Explain how the structure of a villus helps absorption of digested food in the small intestine.

7.2 Human Nutrition – Alimentary Canal

Learning Objective

Explain how the structure of a villus helps the absorption of digested food in the small intestine.

1. Overview of the Alimentary Canal

2. Physical & Chemical Digestion – Quick Recap

• Physical digestion: chewing, segmentation, and peristalsis break food into smaller particles, increasing surface area for enzymes.
• Chemical digestion:
  • Carbohydrates – salivary & pancreatic amylase → maltose → glucose (final breakdown by brush‑border maltase, sucrase, lactase).
  • Proteins – pepsin (stomach) → trypsin & chymotrypsin (pancreas) → di‑/tripeptides → amino acids (final cleavage by brush‑border peptidases).
  • Lipids – lingual & gastric lipase (minor) → pancreatic lipase; bile salts emulsify fats, increasing enzyme access; products are fatty acids and 2‑monoacylglycerols.
  • Pancreatic **bicarbonate** raises duodenal pH, optimising enzyme activity.
• The end‑products are small enough to cross the intestinal epithelium of the villi.

3. Small Intestine – The Principal Site of Absorption

• Absorbs carbohydrates, proteins, lipids, water, electrolytes (Na⁺, K⁺, Cl⁻), vitamins and minerals.
• Water‑soluble nutrients enter the **blood capillaries** → hepatic portal vein → liver for metabolism or distribution.
• Lipid‑soluble nutrients (chylomicrons) enter the **central lacteal** → lymphatic vessels → thoracic duct → subclavian vein → systemic circulation.

4. Structure of a Villus

• Villus (finger‑like projection): projects into the lumen, increasing overall intestinal surface area.
• Enterocytes (simple columnar epithelium):
  • One‑cell‑thick layer – minimises diffusion distance.
  • Apical surface covered by a dense **brush border** of microvilli (≈10 µm long, ~10⁶ per villus).
  • Brush border contains digestive enzymes (maltase, sucrase, lactase, peptidases) and transport proteins.
• Lamina propria (core):
  • Rich network of **capillaries** for water‑soluble nutrients.
  • Central **lacteal** – a lymphatic capillary for lipid‑soluble nutrients.
  • Elastic and smooth‑muscle fibres that give structural support and produce a gentle “pumping” motion.

5. How Villus Structure Facilitates Absorption

1. Maximum Surface Area
   • Villi + microvilli increase the absorptive area by ~600‑fold compared with a smooth tube.
   • More area = more transport proteins = faster nutrient uptake.
2. Thin, Enzyme‑Rich Epithelium
   • One‑cell‑thick layer shortens the diffusion path.
   • Brush‑border enzymes complete carbohydrate and peptide digestion at the site of absorption.
3. Specific Transport Mechanisms
   • Active transport (primary): Na⁺/K⁺‑ATPase on the basolateral membrane creates the Na⁺ gradient.
   • Secondary‑active (cotransport): Na⁺‑glucose (SGLT1) and Na⁺‑amino‑acid transporters use the Na⁺ gradient to move sugars and amino acids against their concentration gradients.
   • Facilitated diffusion: GLUT2 (glucose) and various carrier proteins move nutrients down their gradients.
   • Simple diffusion: Fatty acids and monoglycerides cross the membrane after being re‑esterified.
   • Osmosis & aquaporins: Water follows the osmotic gradient created by solute absorption.
4. Capillary Network – Water‑Soluble Nutrients
   • Glucose, galactose, fructose, amino acids, water‑soluble vitamins and minerals enter the blood capillaries.
   • Rapid removal maintains a low concentration at the basolateral side, sustaining diffusion/transport.
   • Blood → hepatic portal vein → liver (e.g., glucose → glycogen, amino acids → trans‑amination).
5. Lacteal – Lipid‑Soluble Nutrients
   • Inside enterocytes, fatty acids and 2‑monoacylglycerols are re‑esterified to triglycerides and packed into chylomicrons.
   • Chylomicrons are too large for capillaries; they enter the central lacteal.
   • Lacteal → lymphatic vessels → thoracic duct → subclavian vein → systemic circulation.
6. Movement of the Villus
   • Smooth‑muscle fibres in the lamina propria contract rhythmically (segmentation and peristalsis).
   • This gentle “pumping” pushes the absorbed nutrients toward the capillaries and lacteal, enhancing uptake.

6. Summary Table – Structure ↔ Function

7. Nutrient‑Specific Transport Overview

8. Key Points to Remember (AO1)

• Villi and microvilli maximise the surface area available for absorption (≈600‑fold increase).
• The one‑cell‑thick epithelium shortens diffusion distance and contains brush‑border enzymes that finish digestion.
• Active and secondary‑active transport (e.g., SGLT1) move sugars and amino acids against concentration gradients; facilitated diffusion moves other nutrients.
• Water‑soluble nutrients enter blood capillaries and travel via the hepatic portal vein to the liver for metabolism.
• Lipid‑soluble nutrients are packaged into chylomicrons, enter the central lacteal and are transported by the lymphatic system.
• Smooth‑muscle fibres in the villus core create a gentle pumping action that assists the movement of absorbed substances toward the vessels.
• Water and electrolytes are absorbed throughout the small intestine and join the blood capillary network.
• Pancreatic bicarbonate neutralises gastric acid in the duodenum, providing an optimal pH for pancreatic enzymes.
• The liver metabolises most absorbed nutrients (e.g., glucose → glycogen, amino acids → trans‑amination) before they enter the systemic circulation.