Here's why:
* Capillaries are the smallest blood vessels. They have thin walls, only one cell thick, which allows for easy diffusion of substances.
* They form a vast network throughout the body. This ensures that every cell is close to a capillary for exchange.
* Substances move across the capillary walls by diffusion. This means that substances move from areas of high concentration to areas of low concentration. For example, oxygen diffuses from the blood into the cells, while carbon dioxide diffuses from the cells into the blood.
Other vessels, like arteries and veins, are primarily involved in transporting blood. Arteries carry oxygenated blood away from the heart, while veins carry deoxygenated blood back to the heart. They don't have the same structure as capillaries and aren't optimized for material exchange.
