The cardiovascular system includes three types of blood vessel: arteries, veins, and capillaries. Placed end to end, they would circle the Earth nearly four times. The smallest vessels, the capillaries, make up 98 per cent of this length. The largest artery, the aorta, emerges from the heart and branches into a network of progressively smaller arteries that carry blood to every part of the body. The smallest arteries join capillaries, which in turn join a network of tiny veins that merge into larger vein as they return blood to the heart.
Arteries inside the skull
This contrast X-ray shows the arteries that lie under the skull and supply blood to the back of the brain.
Structure of a vein
Veins have thin walls that enable them to expand and hold large volumes of blood when the body is at rest. Large veins contain one-way valves to stop blood from flowing the wrong way.
The smallest vessels
Arteries branch into progressively smaller vessels (arterioles) that eventually join tiny capillaries, which have walls that are only one cell thick. Capillaries join small veins (venules), which gradually merge into larger veins.
Structure of an artery
Arteries have thick, muscular, elastic walls that can resist the wave of high-pressure blood pumped with each heartbeat.