What determines the net direction of net gas diffusion in the body? partial pressure differences ph temperature the size of the gas molecule the solubility of the gas in blood?

The answer is the partial pressure. 
The partial pressure determines the movement of respiratory gases (oxygen and carbon dioxide) between the lungs and the atmosphere, between lungs and blood, and between blood and body cells. Carbon dioxide and oxygen diffuse across a permeable membrane from the area where its partial pressure is greater to the area where the partial pressure is less.