CO2 and O2 are exchanged across capillary membranes in the alveoli via what process?

The exchange of carbon dioxide (CO2) and oxygen (O2) across the capillary membranes in the alveoli primarily occurs through passive diffusion. This process relies on the concentration gradient of gases; oxygen moves from areas of higher concentration in the alveoli to areas of lower concentration in the blood, while carbon dioxide moves from the blood, where its concentration is higher, to the alveoli, where its concentration is lower.

Passive diffusion does not require energy, making it an efficient way for gases to move across cell membranes. The thin walls of the alveoli and the high surface area available facilitate this rapid exchange of gases, ensuring adequate oxygenation of blood and removal of carbon dioxide from the body.

Active transport, on the other hand, involves the movement of substances across a membrane against their concentration gradient and requires energy, which is not the case for gas exchange in the alveoli. Osmosis pertains specifically to the movement of water across a membrane, and while facilitated diffusion involves the use of carrier proteins to assist molecules across a membrane, gas exchange occurs without such mechanisms due to the gases’ small size and lipid solubility, making passive diffusion the correct choice.