Diffusion in body cells and lungs always moves from an area of high concentration to an area of low concentration.

Here's how this applies to specific examples:

* Lungs: Oxygen is at a higher concentration in the alveoli (tiny air sacs) of the lungs than in the blood. So, oxygen diffuses from the alveoli into the blood. Carbon dioxide is at a higher concentration in the blood than in the alveoli, so it diffuses from the blood into the alveoli to be exhaled.

* Body Cells: Oxygen is at a higher concentration in the blood than inside cells. Therefore, oxygen diffuses from the blood into cells. Similarly, carbon dioxide is at a higher concentration inside cells than in the blood, so it diffuses from the cells into the blood to be transported to the lungs.

Essentially, diffusion is driven by the natural tendency of molecules to spread out and reach a state of equilibrium. It's a passive process that doesn't require energy.