Which of the following structures is NOT part of the respiratory membrane involved in gas exchange in the human lung?

• A. Capillary endothelium
• B. Smooth muscle layer
• C. Fused basement membrane
• D. Alveolar epithelium

Answer: (B)

Verified step by step guidance

1. Understand the respiratory membrane: The respiratory membrane is a thin barrier in the lungs where gas exchange occurs between the air in the alveoli and the blood in the capillaries. It consists of specific structures that facilitate this exchange.
2. Identify the components of the respiratory membrane: The respiratory membrane includes the alveolar epithelium, the fused basement membrane (shared by the alveolar epithelium and capillary endothelium), and the capillary endothelium. These structures are directly involved in gas exchange.
3. Analyze the role of smooth muscle: Smooth muscle is not part of the respiratory membrane. Instead, it is found in the walls of bronchi and bronchioles, where it helps regulate airflow by contracting or relaxing. It does not participate in gas exchange.
4. Compare the listed structures: Capillary endothelium, fused basement membrane, and alveolar epithelium are all integral parts of the respiratory membrane. Smooth muscle, however, is not involved in the gas exchange process.
5. Conclude which structure is NOT part of the respiratory membrane: Based on the analysis, smooth muscle is the correct answer as it is not part of the respiratory membrane involved in gas exchange.