Blood vessels are hollow tubes that carry blood around your body, delivering vital oxygen, nutrients and water. Arteries carry oxygen-rich blood away from the heart, minute capillaries deliver it to the tissues, while veins transport the deoxygenated blood and waste back to the heart for replenishing in the lungs.
The biggest vessel – the aorta – is 3,000 times wider than the smallest capillaries, where red blood cells (which carry the oxygen) have to line up in single file to squeeze through. These red blood cells are unusual in that they have no nucleus, meaning they can dedicate even more space to transporting oxygen.
Believe it or not, your blood vessels would circle the world two and a half times if laid end to end.