Name the body structure concerned with the given functional activity :
Provides actual diffusion of respiratory gases in lungs.

Explanation

### Step-by-Step Solution:

1. **Identify the Function**: The question asks for the body structure responsible for the actual diffusion of respiratory gases in the lungs. The key terms here are "diffusion" and "respiratory gases".

2. **Understand the Process of Gas Exchange**: In the lungs, gas exchange occurs primarily through the process of diffusion, where oxygen enters the blood and carbon dioxide is expelled from the blood.

3. **Locate the Structures Involved**: The primary structures involved in this gas exchange process are the alveoli and the capillaries.

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