The blood travels through a closed system of vessels that runs throughout the body, taking in oxygen and nutrients and picking up wastes as it goes. The blood vessels are composed of three main types of vessels:
Arteries: These carry oxygen-rich blood away from the heart to the rest of the body.
Capillaries: These are tiny, thin-walled vessels where the actual exchange of oxygen, nutrients, and waste products occurs between the blood and body tissues.
Veins: These carry oxygen-poor blood back toward the heart to be refilled with oxygen in the lungs.