By what process does oxygen move from the alveoli into the blood of the pulmonary capillaries?

Diffusion

Explanation:

Diffusion is a simple process of molecules' movement, from their higher concentration to he area with their lower concentration. The process of oxygen movement from the alveoli into the blood of the pulmonary capillaries is called gas exchange and it is actually simple diffusion. Alveoli have huge surface area, thin cell walls and a lot of blood vessels around them. All of these are adaptations that facilitate gas exchange. There is a high oxygen gradient:  oxygen concentration in the alveoli is much higher (inhaled air) than in the surrounding capillaries so oxygen moves down its gradient-from the alveoli to the blood.