Although the capillaries are the smallest vessels in the circulatory system, they represent the main exchange site between the blood and the tissues. They can be viewed as both the ultimate destination of the arterial system and the starting point of the venous system. From the heart, blood travels through the arteries to the arterioles, and then to the capilla ries, where exchange occurs.
Nutrients, oxygen, and other materials carried by the blood are traded for waste products from tissue cells. Blood contin ues down the capillaries, soon entering the venules and then the veins on its return trip to the heart.