Published by Patrick Mutisya · 14 days ago
Describe the distribution of the following structures within the human gas‑exchange system:
The respiratory tract can be divided into the conducting zone and the respiratory zone. The distribution of the listed tissues reflects the functional requirements of each zone.
| Structure | Primary Location(s) | Functional Significance |
|---|---|---|
| Cartilage | Trachea, main bronchi, and lobar bronchi (C‑shaped rings in trachea; plate‑like in bronchi) | Provides rigidity to keep airways open during respiration and prevents collapse during negative intrathoracic pressure. |
| Ciliated epithelium | Entire conducting zone – nasal cavity, trachea, bronchi, and bronchioles (up to terminal bronchioles) | Moves mucus and trapped particles toward the pharynx (mucociliary clearance). |
| Goblet cells | Scattered among ciliated columnar epithelium of the nasal cavity, trachea, and larger bronchi. | Secrete mucus that traps dust, microbes, and other inhaled particles. |
| Squamous epithelium of alveoli (type I pneumocytes) | Alveolar walls (respiratory zone) | Forms a thin diffusion barrier (≈0.1 µm) for rapid gas exchange between alveolar air and pulmonary capillary blood. |
| Smooth muscle | Bronchi, bronchioles, and arterioles within the lung parenchyma. | Regulates airway calibre (bronchoconstriction/dilation) and pulmonary vascular resistance, influencing ventilation‑perfusion matching. |
| Capillaries | Surrounding each alveolus; form a dense network in the respiratory zone. | Provide a large surface area for O₂ uptake and CO₂ release; thin walls enable diffusion according to \$J = -D\frac{\Delta C}{\Delta x}\$ where \$J\$ is flux, \$D\$ the diffusion coefficient, \$\Delta C\$ the concentration gradient, and \$\Delta x\$ the diffusion distance. |
1. Air entry and filtration – Nasal hairs and mucus from goblet cells trap particles; ciliated epithelium moves this mucus toward the pharynx.
2. Airway support – Cartilage rings prevent collapse of the trachea and larger bronchi, maintaining a patent airway.
3. Airway calibre control – Smooth muscle contracts or relaxes in response to autonomic signals, altering resistance and airflow distribution.
4. Diffusion surface – In the alveoli, type I pneumocytes provide a minimal barrier, while an extensive capillary network ensures a short diffusion distance and large total surface area.