What is the primary driving force behind gas exchange in the alveoli?

• A. Active transport
• B. Diffusion
• C. Osmosis
• D. Filtration

Answer: (B)

The primary driving force behind gas exchange in the alveoli is diffusion. In the process of gas exchange, oxygen moves from areas of higher concentration (in the alveoli) to areas of lower concentration (in the blood within the capillaries). Conversely, carbon dioxide moves from the blood (where its concentration is higher) into the alveoli (where its concentration is lower) to be exhaled. This movement occurs across the alveolar and capillary membranes due to the natural tendency of gases to move towards equilibrium, following their concentration gradients.

Diffusion is essential in this context because it allows for the efficient transfer of gases without the need for energy expenditure, contrasting with active transport mechanisms that require energy to move substances against their concentration gradients. Osmosis and filtration have different roles in the body and are not directly involved in the gas exchange process in the alveoli.