What is the difference between a vein and an artery?

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In the simplest terms ,an artery carries blood away from the heart; a vein carries blood towards the heart.
The blood in the artery contains dissolved fresh oxygen which has entered the heart chambers, by way of major blood vessels, from tiny air sacs in the lungs.
The blood in the veins contains waste material which it carries up to the heart. The waste gases from the veins filter through to the air sacs in the lungs and are breathed out when you exhale.