Here's why:
* Capillaries are the smallest blood vessels. They are so small that red blood cells have to travel through them in single file. This close proximity to cells allows for efficient exchange of substances.
* Capillary walls are thin and porous. This allows nutrients, oxygen, and other substances to easily pass from the blood into the surrounding tissues.
* Diffusion is the key process. Nutrients move from areas of high concentration (the blood) to areas of low concentration (the cells) through diffusion.
So, while the blood itself carries nutrients, it's the capillaries that actually facilitate the movement of these nutrients into the cells that need them.
