Which physiological process is primarily responsible for oxygen entering the bloodstream?

The physiological process primarily responsible for oxygen entering the bloodstream is diffusion in the alveoli. In the lungs, oxygen from the inhaled air passes through the thin walls of the alveoli and into the surrounding capillaries. This movement is driven by a concentration gradient, where oxygen levels are higher in the alveoli compared to the deoxygenated blood in the capillaries. As a result, oxygen diffuses freely into the blood until equilibrium is reached.

This process is vital because it allows the body to efficiently absorb the oxygen needed for cellular metabolism while removing carbon dioxide, which is a waste product of that metabolism. The thin barrier between the alveolar air and the blood enhances this efficiency, making diffusion an effective means of gas exchange. In contrast to other options, diffusion does not require energy, making it a passive and essential process for normal respiratory function.