How is gas exchanged in the alveoli?

Gas exchange in the alveoli occurs through simple diffusion, which is a passive process. In the alveoli, oxygen and carbon dioxide are exchanged between the air within the alveoli and the blood in the surrounding capillaries. The principle of diffusion states that molecules move from an area of higher concentration to an area of lower concentration.

In this context, oxygen, which is at a higher concentration in the alveoli compared to the blood, diffuses into the blood, while carbon dioxide, which is at a higher concentration in the blood than in the alveoli, diffuses into the alveoli to be exhaled. This process does not require energy input, making it efficient for the rapid exchange of gases during respiration.

Understanding that gas exchange relies on concentration gradients helps clarify why processes like active transport, facilitated diffusion, and osmosis are not applicable here. Active transport would require energy to move molecules against their concentration gradient, while facilitated diffusion involves carrier proteins and is not necessary for the straightforward gas exchange occurring in the alveoli. Osmosis specifically refers to the movement of water across a selectively permeable membrane, further distinguishing it from the gas exchange process.