T12
• Postganglionic cell
bodies: superior
mesenteric ganglion
• Preganglionics:
lumbar splanchnic
nerves, L1−L2
• Postganglionic cell
bodies: inferior
mesenteric ganglion
Referred pain Epigastrium Umbilical Hypogastrium
Derivatives • Esophagus
• Stomach
• Duodenum (first and
second parts)
• Liver
• Pancreas
• Biliary apparatus
• Gallbladder
• Duodenum (second,
third, and fourth
parts)
• Jejunum
• Ileum
• Cecum
• Appendix
• Ascending colon
• Transverse colon
(proximal two-thirds)
• Transverse colon
(distal third - splenic
flexure)
• Descending colon
• Sigmoid colon
• Rectum
• Anal canal (above
pectinate line)

10. Esophagus
• Muscle
– Upper 1/3 Striated muscle
– Middle 1/3 Striated muscle + smooth muscle
– Lower 1/3 Smooth muscle
• Clinical
– Tracheoesophageal fistula
• Connection between esophagus
and trachea
• Failure fusion of tracheoesophageal ridges
• Infant, cyanosis, choking & vomiting with feeding, polyhydramnios
– Hiatal hernia
• Abdominal structures enter thorax
• Sliding hiatal hernia
– Stomach herniates upward through esophageal hiatus, EG junction displaced
• Paraesophageal hernia
– Cardia moves into thorax, EG junction normal position
• Sphincter incompetent → contents reflux into the esophagus

11. Esophagus
• Achalasia
– Incomplete LES relaxation, increased LES tone,
and lack of peristalsis of the esophagus
– Loss of myenteric (Auerbach’s) plexus
– Dysphagia, regurgitation, chest pain
– Investigation: esophageal manometry and barium swallow
• Esophageal cancer
– Adenocarcinoma
• Etiology: Barrett’s esophagus; complication of gastroesophageal reflux disease
• Clinical: Columnar metaplasia of esophageal squamous epithelium, distal 1/3 of
esophagus
– Squamous cell carcinoma
• Etiology: Alcohol & tobacco use; esophagitis
• Dysphagia, anorexia, pain
• Zenker’s diverticulum
– False diverticulum of the mucosa of the esophagus.
– Posterior midline, above the upper sphincter of the esophagus
– Halithosis, dysphagia, obstruction

12. Stomach
• Histology
– Simple columnar epith. No goblet cell
– Gastric gland
• Chief cell: only in fundic gland; nucleus at base, zymogen granule at
apex; produce pepsinogen, rennin, lipase
• Parietal cell: large, pyramidal, central nucleus; produce HCl, intrinsic
factor
– Inner oblique, middle circular, outer longitudinal
• Clinical
– Hypertrophic pyloric stenosis
• Muscularis externa hypertrophies, causing a narrow
pyloric lumen.
• Infant first 6 mo. polyhydramnios;
projectile, nonbilious vomiting;
and a palpable small mass at the right costal margin
• Xray: distended stomach with decreased distal gas

13. Duodenum
• Anatomy
– Second part receives the common bile duct and main pancreatic duct at
the hepatopancreatic ampulla (ofVater).
– Smooth muscle in the wall of the duodenal papilla is sphincter of Oddi.
– Foregut terminates at the point of entry of the common bile duct
• Histology
– Brunner’s gland (only gland in submucosa): alkaline secretion neutralize
acid content
– Goblet cells, Paneth cells, EE cells (CCK & secretin)
• Clinical
– Duodenal atresia
• Failed recanalization of duodenum
• Polyhydramnios, bile-containing vomitus,
and a distended stomach.
• Associated with Down’s syndrome
• Xray: Double bubble sign

14. Jejunum and ileum
• Anatomy
– Simple columnar epith. with microvilli and goblet cells
– In ileum: lamina propria lymphoid follicle: Peyer’s patch
– Plica circulares: much in jejunum, less in ileum
– Jejunum: long vasa recta, simple arcade
Ileum: short vasa recta, complex arcade
• Clinical
– Meckel’s diverticulum
• Remnant of vitelline duct, blind pouch on the antimesenteric border of the ileum
• May contains ectopic gastric, pancreatic, or endometrial tissue, produce ulceration.
• Rule of 2: 2 feet from the ileocecal junction, 2 inches long, incidence 2% of the
population
• Investigation: Tc-99m pertechnitate Meckel scan for ectopic gastric mucosa
– Malrotation of Midgut
• Partial rotation of midgut, resulting in abnormal position of abdominal viscera.
• May be associated with volvulus
– Jejunoileal atresia
• Vascular insult during pregnancy
• Triple bubble sign

15. Colon & Rectum
• Histology of colon
– Simple columnar epith with goblet cell, Lack of villi
– Mainly mucus-secreting and absorptive cells,
abundant lymphoid tissue
– Outer layer muscle bands:
Taenia coli
– Haustra coli
• Appendix
– Intraperitoneum
– Taenia coli converge at
the root of the vermiform appendix.
– McBurney’s point
– Appendicitis
• Inflammation >> increased
intraluminal pressure
• Perforate >> peritonitis
• Psoas sign
• Obturator sign

16. Colon & Rectum
• Intussusception
– A part of the intestine folds into the
section next to it.
– Etiology: Children: unknown cause;
Adult: intraluminal mass
• Volvulus
– Twisting of bowel portion around
mesentery
– Usually in elderly
• Hirschsprung’s disease
– Lack of ganglion cell/enteric nervous
plexuses (Auerbach’s & Meissner’s)
– Failure of neural crest cell migration
– Infant, chronic constipation, failure
to pass meconium
– Dilated part of colon proximal to
aganglionic segment
– Investigation: barium enema

17. Anal canal
• Internal anal sphincter
– Circular smooth muscle
– Sympathetics (lumbar splanchnics)
increase the tone of the muscle
– Parasympathetics (pelvic splanchnics)
relax muscle during defecation
• External anal sphincter
– Circular voluntary skeletal muscle
Above pectinate line Below pectinate line
Visceral (ANS) sensory innervation Somatic sensory innervation
Portal venous drainage Caval venous drainage
Drain to iliac lymph nodes Drain to superficial inguinal nodes
Internal hemorrhoids (painless) External hemorrhoids (painful)
Endoderm Ectoderm

18. Celiac trunk
• Branches: common hepatic, splenic, left gastric

19. Superior and inferior mesenteric arteries

20. Collateral circulation
• If abdominal aorta is blocked
– Internal thoracic artery
(subclavian) ↔ superior
epigastric (internal thoracic) ↔
inferior epigastric (external iliac)
– Superior pancreaticoduodenal
(celiac trunk) ↔ inferior
pancreaticoduodenal (SMA)
– Middle colic (SMA) ↔ left colic
(IMA)
– Superior rectal (IMA) ↔ Middle
rectal (internal iliac)

21. Liver
• Hepatocyte
– Apical surface faces bile canaliculi
Basal surface faces sinusoids
• Hepatic sinusoids
– Fenestrated capillaries
• Space of Disse
– Kupffer cells
– Ito cells (stellate cells): mesenchyme cells
• Collagen type I >> scarring

22. Biliary tract & Pancreas

23. Pancreas
• Annular pancreas
– Defect in the rotation and
fusion of the ventral and dorsal
buds
– Duodenal obstruction,
polyhydramnios

24. Spleen
• Intraperitoneal organ
• Left upper quadrant, deep to
the left 9th, 10th, and 11th ribs
– Spleen may be lacerated with a
fracture of the left 9th, 10th, or
11th rib
• Mesoderm

25. Stomach
Abdominal
aorta
Inferior
vena
cava
Liver
Spleen
Gall-
bladder
Liver
Pancreas
Right kidney Left kidney
Abdominal
aorta
SpleenInferior
vena
cava
Tomography of abdomen

26. Portocaval anastomosis
Site Portal Caval Clinical signs
Esophagus Esophageal veins
(left gastric vein)
Veins of thoracic esophagus,
draining into azygos system
Esophageal varices
Umbilicus Paraaumbilical vein Superficial veins of anterior
abdominal wall
Caput medusae
Rectum Superior rectal veins
(inferior mesenteric vein)
Inferior rectal veins (internal
iliac vein)
Internal hemorrhoids

28. References
• Tao, et al. First aid for the USMLE step 1 2014 : a student-to-
student guide. NewYork: McGraw-Hill Medical, 2014.
• Moore, Keith L., Arthur F. Dalley, and A. M. R. Agur. Clinically
oriented anatomy. Philadelphia: Wolters Kluwer
Health/Lippincott Williams &Wilkins, 2014.
• White, James, and David Seiden. USMLE® step 1 lecture notes
2016 : anatomy. NewYork: Kaplan Medical, 2016.
• Netter, Frank H. Atlas of human anatomy. Philadelphia, PA:
Saunders/Elsevier, 2014.
• Tank, Patrick W., and Grant. Grant's dissector. Philadelphia:
Wolter Kluwer Health/Lippincott Williams &Wilkins, 2013.
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