The blood vessels conduct blood from the heart to the tissues and back, thus achieving continuous supply of oxygen and nutrients, removal of waste products, and—when needed—delivery of leukocytes to the organs. In one overall circulation cycle, the heart is passed two times in order to pump the blood through the other tissues and lungs (see chapter “ Heart Overview ” under part “Heart”). Starting in the left ventricle of the heart, oxygenated blood flows into the systemic circulation through the aorta, and then into the large conduit arteries, which subsequently divide into smaller conduit arteries, resistance arteries, and the microcirculation. There, arterioles branch out into capillaries, the smallest blood vessels and site of solute and gas exchange. Capillaries merge into venules, and those merge into veins, conducting the blood toward the right heart. From the right heart, blood flows into the pulmonary artery to enter the pulmonary circulation, where it is reoxygenated. The blood then returns to the left heart.