Chapter 24: The Digestive System (Part 6) - Small Intestine

Chapter 24: The Digestive System (Part 6) - Small Intestine

Functions of Small Intestine

  • The primary functions of the small intestine include:

    • Peristalsis: Propels chyme through the small intestine.

    • Mixing: Mixes chyme with digestive juices.

    • Digestion: Completes digestion of carbohydrates, proteins, and lipids.

    • Contact for Absorption: Brings food into contact with the mucosa for absorption.

    • Nucleic Acids: Begins and completes digestion of nucleic acids.

    • Nutrient Absorption: Absorbs approximately 90% of nutrients passing through the gastrointestinal (GI) tract.

Anatomy of the Small Intestine

  • The small intestine extends from the pyloric sphincter (gastroduodenal junction) to the ileocecal sphincter, where it connects to the large intestine.

  • Length: Approximately 10 feet, providing a large surface area conducive to digestion and absorption.

Structural Features Increasing Surface Area

  1. Plicae Circulares (Circular Folds)

    • Permanent ridges found in the mucosa and submucosa.

    • These folds cause chyme to “spiral,” which enhances mixing and absorption efficiency.

  2. Villi

    • Fingerlike projections of the mucosa.

    • Structure includes:

      • Epithelium: Simple columnar epithelium.

      • Lamina Propria: Contains blood vessels and lymphatic nodules.

      • Muscularis Mucosae: Muscular layer to assist in movement.

      • Arteriole and Venule: For blood flow.

      • Lacteal: A lymphatic vessel for fat absorption.

      • Capillaries: For nutrient transport.

  3. Microvilli

    • Projections of the apical membrane of absorptive cells.

    • Form a structure known as the brush border, which contains various brush border enzymes critical for digestion.

Histological Features of the Small Intestine

Mucosa (Duodenum)

  • Composed of several cell types:

    • Absorptive cells: Absorb nutrients.

    • Goblet cells: Secrete mucus to aid in lubrication.

    • Intestinal glands: Secrete intestinal juice, containing various digestive enzymes.

    • Paneth cells: Secrete lysozyme and perform phagocytosis to combat pathogens.

    • Enteroendocrine cells: Secrete hormones such as secretin and cholecystokinin (CCK), which regulate digestive processes.

Submucosa (Duodenum)

  • Contains Duodenal (Brunner’s) glands:

    • These glands secrete alkaline mucus to neutralize gastric acid entering the chyme from the stomach.

    • Intestinal juice maintains a pH of approximately 7.6.

Submucosa (Ileum)

  • Contains aggregated lymphatic nodules known as Peyer’s patches:

    • Function in immune surveillance of the intestinal lumen, protecting against pathogens.

    • Presence of Peyer’s patches distinguishes the ileum from the duodenum and jejunum.

    • Encounter macrophages, dendritic cells, B-lymphocytes, and T-lymphocytes that help in immune responses to antigens in the intestinal tract.

Digestive Processes in the Small Intestine

Mechanical Digestion: Segmentation

  • Defined as:

    • Stationary, oscillatory, alternating contractions.

    • Mixes chyme with intestinal juices and enhances contact with absorptive epithelium.

    • Does not propel chyme down the intestine.

    • Breaks up triglycerides into smaller fat droplets for absorption.

Composition of Chyme Entering the Duodenum

  • Contains:

    • Partially digested carbohydrates (due to salivary amylase action in the mouth).

    • Partially digested proteins (activated pepsinogen from the stomach).

    • Partially digested lipids (influenced by lingual lipase and gastric lipase).

Brush Border Enzymes

  • Synthesized by absorptive cells:

    • Inserted into the plasma membrane of cells.

    • Participate in enzymatic digestion at the surface instead of only the lumen.

    • Include enzymes such as:

    • α-Dextrinase

    • Maltase

    • Sucrase

    • Lactase

    • Enterokinase

    • Aminopeptidase

    • Dipeptidase

    • Nucleosidases and phosphatases.

    • These enzymes along with those from the pancreas and bile finalize the digestion of carbohydrates, proteins, lipids, and nucleic acids.

Fluid Balance and Water Absorption

  • Approximately 9.3 liters of fluid come from ingestion (2.3 L) and GI secretions (7.0 L).

  • Most of this fluid is absorbed in the small intestine, with some absorption occurring in the large intestine.

  • Only about 100 mL is excreted in feces.

  • All absorption occurs by osmosis, emphasizing the small intestine's role in maintaining fluid balance and absorbing considerable volumes of water.

Enzymatic Disorders

  • Lactose Intolerance:

    • A genetic disorder that leads to the failure of intestinal mucosal cells to produce lactase.

    • Resulting issues include an inability to digest lactose, leading to symptoms like diarrhea, gas, bloating, and abdominal cramps after dairy consumption.

  • Enteritis:

    • Chronic inflammation of the gastrointestinal tract, typically affecting both the small and large intestines.

    • Causes are often unknown but may involve:

    • Viral or bacterial infection

    • Immune disorder

    • Defects in the intestinal barrier

    • Dysfunctional mucosal repair

    • Genetic predisposition

    • Dietary and environmental factors.

Capsule Endoscopy

  • Describes a diagnostic tool: a capsule equipped with a mini video camera that can explore parts of the small intestine not accessible by standard scopes.

  • Designed for evaluating unexplained bleeding or other abnormalities in the GI tract.

  • The camera transmits video data to a recorder worn on a belt, which is later downloaded to a computer for analysis.