The circulatory system, which has the heart at its centre, is responsible for transporting blood around the whole body. There are five types of blood vessels.

1. Arteries

Arteries carry blood away from the heart. They have thick muscular walls that can withstand the high pressure created by pumping action of the heart. The blood in arteries is oxygenated with one exception: the pulmonary artery carries deoxygenated blood from the heart to the lungs.

2. Arterioles

Arterioles are subdivisions of arteries, smaller in diameter as they get closer to the body tissues.

3. Capillaries

Capillaries are the smallest blood vessels, incredibly fine and narrow with walls just a cell thick in order that nutrients, gases and other substances can transfer between the blood inside them and the interstitial fluid that surrounds them, and then onwards to the body’s tissues.

4. Venules

Venules are larger blood vessels formed by capillaries joining up as they start going back towards the heart.

5. Veins

Veins are the larger vessels that transport blood back to the heart. They have much thinner walls than arteries, because the blood is no longer under such high pressure, but they contain valves to keep blood flowing in the correct direction. Veins are also sandwiched between the body’s skeletal muscle in order to force blood to flow as needed, and contain deoxygenated blood other than the pulmonary vein which carries oxygen-rich blood from the lungs to the heart.