Anatomy and Physiology - Digestive System

Overview

Digestive physiology is an integrated system that converts ingested macromolecules into absorbable units through coordinated mechanical processing, chemical digestion, epithelial transport, and regulated motility. These processes occur across anatomically specialized regions, with tight neural and hormonal control ensuring that secretion, mixing, propulsion, and absorption are matched to nutrient load and composition.

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Anatomy and secretions (functional survey)

Mouth

• Mechanical: mastication increases surface area and forms a bolus.
• Chemical:
  • Salivary amylase begins starch digestion (α-1,4 glycosidic bonds).
  • Lingual lipase (minor in adults, important in infants) initiates triglyceride hydrolysis.
• Other: mucus lubricates; bicarbonate buffers oral pH.

Esophagus

• Function: propulsion via primary and secondary peristalsis.
• Lower esophageal sphincter (LES) prevents reflux; failure → GERD.

Stomach

• Mechanical: mixing waves grind food into chyme.

• Secretory cells:

  • Parietal cells: HCl (protein denaturation, antimicrobial) + intrinsic factor (B₁₂ absorption).
  • Chief cells: pepsinogen → pepsin (activated at low pH).
  • Mucous cells: mucus + HCO₃⁻ barrier prevents autodigestion.

• Physiologic roles:

  • Protein digestion initiation.
  • Regulated gastric emptying via pyloric sphincter.

• Clinical tie-in: loss of parietal cells → achlorhydria + B₁₂ deficiency.

Small Intestine

Primary site of digestion and absorption

Duodenum

• Neutralizes acidic chyme via pancreatic HCO₃⁻.

• Receives:

  • Bile (emulsifies fats).
  • Pancreatic enzymes (amylase, lipase, proteases).

• Enteropeptidase (brush border) activates trypsinogen → trypsin.

Jejunum

• Major site for:

  • Carbohydrates
  • Amino acids/peptides
  • Lipids
  • Water-soluble vitamins (except B₁₂)

• Dense villi and microvilli maximize surface area.

Ileum

• Specialized absorption of:

  • Vitamin B₁₂–intrinsic factor complex
  • Bile salts (enterohepatic circulation)

• Peyer patches provide immune surveillance.

Accessory Organs

• Liver:

  • Produces bile acids from cholesterol.
  • Processes absorbed nutrients (first-pass metabolism).

• Gallbladder:

  • Stores and concentrates bile.
  • Contracts in response to CCK.

• Pancreas:

  • Exocrine: digestive enzymes + bicarbonate.
  • Proteases secreted as zymogens to prevent autodigestion.

Large Intestine

• Absorbs water and electrolytes.

• Microbiota:

  • Ferment indigestible carbohydrates → short-chain fatty acids.
  • Synthesize vitamin K and some B vitamins.

• Forms and stores feces.

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Motility patterns

• Peristalsis: propulsive waves move contents forward.

• Segmentation: mixing contractions enhance digestion and absorption.

• Migrating Motor Complex (MMC):

  • Occurs during fasting every ~90 minutes.
  • Clears residual contents (“intestinal housekeeping”).

• Sphincter coordination:

  • LES, pyloric, ileocecal, internal/external anal sphincters.
  • Dysfunction leads to reflux, dumping syndrome, or constipation.

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Digestion and absorption (mechanistic detail)

Carbohydrates

1. Luminal digestion:

   • Salivary + pancreatic amylase → maltose, maltotriose, α-limit dextrins.

2. Brush-border digestion:

   • Lactase, sucrase-isomaltase, maltase → monosaccharides.

3. Transport:

   • SGLT1: glucose + galactose (Na⁺-dependent).
   • GLUT5: fructose (facilitated diffusion).
   • GLUT2: basolateral exit to blood.

Proteins

1. Stomach: pepsin cleaves large proteins.

2. Pancreatic proteases:

   • Trypsin, chymotrypsin, elastase, carboxypeptidases.

3. Brush-border and cytosolic peptidases complete digestion.

4. Absorption:

   • Free amino acids.
   • Di- and tri-peptides via PEPT1 (H⁺-coupled).

Lipids

1. Emulsification by bile salts.

2. Pancreatic lipase + colipase:

   • Triglycerides → 2-monoacylglycerol + free fatty acids.

3. Micelles deliver lipids to enterocyte membrane.

4. Enterocyte processing:

   • Re-esterification → triglycerides.
   • Packaging into chylomicrons.

5. Transport:

   • Lymphatic system → systemic circulation.

Vitamins and Minerals

• Iron:

  • Absorbed in duodenum as Fe²⁺ (acidic environment favors solubility).
  • Heme iron absorbed via separate transporter.

• Calcium:

  • Vitamin D-regulated transcellular absorption.

• Folate: jejunum.

• Vitamin B₁₂: ileum with intrinsic factor.

• Bile acids: reabsorbed in ileum → recycled.

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Regulation of digestion

Neural

• Enteric nervous system (ENS):

  • Myenteric plexus: motility.
  • Submucosal plexus: secretion and blood flow.

• Parasympathetic (vagus): stimulatory.

• Sympathetic: inhibitory.

Hormonal

• Gastrin: ↑ gastric acid and mucosal growth.

• CCK:

  • ↑ pancreatic enzyme secretion.
  • ↑ gallbladder contraction.
  • ↓ gastric emptying.

• Secretin:

  • ↑ pancreatic and biliary bicarbonate.
  • ↓ gastric acid.

• GIP / GLP-1:

  • Incretin effect → ↑ insulin.
  • Slow gastric emptying.

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Worked micro-examples (expanded)

1) Steatorrhea (fatty stools)

• Pancreatic insufficiency:

  • ↓ lipase → undigested triglycerides.
  • Deficiency of fat-soluble vitamins (A, D, E, K).

• Bile salt deficiency (e.g., ileal disease):

  • Poor micelle formation.
  • ↑ gallstone risk.

• Mucosal disease (e.g., celiac):

  • Reduced absorptive surface area.

2) Lactase deficiency

• Undigested lactose → colonic fermentation.
• ↑ osmotic load → diarrhea.
• ↑ H₂ on breath test.
• Normal intestinal histology in primary deficiency.

3) Vitamin B₁₂ malabsorption

• Causes:

  • Pernicious anemia (intrinsic factor loss).
  • Ileal resection or Crohn disease.

• Effects:

  • Macrocytic anemia.
  • Neurologic deficits (demyelination).

• Key distinction:

  • Folate fixes anemia but not neurologic damage.

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Common pitfalls

• Mixing up sites of absorption (iron ≠ B₁₂).
• Forgetting brush-border enzymes complete digestion.
• Ignoring enterohepatic circulation when predicting lipid and gallstone outcomes.
• Assuming enzymes alone are sufficient without proper motility and pH.

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Practice prompts

1. Predict nutrient deficiencies after pancreatic enzyme inhibition.
2. Contrast effects of CCK vs secretin on pancreas and bile.
3. Explain mechanistically why ileal resection increases gallstone risk.
4. Trace the fate of a triglyceride molecule from ingestion to systemic circulation.
5. Explain why achlorhydria impairs iron absorption.

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References

• SciOly Wiki – Anatomy & Physiology (Digestive)