What is the purpose of blood capillaries, arteries, veins, and blood vessels as a whole?
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Blood capillaries are the smallest diameter blood vessels of living organisms. They have a wall formed by a single layer of endothelial cells, which allows the exchange of substances between blood and surrounding tissues, a phenomenon called capillary exchange, thanks to which O2 and nutrients penetrate cells and CO2 and Waste substances pass into the blood for disposal.