Gas exchange of oxygen and carbon dioxide is normally limited by:

• A. Diffusion
• B. Perfusion
• C. Both diffusion and perfusion equally
• D. Vascular resistance

Answer: (B)

Gas exchange of oxygen and carbon dioxide in the lungs primarily relies on perfusion, which is the process of blood flow through the pulmonary capillaries surrounding the alveoli. While diffusion — the movement of gases across the alveolar membrane — does play a significant role, it is the availability and distribution of blood flow (perfusion) that is commonly the limiting factor for efficient gas exchange.

In healthy lungs, when perfusion is optimal, oxygen can rapidly diffuse into the bloodstream while carbon dioxide diffuses out. However, if perfusion is inadequate due to conditions like pulmonary embolism or other circulatory issues, the efficiency of gas exchange is compromised. In such cases, even if the alveoli are functioning well for diffusion, the lack of blood flow means less gas exchange will occur because there is insufficient blood to carry the gases away or bring them in.

This principle is fundamental in understanding conditions such as V/Q (ventilation-to-perfusion) mismatch, where normal ventilation occurs, but insufficient perfusion limits gas exchange. Therefore, recognizing that perfusion serves as the primary limiting factor is key in respiratory physiology.