Ingestion and Digestion
High-Yield Summary
- Oral cavity: mastication (mechanical) + salivary amylase (chemical, starts carb digestion) → shapes food into a bolus. Saliva regulated by parasympathetic NS.
- Pharynx and esophagus do NO digestion or absorption — pure transport/protection. Epiglottis blocks the airway during swallowing; peristalsis moves food independent of gravity.
- Lower esophageal sphincter relaxes to admit food, contracts to prevent reflux.
- Stomach regions: fundus (storage), body (chemical digestion), antrum (grinding + gastrin), pylorus (pyloric sphincter). Gastric juice: HCl (parietal), pepsinogen (chief), mucus/bicarbonate (mucous cells).
- Pepsinogen → pepsin (activated by HCl); intrinsic factor (parietal cells) required for later B-12 absorption; gastrin (G cells) stimulates acid secretion + motility.
- Duodenum = regulatory hub: secretin → pancreatic bicarbonate (neutralizes acid); brush-border enzymes finish digestion; enteropeptidase activates trypsinogen → trypsin → cascade; CCK → pancreatic enzymes + gallbladder contraction (bile → emulsification → micelles).
Key Terms
- Bolus
- Compact, rounded mass of chewed food shaped for safe swallowing.
- Epiglottis
- Cartilage flap that folds down during swallowing to block the airway.
- Chyme
- Semi-liquid mixture the stomach transforms food into before releasing it to the duodenum.
- Zymogens
- Inactive precursor forms of pancreatic enzymes, preventing premature/self-digestion.
- Micelles
- Structures formed when lipid digestion products associate with bile salts, enabling lipid absorption.
Secretin vs. CCK
| Secretin | Cholecystokinin (CCK) |
|---|---|
| Triggered by acidic chyme | Triggered by fats and amino acids in the lumen |
| Stimulates pancreatic bicarbonate release | Stimulates pancreatic enzyme release |
| Neutralizes gastric acid | Signals gallbladder to contract, release bile; slows gastric emptying |
Gastric Juice Components (by Cell Type)
- Parietal cells
- Secrete hydrochloric acid (HCl) and intrinsic factor.
- Chief cells
- Secrete pepsinogen, the inactive precursor to pepsin.
- Mucous cells
- Secrete mucus and bicarbonate, forming the mucus-bicarbonate barrier that protects the stomach lining.
Enteropeptidase → Trypsin Cascade
- 1Pancreas releases proteases as inactive zymogens (e.g., trypsinogen) into the duodenum.
- 2Brush-border enzyme enteropeptidase converts trypsinogen → active trypsin.
- 3Trypsin activates the remaining pancreatic zymogens in a cascade.
- 4Protein digestion proceeds only in the intestinal lumen — protects the pancreas from self-digestion.
Common MCAT Trap
- Neither the pharynx NOR the esophagus digest or absorb anything — both are transport-only. Don't attribute any enzyme secretion to them.
- Pepsinogen is made by CHIEF cells; HCl and intrinsic factor come from PARIETAL cells — these are commonly swapped on test questions.
- Enteropeptidase (a brush-border enzyme, not pancreatic) activates trypsinogen → trypsin; trypsin then activates the REST of the cascade. The pancreas itself never activates its own zymogens.
Quick Recall
What activates pepsinogen into pepsin?
What's the difference between secretin and CCK triggers/effects?
What enzyme activates trypsinogen, and what happens next?
Why doesn't digestion occur in the pharynx or esophagus?