Question

knowing what you know about how a substance travels in diffusion in order for carbon dioxide to leave the blood and enter the alveoli there would be higher or lower concentration of carbon dioxide

Answer 1

CO2 moves down its concentration gradient by simple diffusion. Therefore, CO2 would have to be high in the blood, and low in the alveoli for simple diffusion to happen, this would be done by passive diffusion and thus, simple diffusion.

Remember that in simple diffusion you go from an area of a high concentrated solute towards an area of low concentrated solute.