Lecture on the Small Intestine and Digestion

Introduction to the Small Intestine

• Food, or chyme, exits the stomach through the pyloric sphincter into the small intestine.
• The small intestine is a large tissue folded in the abdominal region, crucial for digestion and nutrient absorption.

Regions of the Small Intestine

1. Duodenum:
   • First section, about 12 inches long.
   • Pronounced as 'duodenum' by clinicians and 'dadum' by anatomists.
   • Key role in neutralizing stomach acid with alkaline mucus.
2. Jejunum:
   • Middle section, involved in nutrient absorption.
3. Ileum:
   • Largest and distal section, major site for nutrient absorption.

Structural Features

• Folding:
  • Provides large surface area for absorption.
  • Similar to brain folding.
• Circular Folds and Villi:
  • Inner surface has circular folds with projections called villi.
  • Villi covered with simple columnar epithelial tissue and microvilli forming the brush border.
• Brush Border Enzymes:
  • Embedded in microvilli, aid in breaking down nutrients into absorbable units.

Nutrient Absorption

• Capillaries in Villi:
  • Absorb amino acids and saccharides.
  • Transported via hepatic portal system to liver.
• Lymphatic Lacteals:
  • Absorb lipids packaged as chylomicrons.
  • Chylomicrons enter the lymphatic system and then the bloodstream.

Cellular Components

• Enterocytes:
  • Intestinal cells responsible for digestion and absorption.
• Goblet Cells:
  • Secrete mucus.
• Paneth Cells:
  • Release antimicrobial enzymes.
• Enteroendocrine Cells:
  • Release hormones into blood to regulate digestion.

Duodenum Features

• Neutralization of Acidic Chyme:
  • Alkaline mucus from submucosal glands neutralizes acid.
  • Intestinal juice includes digestive enzymes and buffers.

Digestive Enzymes and Processes

• Brush Border Enzymes:
  • Breakdown disaccharides and dipeptides.
• Pancreatic Enzymes:
  • Activated in the small intestine, e.g., trypsinogen to trypsin.

Regulation of Digestion

• Neural Control:
  • Parasympathetic stimulation enhances digestion, sympathetic inhibits.
• Reflexes:
  • Gastroileal reflex: stomach fullness increases small intestine activity.
• Hormonal Control:
  • Gastrin: Increases stomach motility.
  • Secretin: Stimulates pancreatic buffer release.
  • Cholecystokinin (CCK): Stimulates bile release and enzyme secretion.

Transition to Large Intestine

• Ileocecal Valve:
  • Regulates passage from ileum to large intestine.
• Nutrient Absorption in Ileum:
  • Absorbs fats, bile, and remaining nutrients.

This lecture covered the structure, function, and regulation of the small intestine, highlighting its critical role in digestion and nutrient absorption.