Ch 23 Digestive System Functional Anatomy Pt III

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

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

Key features: Circular folds, Villi, Microvilli

Ciruclar 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.

Villus

Multicellular structure containing blood vessels and lymphatic vessels within and lining epithelium composed of enterocytes and microvilli.

Intestinal crypts

Enterocytes, goblet cells, paneth cells, enteroendocrine cells, stem cells

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 mechanism

• 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.