OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

How are alveoli adapted for efficient gas diffusion?

1. Thick walls

2. Large air sacs

3. Moist lining

4. Low blood supply

The correct answer is: Moist lining

Alveoli are small air sacs in the lungs that are intricately designed to facilitate efficient gas exchange. One of the key adaptations that contribute to their effectiveness is the presence of a moist lining. This moisture is essential because gases, including oxygen and carbon dioxide, can only diffuse through a liquid phase. The presence of a thin layer of moisture on the alveolar surface allows for the gases to dissolve before diffusion occurs, enhancing the overall rate of gas exchange. The structure of alveoli further supports their function. They are lined with a single layer of epithelial cells, which minimizes the distance that gases must travel, allowing for quick and efficient exchange. Moreover, the large surface area of the extensive network of alveoli increases the capacity for gas exchange, while the thin walls (not thick) promote rapid diffusion. Adequate blood supply is also a crucial factor, as it enables efficient transport of gases to and from the alveoli, but in this context, the low blood supply option would hinder rather than facilitate efficiency.