How do arteries and veins differ?

Arteries have thicker, more muscular walls because they must handle the force of blood leaving the heart. Veins usually carry blood at lower pressure and often contain valves, especially in the legs, to keep blood moving upward against gravity. Capillaries connect the two systems and are the sites where oxygen, nutrients, and wastes are exchanged with tissues. In the usual circulation pattern, arteries carry oxygen-rich blood and veins carry oxygen-poor blood, although the pulmonary circulation is the important exception. Together, the vessels form an organized transport network.