Capillaries are the crossroads between arteries and veins. In mammals capillaries made it possible for larger, more complex life forms to evolve thru more efficient gas exchange. The gas exchange that occurs between arteries, veins and the atmosphere is critical to the survival of all living organisms. Plants produce gases which animals need to survive and animals create gases which plants need to survive. This results in a web of crossroads between all living things in our world.
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