1) Arteries branch into capillaries.
2) Capillaries are really tiny - too small to see.
3) They carry the blood really close to every cell in the body to exchange substances with them.
4) They have permeable walls, so substances can diffuse in and out.
5) They supply food and oxygen, and take away waste like carbon dioxide.
6) Their walls are usually only one cell thick. This increases the rate of diffusion by decreasing the distance over which it occurs.
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