Blood is transported by arteries, veins, and capillaries.

Arteries have muscular walls that have 3 layers to keep nutrients an gases from passing through.

The veins also have 3 layers, but these are thinner and narrower than the arteries. Like the heart, veins in the arms and legs have valves that prevent blood from flowing in the wrong direction.

Capillaries are tiny vessels that connect to arteries and veins. Gas exchange of carbon dioxide and oxygen take place in the thin walls of the capillaries. The oxygen and carbon dioxide pass through the walls of the capillaries. Then they are picked up by red blood cells.