Final answer:
Gas exchange between the alveoli and capillaries occurs via diffusion, where oxygen moves from the alveoli into the blood, and carbon dioxide moves from the blood into the alveoli.
Step-by-step explanation:
Gas exchange in the capillary beds next to the alveoli involves the diffusion of gases between the blood and the lungs. This process is precisely described by answer choice (a): Oxygen diffuses from alveoli to capillaries; carbon dioxide diffuses from capillaries to alveoli. When you inhale, oxygen-rich air fills the alveoli, which are surrounded by capillaries. The oxygen moves from the alveoli, where its concentration is high, into the capillaries, where its concentration is low. Red blood cells within these capillaries carry the oxygen to the tissues of the body. Conversely, carbon dioxide, which is more concentrated in the blood within the capillaries, diffuses into the alveoli from where it can be exhaled. This exchange is driven by differences in partial pressure and does not require energy, thus it occurs via simple diffusion rather than active transport.