Final answer:
Oxygen enters the bloodstream from the alveoli through a process called diffusion, moving down a concentration gradient without the need for energy.
Step-by-step explanation:
The movement of O2 from the alveoli of the lungs into the bloodstream is a process called diffusion. This occurs because oxygen molecules move down a concentration gradient, from an area where they are in higher concentration in the alveoli to an area where they are in lower concentration in the capillary blood. The gas exchange of oxygen for carbon dioxide is facilitated by the special characteristics of the respiratory membrane, which is optimized for gas diffusion due to its thinness and large surface area.