9.03 Trachea, Bronchi, and Bronchioles

1. Overview of Airway Structure

Airway Pathway

• Air travels from the throat to the lungs through a hierarchical branching system of airways:

Main Pathway:

Trachea → Bronchi → Bronchioles → Alveolar Ducts → Alveoli

Trachea (Windpipe)

• Connection: Links the larynx (voice box) to the bronchi.
• Structure:
  • Shape: Tube-like.
  • Support: C-shaped rings of cartilage maintain an open airway.
• Function:
  • Facilitates the movement of air in and out of the lungs.
  • Maintains low air resistance during breathing.

Bronchi (Singular: Bronchus)

• Branches: Two primary bronchi branch from the trachea, each entering one lung.
• Structure:
  • Support: Irregular blocks of cartilage prevent collapse and keep airways open.
• Function:
  • Directs air into each lung.
  • Further subdivides to form the bronchial tree, increasing the surface area for air distribution.

Bronchioles

• Description: Smaller branches stemming from the bronchi within each lung.
• Types:
  • Terminal Bronchioles:
    • Role: Do not participate in gas exchange.
    • Function: Supply air to respiratory bronchioles.
  • Respiratory Bronchioles:
    • Role: Lead to alveolar ducts.
    • Function: Supply air directly to alveoli for gas exchange.
• Structure:
  • Lack Cartilage: Unlike larger airways.
  • Presence of Smooth Muscle: Controls airway diameter to regulate airflow.

2. Structural Components and Their Functions

Cartilage

• Locations:
  • Trachea: C-shaped rings.
  • Bronchi: Irregular blocks.
• Functions:
  • Airway Support: Keeps airways open, preventing collapse or obstruction during breathing.
  • Airflow Efficiency: Reduces air resistance, ensuring smooth passage of air.

Smooth Muscle

• Location: Predominantly in bronchioles.
• Appearance: Thin bands or layers observable under a microscope.
• Functions:
  • Airway Regulation: Contracts or relaxes to adjust bronchiole diameter.
  • Flow Control: Regulates airflow distribution to different lung regions, optimizing gas exchange.

3. Role and Structure of Alveoli

Alveoli

• Description: Tiny, balloon-like air sacs at the end of the respiratory pathway.
• Surface Area: Approximately 70–75 m² in total, providing a vast area for gas exchange.
• Function:
  • Oxygen Uptake: Facilitates the diffusion of oxygen into the blood.
  • Carbon Dioxide Removal: Allows carbon dioxide to diffuse out of the blood.
• Importance:
  • Efficient Gas Exchange: Large number and surface area compensate for oxygen’s low solubility in water, ensuring effective diffusion necessary for cellular respiration.