small initestin / anatomy

Overview of the Gastrointestinal System

  • The gastrointestinal system (GI system)
      - Responsible for the intake and breakdown of consumed substances into nutrients.
      - Functions involve both motility (movement through the tract) and secretions (enzymes and acids) facilitating digestion and absorption.

Anatomy of the Gastrointestinal System

  • The GI system consists of:
      - Gastrointestinal Tract (GI tract)
      - Accessory glands

  • Accessory glands provide secretions that assist in digestion and absorption.

Layers of the GI Tract Wall

  • The wall of the GI tract is composed of four main layers:
      1. Mucosa
      2. Submucosa
      3. Muscularis externa
      4. Serosa

1. Mucosa

  • Depicted in yellow, lining the lumen (inner cavity) of the GI tract.
      - Functions as a barrier between the body and lumen contents.
      - Composed of three sub-layers:
        - Epithelial lining
          - Separates inside body from outside body.
          - Contains specialized cells:
            - Endocrine cells: Secrete hormones.
            - Exocrine cells: Secrete digestive enzymes.
            - Absorptive cells: Facilitate nutrient absorption.
        - Lamina propria
          - Connective tissue with blood vessels, lymphatic vessels, and nervous tissue.
          - Contains Peyer's patches (lymph nodes) for immune response against foreign pathogens.
        - Muscularis mucosa
          - Thin layer of smooth muscle with:
            - Longitudinal fibers: Shorten GI tract length.
            - Circular fibers: Change lumen diameter for mixing content.

2. Submucosa

  • Thicker layer of connective tissue that is distensible and elastic.
      - Contains larger blood and lymphatic vessels, as well as the submucosal plexus (Meissner's plexus) that controls GI motility.

3. Muscularis Externa

  • Composed of two layers of smooth muscle:
      - Inner circular layer: Alters the diameter of the GI tract.
      - Outer longitudinal layer: Alters the length of the GI tract.

  • Contains the myenteric plexus (Auerbach's plexus) that regulates muscle contractions and GI motility.

4. Serosa

  • The outer layer consists of:
      - Inner connective tissue layer
      - Outer mesothelium (epithelial tissue): Connects to the abdominal cavity lining (peritoneum).
      - The peritoneum reduces friction during GI tract movements.

Functions and Sections of the GI Tract

Mouth

  • Digestion begins with chewing (mastication), aided by saliva:
      - Saliva composition:
        - Mucus: Lubricates food for easier swallowing.
        - Salivary amylase: Enzyme that begins carbohydrate digestion.

Pharynx

  • Throat region, a passage for both air and food.

Esophagus

  • Muscular tube connecting pharynx to stomach.
      - Contains:
        - Upper esophageal sphincter: Regulates entry of food.
        - Lower esophageal sphincter: Prevents gastric contents from refluxing into the esophagus.

Stomach

  • Stores food and begins digestion:
      - Volumes:
        - Empty volume: 50 mL
        - Full volume: 1000 mL (capable of expanding up to 20 times its empty volume).

  • Gastric secretions include:
      - Gastric juices: Break down nutrients into chyme.
      - Contractions: Mix food with gastric juices, forming chyme.
      - Pyloric sphincter: Controls the passage of chyme into the small intestine.

  • Rugae: Folds in the stomach that allow for expansion.

Key Secretions in the Stomach:

  1. Pepsinogen
       - Inactive enzyme secreted by chief cells.
       - Converts to pepsin in acidic conditions for protein digestion.

  2. Hydrochloric acid (HCl)
       - Secreted by parietal cells to create an acidic environment (pH ~ 2).
       - Functions:
         - Activates pepsinogen.
         - Denatures proteins and kills bacteria.

  3. Intrinsic factor
       - Binds to vitamin B12 for absorption in the small intestine, secreted by parietal cells.

  4. Gastrin
       - Hormone from G cells, stimulates chief and parietal cells to prepare for food arrival.

  5. Mucus
       - Secreted by neck cells to protect the stomach lining from HCl's acidity.

Peptic Ulcers

  • Caused by H. Pylori bacteria which can survive stomach acidity and damage stomach lining.

  • Symptoms can worsen with NSAID use, leading to less mucus production and increased susceptibility to damage.

Small Intestine

  • Primary site for nutrient digestion and absorption, consists of three segments:
      1. Duodenum (first section)
         - Receives pancreatic juices and bile.
         - Bicarbonate in pancreatic juices neutralizes acidic chyme.
         - Bile salts emulsify fats.
      2. Jejunum (middle section, main absorptive site).
      3. Ileum (final section, absorbs remaining nutrients).

  • Villi and microvilli increase surface area for absorption, creating a brush border.

  • Crypts of Lieberkühn between villi secrete bicarbonate to further neutralize acidic chyme and absorb it at the distal end.