The Digestive System: Step-by-Step Journey from Mouth to Anus

Digestion is a coordinated process involving mechanical and chemical breakdown, nutrient absorption, and waste elimination.

1. Ingestion & Mechanical Digestion (Mouth): Teeth macerate food; salivary amylase (ptyalin) begins starch hydrolysis. Tongue forms bolus.
2. Swallowing & Esophageal Transit: Voluntary oral phase → involuntary pharyngeal/esophageal phases. Peristalsis (sequential circular + longitudinal muscle contractions) propels bolus in 8–10 sec. Lower esophageal sphincter (LES) prevents reflux.
3. Gastric Phase (Stomach): Reservoir function (accommodates ~1.5 L). Chief cells secrete pepsinogen → pepsin (protein digestion at pH 1.5–2.5). Parietal cells secrete HCl and intrinsic factor (B₁₂ absorption). Mucus protects mucosa. Chyme released gradually via pyloric sphincter.
4. Small Intestine (Duodenum, Jejunum, Ileum): Primary digestion & absorption site.
   • Pancreatic juice: Amylase (carbs), lipase (fats + colipase), proteases (trypsinogen → trypsin), bicarbonate (neutralizes acid).
   • Bile (liver → gallbladder → duodenum): Emulsifies fats → micelles for lipase access.
   • Brush-border enzymes: Disaccharidases (lactase, sucrase-isomaltase), peptidases.
   • Villi & microvilli → massive surface area (~250 m²). Active & passive transport of monosaccharides, amino acids, fatty acids, vitamins, minerals, water.
5. Large Intestine (Cecum, Colon, Rectum):
   • Microbiome ferments fiber → SCFAs (butyrate, acetate, propionate) → colonocyte energy, systemic benefits.
   • Water/electrolyte absorption (1.5 L/day → 100–200 mL stool).
   • Vitamin K, biotin synthesis by bacteria.
   • Mass movements (1–3×/day) propel stool to rectum.
6. Defecation (Rectum & Anus): Rectal distension → urge. Internal anal sphincter (involuntary) relaxes; external sphincter (voluntary) controls timing. Puborectalis sling maintains continence.

Regulatory Hormones

• Gastrin (G cells): Stimulates HCl & pepsinogen.
• CCK (I cells): Gallbladder contraction, pancreatic enzyme secretion, satiety.
• Secretin (S cells): Pancreatic bicarbonate, inhibits gastric emptying.
• GIP, GLP-1: Incretins (insulin release).
• Motilin: Migrating motor complex (fasting cleanup waves).

Total transit time: 24–72 hours (highly variable). Fiber, hydration, activity, and microbiome diversity promote regularity.