What role do capillaries play in the circulatory system?

Capillaries are the smallest blood vessels. They are made up of only one layer of epithelial cells. The way blood circulates is that:

1. The heart pumps blood to the arteries.
2. The capillaries take it on from there. Materials are exchanged between the blood and the cells in the capillaries
3. The veins take the blood back to the heart, and the whole process starts over again.

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