What is the digestive process from ingestion to excretion?

The Digestive Process: From Ingestion to Excretion

The digestive process involves a coordinated sequence of mechanical and chemical events that transform food into absorbable nutrients, with approximately 10% of ingested nutrients physiologically escaping absorption and reaching the colon. 1, 2

Oral Phase

• Mechanical digestion: Food particle size reduction through chewing
• Chemical digestion: Begins with salivary amylase (0.5 liters of saliva daily) 1
• Bolus formation: Mixing of food with saliva for swallowing

Gastric Phase

• Mechanical digestion: Stomach churning mixes food with gastric secretions
• Chemical digestion:
  • Gastric acid (2.0 liters daily) creates acidic environment (pH 1-3) 1
  • Gastric lipase initiates fat digestion 3
  • Pepsin begins protein breakdown
• Regulation: Gastric emptying controlled by neural and hormonal factors
• Protective mechanisms: Mucus layer prevents self-digestion

Small Intestinal Phase

Duodenum

• Chemical digestion:
  • Pancreatic enzymes (1.5 liters daily) neutralize acid and digest macronutrients 1
  • Bile emulsifies fats to increase surface area for enzymatic action 3
• Regulation: CCK and secretin stimulate pancreatic and biliary secretions 1

Jejunum

• Absorption:
  • Primary site for nutrient absorption
  • Carbohydrates: Glucose via SGLT1 (sodium-dependent); fructose via GLUT5 2
  • Proteins: Amino acids and di/tripeptides
  • Fats: Free fatty acids and monoglycerides form micelles for absorption
  • Water and electrolytes: Majority reabsorbed in upper jejunum 1

Ileum

• Specialized absorption:
  • Vitamin B12 and bile salts (60-100 cm of terminal ileum required) 1
  • Continued absorption of nutrients not absorbed in jejunum

Colonic Phase

• Fermentation: Unabsorbed carbohydrates (1-30% of intake) fermented by microbiota 1, 2
• Production: Short-chain fatty acids provide energy to colonocytes 4
• Absorption: Water, electrolytes, and microbial metabolites
• Formation: Fecal matter through water reabsorption
• Elimination: Coordinated defecation through neural reflexes

Regulatory Mechanisms

• Neural control:
  • Vagal-cholinergic pathways (contributes up to 40% of pancreatic response) 1
  • Enteric nervous system coordinates motility
• Hormonal control:
  • CCK: Stimulates pancreatic enzyme secretion and gallbladder contraction 1
  • Secretin: Stimulates bicarbonate secretion
  • GLP-1 and PYY: Slow gastric emptying and intestinal transit 1
• Interdigestive activity: Migrating motor complex (MMC) during fasting state maintains intestinal homeostasis 1

Key Physiological Concepts

• Motor-secretory coupling: Coordinated motility and secretion optimize nutrient digestion and absorption 1
• Physiological malabsorption: 1-30% of ingested nutrients normally escape absorption 1, 2
• Intestinal adaptation: Process that attempts to restore absorptive capacity after intestinal resection 1

Clinical Considerations

• Malabsorption: Can result from pancreatic insufficiency, bile acid disorders, or intestinal disease 3
• Short bowel syndrome: Reduced absorptive capacity after intestinal resection affects fluid and nutrient balance 1
• pH considerations: Brush border enzyme activity optimal at pH 8.0; iron absorption occurs at pH 6.2-6.7 1

Note: This chart provides a comprehensive overview of the digestive process for clinical reference and patient education.