Study Notes on the Small Intestine and its Functions

Introduction to the Small Intestine

The small intestine (intestino delgado) is a crucial segment of the digestive system situated between the pylorus (the opening of the stomach) and the ileocecal valve (valvula ileocecal). It is responsible for the majority of nutrient digestion and absorption in all species. The small intestine is suspended in the abdominal cavity by the mesenteries.

Functions of the Small Intestine

The small intestine fulfills several vital functions:

  • Production of Intestinal Juices and Hormones: It produces various digestive entities that aid in digestion.
  • Reception of Pancreatic and Hepatic Juices: It receives enzymes and other substances from the pancreas and liver, essential for digestion.
  • Mechanical and Chemical Digestion: The small intestine conducts both mechanical and chemical processes to break down food into absorbable units.
  • Nutrient Absorption: One of its primary roles is to absorb nutrients into the bloodstream.
  • Transport of Undigested Material to the Large Intestine: It aids in the movement of indigestible substances to the large intestine for elimination.
  • Barrier Against Microbial and Toxic Infiltration: It helps prevent the entry of harmful microbes and toxins into the bloodstream.

Anatomy of the Small Intestine

The small intestine is divided into three main regions based on function and histological structure:

  1. Duodenum
  2. Jejunum
  3. Ileum
    In birds, it comprises only two sections: the duodenum and jejunum.
Regions and Their Structures
  • Duodenum: Short and C-shaped, it receives digestive juices from the pancreas and liver through the ampulla of Vater. It also contains Brunner's glands, which secrete alkaline mucus that protects the duodenum from acidic chyme.
  • Jejunum: Known for its role in nutrient absorption, it is longer than the duodenum.
  • Ileum: The final segment of the small intestine, it contains Peyer’s patches, which are clusters of lymphoid tissue that play a role in immune response.

Cross-Sectional Structure of the Small Intestine

The small intestine has multiple layers:

  • Mucosa: The innermost layer containing folds known as circular folds (plicae circulares), which increase the absorptive surface area.
  • Submucosa: Contains connective tissue, blood vessels, and glands.
  • Muscularis External: Composed of muscle layers that facilitate peristalsis.
  • Serosa: The outer layer, also known as the visceral peritoneum.

Microscopic Features

  • Villi: Finger-like projections that increase the surface area for absorption, each villus contains:
    • Enterocytes: Specialized epithelial cells involved in nutrient absorption.
    • Lacteals: Lymphatic vessels that absorb fats.
    • Goblet Cells: Secrete mucin for lubrication.
  • Microvilli: Smaller projections on enterocytes that further increase surface area and are collectively known as the brush border.
  • Crypts of Lieberkühn: Located at the base of villi, these intestinal glands produce digestive enzymes and renew enterocytes every 3-5 days.

Digestive Enzymes and Hormones

Enzyme Production

  • Digestive Enzymes: Produced in the crypts of Lieberkühn, essential for digestion including:
    • Enterokinase: Converts trypsinogen to trypsin.
    • Aminopeptidases: Hydrolyze polypeptides to terminal amino acids.
    • Maltase: Hydrolyzes maltose into glucose.
    • Lactase: Hydrolyzes lactose into glucose and galactose.
    • Sacarasa: Hydrolyzes sucrose (table sugar) into glucose and fructose.

Hormones of the Small Intestine

Cholecystokinin (CCK)
  • Origin: Produced by enteroendocrine I cells in the duodenum.
  • Stimulus for Secretion: Triggered by HCl, peptides, and lipids in chyme.
  • Functions:
    • Stimulates secretion of pancreatic juices and bile.
    • Inhibits gastric emptying.
    • Enhances intestinal mucosal growth and enzyme secretion.
Secretin
  • Origin: Produced by S cells in the duodenum.
  • Stimulus for Secretion: Triggered by acid in the chyme.
  • Functions:
    • Inhibits gastric motility and secretion.
    • Promotes bicarbonate secretion from the pancreas and liver.
GIP (Gastric Inhibitory Polypeptide)
  • Origin: Produced by K cells in the duodenum and jejunum.
  • Stimulus for Secretion: Glucose and fat presence in the intestine.
  • Functions:
    • Stimulates insulin secretion from the pancreas.
VIP (Vasoactive Intestinal Polypeptide)
  • Origin: Found throughout the gastrointestinal tract.
  • Functions:
    • Stimulates electrolyte and water secretion.
    • Promotes vasodilation and inhibits secretions in the stomach.
Motilin
  • Origin: Produced by M cells in the duodenum.
  • Functions:
    • Stimulates intestinal peristalsis.

Summary of the Small Intestine

The small intestine is integral to digestion and nutrient absorption within the digestive system. It comprises multiple regions that work cohesively and is characterized by specialized structures that enhance absorption. The roles played by various hormones and enzymes produced within the small intestine further illustrate its vital contributions to overall digestive health and efficiency. Through these mechanisms, the small intestine ensures proper absorption of nutrients while safeguarding against harmful microbes and toxins.