Lecture Notes: Small Intestine

Overview

• The lecture covers the structure and function of the small intestine.
• Focus on cellular composition, absorption processes, and interaction with other digestive organs.

Structure of the Small Intestine

• Location: Begins at the pyloric sphincter of the stomach.
• Sections: Consists of three parts: duodenum, jejunum, and ileum.
  • Duodenum: Shortest, fixed, receives chyme and digestive secretions.
  • Jejunum: Thicker, more mobile, major site for absorption.
  • Ileum: Contains Peyer's patches, connects to the large intestine.

Functionality

• Digestive Assistance: Receives chyme from the stomach, liver, and pancreatic secretions.
• Absorption: Main site for nutrient absorption.
• Transport: Mesentery supports the intestine, transporting blood, lymphatic vessels, and nerves.

Cellular Composition and Surface Area

• Villi and Microvilli: Increase surface area for absorption.
  • Villi: Projections on the intestinal wall.
  • Microvilli: Further increase surface area on villi.
• Epithelium: Simple columnar epithelium with blood vessels and lacteals for nutrient absorption.

Digestive Enzymes and Processes

• Mucus Secretion: By goblet cells and Brunner's glands.
  • Alkaline mucus neutralizes stomach acid.
• Enzymes: Aid in breaking down macromolecules.
  • Peptidases: Break down proteins into amino acids.
  • Sucrase, Maltase: Break down disaccharides to monosaccharides.
  • Lipases: Break down fats into fatty acids and glycerol.

Regulation of Secretion

• Mechanical Stimulation: Presence of chyme stimulates secretion.
• Nerve Plexuses: Activated by distension of the intestine.
• Parasympathetic Influence: Stimulates enzyme release.

Absorption Mechanics

• Carbohydrate Digestion: By amylases, breaking down starch into simpler sugars.
• Protein Digestion: By pepsin and proteases, further broken down by intestinal peptidases.
• Fat Digestion: Emulsified by bile salts, absorbed as micelles.

Movement in the Small Intestine

• Peristalsis: Moves contents down the intestine.
• Segmentation: Mixes contents to enhance digestion.
• Parasympathetic Stimulation: Enhances both movement types.

Key Terms

• Ileocecal Sphincter: Regulates chyme entry into the large intestine.
• Lacteals: Lymphatic vessels involved in fat absorption.

Conclusion

• The small intestine is vital for digestion and nutrient absorption.
• The lecture concludes with a preview of the large intestine.