capillaries. permeable, microscopic vessels between arteries and veins. serve as the sites of exchange between the blood and body tissues; it is from them that oxygen and nutrients exit the blood, and CO2 and cellular waste enter the blood.apillaries. permeable, microscopic vessels between arteries and veins. serve as the sites of exchange between the blood and body tissues; it is from them that oxygen and nutrients exit the blood, and CO2 and cellular waste enter the blood.