Here's why:
* Capillaries are tiny blood vessels that form a vast network throughout the body. They are so small that red blood cells can only travel through them in single file.
* They are the site of diffusion: The thin walls of capillaries allow for the easy movement of gases (oxygen and carbon dioxide) between the blood and the surrounding tissues.
* Oxygen delivery: Oxygen-rich blood from the lungs travels through the arteries to the capillaries, where it diffuses into the surrounding tissues to fuel cellular processes.
* Carbon dioxide removal: Carbon dioxide, a waste product of cellular respiration, diffuses from the tissues into the capillaries, where it is transported back to the lungs to be exhaled.
So, while the lungs are responsible for bringing oxygen into the body and removing carbon dioxide, the actual exchange of these gases with the body's cells happens at the level of the capillaries.
