Which of the following structures is directly involved in gas exchange within the alveoli of the human lung?

• A. Ciliated epithelial cells
• B. Smooth muscle cells
• C. Type I alveolar cells (pneumocytes)
• D. Goblet cells

Answer: (C)

Verified step by step guidance

1. Understand the function of alveoli: Alveoli are tiny air sacs in the lungs where gas exchange occurs. Oxygen from inhaled air diffuses into the blood, and carbon dioxide from the blood diffuses out into the alveoli to be exhaled.
2. Identify the role of Type I alveolar cells: These cells are thin and flat, forming the majority of the alveolar surface area. Their structure facilitates efficient gas exchange by minimizing the distance for diffusion of oxygen and carbon dioxide.
3. Differentiate Type I alveolar cells from other cell types: Ciliated epithelial cells are involved in moving mucus and trapped particles out of the respiratory tract, smooth muscle cells control the diameter of airways, and goblet cells produce mucus. None of these are directly involved in gas exchange.
4. Recognize the importance of diffusion: Gas exchange in the alveoli occurs via passive diffusion, which is driven by concentration gradients of oxygen and carbon dioxide. Type I alveolar cells provide the structural support for this process.
5. Conclude that Type I alveolar cells are the correct answer: Their specialized structure and function make them directly responsible for gas exchange within the alveoli of the human lung.