Your circulatory system

Freshly oxygenated blood is pumped out from your heart through your arteries and depleted blood returns to your heart via your veins.

As your blood vessels travel further away from your heart, they branch out and get smaller and smaller, eventually becoming tiny, delicate vessels called capillaries.

The capillaries transport blood to your individual cells, delivering all the nutrients, hormones, and oxygen they need to survive.

Capillaries are the smallest blood vessels in your vascular system. They are only about 8 to 10 microns in diameter (a micron is 0.001 mm), about a 10th of the diameter of a human hair.

The walls of the capillaries are very thin and this allows nutrients to pass through and be absorbed into your muscles and other organs.

And in addition to delivering the nutrients your cells need, the blood in your capillaries also carries away waste products and carbon dioxide.

Your circulatory system is huge. If you were to take all of your arteries, veins, and capillaries and arrange them end to end they would stretch for 100,000 kilometers (62,000 miles).

To learn more about how the heart pumps blood around your body, take a look at How your heart pumps.