What causes molecules of oxygen and carbon dioxide to move between the air and blood ?
Just the fact that there is much more O2 in the air than there is in blood, this causes a gradient and makes the O2 move towards the blood where there is less of it. CO2 is abundant in blood because of respiration, so since there is more of it in the blood, it tends to get exchanged in the lungs thru the alveoli where there is less CO2, so that it may be released during expiration to the environment.
