diffusion is a movement of atoms or molecules from high concentrated ( or higher pressure) region to low concentration ( or lower pressure) region, no external source of energy is used
respiratory surface, also known as the respiratory membrane, includes the alveolar epithelial cells as well as the pulmonary capillary endothelial cells.
Thus diffusing gas should have more pressure. Now when oxygen enters in the blood then pressure of oxygen must be more in blood than alveoli
And CO2 moves out of blood, pressure of CO2 must be more in blood than alveoli.
Tissues consume oxygen and produce CO2 and it is waste product of tissue. Concentration of CO2 is always more in tissue than blood