Blood vesselsSpec 4.2.2.2
The body contains three different types of blood vessel: arteries, veins and capillaries. Their structures are related to their functions.
| Artery | Vein | Capillary | |
|---|---|---|---|
| Function | Carries blood away from the heart at high pressure | Carries blood back to the heart at low pressure | Exchanges substances with body cells |
| Wall | Thick, with muscle and elastic fibres | Thinner walls | One cell thick, so substances pass through easily |
| Lumen (inside space) | Narrow compared with the thickness of the wall | Wide | Very narrow |
| Valves | No | Yes, to stop backflow | No |
Arteries have thick, strong walls because the blood is at high pressure. The elastic fibres let the wall stretch and spring back. The muscle strengthens the wall.
Veins have a wide lumen and valves. The blood is at low pressure, so the valves stop it flowing backwards.
Capillaries are tiny and have very thin walls. This gives a short diffusion distance, so oxygen and nutrients can pass to the cells and carbon dioxide and waste can pass back into the blood. Capillary walls are permeable.
Blood flow rate
Calculating blood flow
300 cm³ of blood flows through an artery in 4 minutes. Calculate the rate of blood flow in cm³ per minute.
- Rate = volume ÷ time.
- Rate = 300 ÷ 4.
Answer: 75 cm³/min
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