The main function of the blood vascular system in higher vertebrates is to ensure and regulate the delivery of oxygen and nutrients to every cell in the peripheral tissues, as well as the removal of their metabolic waste. In this system, the arteries carry blood away from the heart through a divergent arborescence; the capillaries are the main sites for exchanges between blood and tissue and have therefore to supply the entire volume of the organism; and the veins carry blood back to the heart through a convergent arborescence. Functionally, these compartments are organized in series: blood flow proceeds from the upstream arteries to the downstream veins through the capillaries.