Food's Journey: From Mouth to Small Intestine – Digestion Pathway

Food follows a precise anatomical sequence from ingestion to absorption. It does not pass directly through the stomach; the stomach is a processing chamber.

Step-by-Step Pathway

1. Mouth: Mastication (mechanical) + salivary amylase (chemical, starch → maltose). Bolus formed.
2. Pharynx: Voluntary swallowing → involuntary peristalsis. Epiglottis closes airway.
3. Esophagus: 25 cm muscular tube. Primary peristalsis (swallow-initiated) + secondary peristalsis (distension-initiated). Lower esophageal sphincter (LES) relaxes → bolus enters stomach.
4. Stomach: Temporary storage (1.5 L capacity). Mechanical churning (oblique/circular/longitudinal muscle). Chemical: HCl + pepsin (protein → peptides). Output: chyme (semifluid).
5. Pyloric sphincter: Regulates chyme release into duodenum (~3 mL per wave).
6. Small Intestine (Duodenum, Jejunum, Ileum): Primary digestion/absorption site. Pancreatic enzymes, bile, brush border enzymes. Villi/microvilli → huge surface area. Nutrients → blood (capillaries) & lymph (lacteals).

Key Sphincters

• Upper esophageal sphincter (UES) – prevents air entry.
• Lower esophageal sphincter (LES) – prevents reflux (GERD if incompetent).
• Pyloric sphincter – controls gastric emptying rate.
• Ileocecal valve – prevents colonic backflow.

Transit Times (Approximate)

• Esophagus: 5–10 seconds
• Stomach: 2–4 hours (liquids < solids; fat delays)
• Small intestine: 3–5 hours
• Large intestine: 12–48 hours