Diffusion is defined as:

• A. Movement of gas from an area of higher concentration to an area of lower concentration
• B. The exchange of gases between the lungs and the blood
• C. The body's use of oxygen for metabolism
• D. The diffusion of carbon dioxide within the blood

Answer: (A)

Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration, driven by random molecular motion and requiring no energy. In the lungs, this principle explains gas movement: oxygen moves from the alveoli, where its concentration is higher, into the blood, where it is lower, while carbon dioxide moves from the blood (higher concentration) into the alveolar air (lower concentration). The broader idea of gas exchange describes how gases move between lungs and blood, but diffusion is the mechanism that makes that exchange possible. The body's use of oxygen for metabolism is a separate cellular process, and diffusion of carbon dioxide within the blood refers to a more specific, localized movement rather than the general definition of diffusion.