Overview
This lecture covers the anatomical and cellular changes from the trachea through the bronchial tree to the alveoli, emphasizing how the structure adapts for efficient gas exchange in the respiratory zone.
Lower Respiratory Tract Structure
- The trachea divides into paired primary bronchi, then lobal, segmental, and smaller bronchi, ending in alveoli.
- Lung lobules are roughly hexagonal and about the size of an eraserhead; each contains many air sacs.
- Respiratory bronchioles get progressively smaller through about 23 divisions before reaching alveoli.
Cellular and Structural Changes Along the Airway
- Trachea has a large lumen with thick cartilage to keep airways open; lined by tall, ciliated pseudostratified columnar epithelium.
- Moving downward, cartilage decreases and smooth muscle increases relatively (though less in absolute terms).
- Epithelial cells get shorter and less ciliated, transitioning from columnar to cuboidal, then to squamous epithelium.
- Terminal bronchioles have little to no cartilage, high relative smooth muscle for airway control, and minimal cilia.
- Macrophages begin to patrol distal bronchioles as cilia disappear.
Respiratory Zone and Alveoli
- Respiratory bronchioles and alveolar ducts are lined by thin, single-cell epithelium with very little smooth muscle.
- Alveoli are grape-like air sacs where gas exchange occurs, lined by Type I and Type II pneumocytes.
- Type I pneumocytes form the structure, while Type II pneumocytes produce surfactant to reduce surface tension.
- Elastic fibers surround alveoli, promoting recoil for passive exhalation.
- Capillaries surround alveoli, facilitating external respiration (gas exchange with blood).
- Macrophages move between alveoli via small pores to help keep air sacs clean.
Key Terms & Definitions
- Lobule — Smallest lung unit visible to the naked eye, containing many alveoli.
- Pseudostratified Columnar Epithelium — Tall, ciliated cells in the trachea and bronchi for airway cleaning.
- Cuboidal/Squamous Epithelium — Shorter, flatter cells found deeper in the airway for thinner barriers.
- Terminal Bronchiole — Last airway with abundant smooth muscle, minimal/no cartilage, leads to respiratory zone.
- Alveolus (plural: Alveoli) — Tiny air sac for gas exchange.
- Type I Pneumocyte — Thin cell forming alveolar wall, enables gas exchange.
- Type II Pneumocyte — Cell producing surfactant to prevent alveolar collapse.
- Surfactant — Lipid substance reducing water surface tension in alveoli.
- External Respiration — Gas exchange between lungs and blood.
Action Items / Next Steps
- Review histological slides of airway structures from trachea to alveoli.
- Study the functions of different cell types in the respiratory system.