Why does oxygen diffuse from the alveoli to the capillaries?

Oxygen diffuses from the alveoli to the capillaries primarily because the concentration of oxygen is lower in the capillaries compared to the alveoli. This process is driven by the principle of diffusion, where molecules move from an area of higher concentration to an area of lower concentration in order to achieve equilibrium.

In the lungs, the alveoli are filled with fresh air, which has a high concentration of oxygen, while the blood in the capillaries that surround the alveoli has just delivered its oxygen to the tissues and therefore has a lower oxygen concentration. This gradient encourages the movement of oxygen from the alveoli into the capillaries, where it can then be transported throughout the body to meet the metabolic needs of tissues.

Understanding this mechanism is crucial, as it highlights how the body's respiratory and circulatory systems work together to maintain essential processes like cellular respiration.