The Digestive Tract: Sequential Anatomy and Physiology from Ingestion to Elimination

Digestion is a coordinated sequence of mechanical and chemical processes across specialized organs, converting food into absorbable nutrients and eliminating waste.

1. Oral Cavity (Ingestion & Mechanical/Chemical Initiation)

• Mastication: Teeth reduce particle size; tongue manipulates bolus
• Saliva: Serous (amylase → starch digestion), mucous (lubrication), antimicrobial (lysozyme, IgA)
• Swallowing: Voluntary oral phase → involuntary pharyngeal/esophageal phases

2. Esophagus (Transit)

• Muscular tube (skeletal → smooth muscle transition)
• Peristalsis: Primary (swallow-initiated) and secondary (distension-initiated) waves
• Lower esophageal sphincter (LES): Prevents reflux; relaxes for bolus passage
• Transit time: ~6-10 seconds

3. Stomach (Storage, Mixing, Chemical Digestion)

• Regions: Fundus (accommodation), body (acid/enzyme secretion), antrum (grinding), pylorus (gatekeeper)
• Gastric juice: HCl (pH 1.5-3.5), pepsinogen → pepsin, intrinsic factor, lipase
• Chyme formation: 2-4 hours for mixed meal; liquids faster
• Pyloric sphincter: Regulates chyme delivery to duodenum (2-3 mL/peristaltic wave)

4. Small Intestine (Primary Digestion & Absorption)

• Duodenum: Bile (emulsification), pancreatic enzymes, Brunner's glands (alkaline mucus)
• Jejunum: Peak nutrient absorption (villi density highest)
• Ileum: Bile acid/B12 absorption, Peyer's patches (immune surveillance)
• Brush border enzymes: Disaccharidases, peptidases
• Transit: 3-5 hours

5. Large Intestine (Water/Electrolyte Absorption, Microbiome)

• Cecum, ascending, transverse, descending, sigmoid colon, rectum
• Microbiota: Ferment fiber → SCFAs (butyrate, propionate, acetate); synthesize vitamin K, biotin
• Stool formation: Dehydration of residue; mass movements 1-3x/day
• Transit: 12-48 hours

6. Defecation

• Rectal distension → defecation reflex (parasympathetic S2-4)
• Internal anal sphincter (involuntary) + external anal sphincter (voluntary)
• Puborectalis sling maintains anorectal angle; relaxes for evacuation

Hormonal Regulation

Gastrin, CCK, secretin, GIP, motilin, ghrelin coordinate motility, secretion, satiety, and metabolic responses.