The direction of gas movement is determined by solubility of the gas in blood size of the gas molecule temperature ph partial pressure differences

The direction of gas movement is determined by Partial Pressure Difference.

Explanation:
                   According to phenomenon of Diffusion and Effusion gases always move from level of higher concentration to level of lower concentration. In lungs the oxygen moves from external atmosphere to inner body due to difference in partial pressure of Oxygen. Oxygen present in higher quantity outside body moves in through alveoli. And CO₂ present in body moves out through lungs. Same process takes place at tissue level.