What is the difference between an artery, a vein, and a capillary?

Arteries carry oxygenated blood away from the heart to the body's tissues under high pressure, with thick muscular walls that expand and contract with each heartbeat. The combining form is arterio. Veins carry deoxygenated blood back to the heart under low pressure, with thinner walls and one-way valves that prevent backflow. The combining form is veno or phlebo. Capillaries are microscopic vessels connecting arteries to veins where gas exchange occurs, with walls only one cell thick allowing oxygen and nutrients to pass to tissues and waste products to enter the blood.