Ch 23 Digestive System Functional Anatomy Pt III

Chapter 23: The Digestive System, Functional Anatomy of the Digestive System, Part Three

Overview of the Small Intestine

• Runs from the pyloric sphincter (at the stomach) to the ileocecal valve (to the large intestine).

• Regulates flow of contents using sphincters and valves.

• Length: Approximately 21 feet:

  • Duodenum: ~1 foot.

  • Jejunum: ~8 feet.

  • Ileum: ~12 feet.

• Named "small" due to its diameter, not its length (smaller than the large intestine).

Functions of the Small Intestine

• Site of chemical digestion for proteins, fats, and carbohydrates.

• Chyme enters the duodenum where it mixes with digestive secretions facilitating digestion.

• Absorption of nutrients occurs primarily in the jejunum, continues into the ileum.

  • The small intestine is the primary organ for nutrient absorption from the diet.

Structural Features Enhancing Absorption

• Increased Surface Area: Vital for nutrient absorption.

• Three Key Features:

  • Circular Folds: Large grooves that spiral down the small intestine.

  • Villi: Multicellular structures on the folds that increase surface area.

  • Microvilli: Extensions on the enterocytes that form the brush border, further increasing surface area by 600-fold, equivalent to a tennis court.

Anatomy of Villi and Associated Structures

• Villus: Multicellular structure containing:

  • Blood vessels and lymphatic vessels within.

  • Lining epithelium composed of enterocytes and microvilli.

• Intestinal Crypts (Crypts of Lieberkühn): Contains various cell types:

  • Enterocytes: Absorptive cells that also secrete intestinal juice.

  • Goblet Cells: Secrete mucin, forming a protective mucus coat.

  • Paneth Cells: Secrete lysozymes and defensins to prevent intestinal infections.

  • Enteroendocrine Cells: Secrete signaling chemicals like secretin that regulate digestive functions.

  • Stem Cells: Responsible for cell turnover and replacement in the intestinal lining.

Role of Enterocytes in Absorption

• Enterocytes possess microvilli to enhance absorption.

• Contain many mitochondria for energy required for active transport of nutrients.

• Actively move nutrients from the lumen into the bloodstream and lymphatic system.

Mucus Layer and Protecting Mechanisms

• Goblet cells continuously produce mucus to prevent localized inflammation and facilitate movement of bacteria.

• MALT (Mucosa-Associated Lymphoid Tissue) offers immune protection within the small intestine.

Summary of Digestive Processes

• Chemical and mechanical digestion occurs as chyme moves through the small intestine, initiated in the duodenum.

• Nutrient absorption starts in the jejunum and continues as chyme travels towards the large intestine.