Which of the following structures is NOT a component of the respiratory membrane in the lungs?

• A. Fused basement membrane
• B. Alveolar epithelial cell
• C. Smooth muscle cell
• D. Capillary endothelial cell

Answer: (C)

Verified step by step guidance

1. Understand the respiratory membrane: The respiratory membrane is the site of gas exchange in the lungs, consisting of structures that facilitate the diffusion of oxygen and carbon dioxide between the alveoli and capillaries.
2. Identify the components of the respiratory membrane: The respiratory membrane includes the alveolar epithelial cell, the fused basement membrane (shared by alveolar epithelium and capillary endothelium), and the capillary endothelial cell.
3. Recognize the role of smooth muscle cells: Smooth muscle cells are not part of the respiratory membrane. They are found in the walls of bronchi and bronchioles, where they help regulate airflow by contracting or relaxing, but they do not participate in gas exchange.
4. Compare the listed structures: Fused basement membrane, alveolar epithelial cell, and capillary endothelial cell are all components of the respiratory membrane, while smooth muscle cells are not.
5. Conclude the correct answer: Smooth muscle cells are not a component of the respiratory membrane because they are not involved in the gas exchange process.