Answer
This diffusion of carbon dioxide is a case of flowing down a gradient. Active tissues that are generating carbon dioxide have a higher concentration of CO2 than the surrounding blood. Since the CO2 is moving from a high concentration area to a lower concentration area, it's diffusing down the gradient.
Work Step by Step
You can think of the gradient as a hill, with your high value at the top and your low value at the bottom. Normally, things diffuse or move from the top of the hill to the bottom without any issue (water flows down the hill no problem because of gravity). It will actually take work or energy to move things up the gradient.
In the question, our two places are in the active tissues, which generate CO2, and the bloodstream, which removes CO2 from the site. Of these two places, the active tissues are the top of the hill and the bloodstream is the bottom. Therefore the CO2 is moving down the gradient when it moves from the active tissues to the bloodstream.
