Every organ, enzyme, and hormone lesson in this section builds on this one pathway. Learn the order once, and everything else slots into place around it.
Mouth
Start
Esoph.
Transport
Stom.
Protein
SI
Absorb
LI
Water
Rectum
Store
📖 Full Breakdown
The full 30-foot journey, stop by stop
Each organ has one dominant job — knowing that job tells you what happens if it fails.
Mouth
Mechanical + chemical digestion begins
Chewing breaks food down mechanically while salivary amylase begins breaking down starch chemically — the only digestion that starts before swallowing.
Esophagus
Pure transport, no digestion
Peristalsis (rhythmic muscle contraction) pushes food down. The lower esophageal sphincter prevents stomach acid from refluxing back upward.
Stomach
Protein digestion begins
HCl and pepsin break down proteins, and churning turns food into a semi-liquid mixture called chyme. The pyloric sphincter controls how fast chyme empties into the duodenum.
Small intestine
Where 90% of absorption happens
Duodenum (enzymes + bile mix in), Jejunum (most nutrient absorption), Ileum (vitamin B12 and bile salt absorption) — three regions, each with a distinct job covered in the next lesson.
Large intestine
Water absorption, no digestion
Cecum through sigmoid colon absorb water and electrolytes and host bacteria that produce vitamin K — no further chemical digestion occurs here.
Rectum & anus
Storage and elimination
The rectum stores formed stool until voluntary elimination through the anal canal.
🩺 Clinical / Exam Application
A patient with Crohn's disease has inflammation concentrated in the terminal ileum. Because you know the ileum is the only site where vitamin B12 and bile salts are absorbed, you can predict this patient is at risk for B12 deficiency (leading to anemia) and fat malabsorption (since bile salts are needed to recycle for fat digestion) — even before any lab results come back. The pathway isn't just sequence; each stop has consequences if damaged.
⚠️ Exam Alert
A classic exam distinction: digestion (breaking down food chemically) occurs in the mouth, stomach, and small intestine, while the large intestine performs no digestion at all — only water absorption. Don't assume every GI organ digests something.
🚧 Common Trap
Students often think the stomach is where most absorption happens because it's where food "dissolves." In reality the stomach absorbs almost nothing (a small amount of water, alcohol, and some drugs) — the small intestine is responsible for roughly 90% of nutrient absorption.
✅ Quick Check
List the 4 main organs of the GI tract in order and state the ONE dominant function of each (transport, protein digestion, absorption, or water absorption).
📝 Exam Prep
Common Exam Questions
❓ In what order does food travel through the GI tract?