Overview
This lecture covers the anatomy, physiology, vascular supply, and ultrasound appearance of the gastrointestinal (GI) tract, focusing on its relevance to sonography and pediatric imaging.
GI Tract Anatomy
- The GI (alimentary) tract is a continuous tube from mouth to anus, about 8 meters long.
- Key ultrasound-relevant sections: esophagus, stomach, small intestine, large intestine.
- Esophagus runs from the pharynx to the stomach, passing through the diaphragm at the gastroesophageal junction (sphincter).
- Peristalsis (muscular contractions) moves food through the GI tract and helps identify bowel in ultrasound images.
- Stomach connects to the esophagus and empties into the duodenum; it has four regions: fundus, body, antrum, pylorus.
- Stomach wall layers: mucosa, submucosa, muscularis (3 muscle types), serosa; mucosal folds called rugae expand with food.
- Stomach cell types: goblet (mucus), parietal (acid), chief (pepsinogen), G cells (gastrin), D cells (somatostatin).
- Small intestine (~5 meters): duodenum (four parts), jejunum, ileum; attached to the posterior wall by the mesentery.
- Small intestine has five wall layers (serosa, muscularis, submucosa, mucosa, superficial mucosa/lumen); villi increase surface area for absorption.
- Large intestine: cecum (with appendix), ascending, transverse, descending, sigmoid colon, rectum; lacks villi, absorbs water, has haustra (saccules).
- Appendix location: near the ileocecal junction, typical position at McBurney’s point (2/3 from umbilicus to right iliac crest).
GI Tract Vasculature
- Esophagus blood supply: esophageal branches of the left gastric artery; potential for esophageal varices if blood backs up from the liver.
- Stomach supplied by left and right gastric arteries and branches of the splenic artery.
- Small intestine mainly supplied by the superior mesenteric artery; large intestine by branches of inferior mesenteric artery.
- Venous drainage via superior and inferior mesenteric veins and splenic vein, joining to form the portal vein to the liver.
Physiology of Digestion
- Mechanical and chemical breakdown of food starts in the mouth; stomach acid and enzymes further digest food into chyme.
- Chyme passes into the duodenum, where bile and pancreatic enzymes aid digestion.
- Small intestine absorbs nutrients; remainder passes to large intestine for water/salt absorption and feces formation.
- Stool color can indicate GI pathology: black (upper GI bleed), red (lower GI bleed), pale/clay (gallbladder disease).
- Blood tests (CBC) can suggest GI bleeding or infection.
Ultrasound Appearance
- Esophagus appears as a target shape posterior to the thyroid in the neck.
- Stomach appearance varies by content; collapsed stomach may show gut wall layers and rugae.
- Pylorus: canal <3 cm, muscle <1.4 cm; thickening indicates pyloric stenosis in pediatrics.
- Duodenum may obscure nearby structures due to air/gas; C-loop surrounds pancreatic head.
- Small/large intestine: five alternating echogenic/hypoechoic wall layers (SMSMS sign); peristalsis helps identify bowel.
- Appendix: normal <6 mm, compressible, blind end; easier to see in pediatric patients.
- Use graded compression and high-frequency probes for best bowel visualization.
Key Terms & Definitions
- Peristalsis — Rhythmic contractions that move contents through the GI tract.
- Rugae — Folds in the stomach mucosa allowing expansion.
- Haustra — Saccular segments of the colon wall.
- Mesentery — Fold of peritoneum anchoring intestines to the abdominal wall.
- Portal vein — Vessel carrying nutrient-rich blood from GI tract to the liver.
- Chyme — Semi-liquid mass of partially digested food from the stomach.
Action Items / Next Steps
- Complete workbook labeling activities and nerd check questions for review.
- Practice identifying key anatomical features and pathology on ultrasound images.
- Consider making flashcards from key terms and anatomy points.