The structure and function of blood vesselsSpec 8.7
There are three types of blood vessel. Each is adapted to its job.
| Artery | Vein | Capillary | |
|---|---|---|---|
| Direction of blood | Away from the heart | Towards the heart | Between arteries and veins, through the tissues |
| Blood pressure | High | Low | Falls as blood passes through |
| Wall | Thick, muscular, with elastic fibres | Thinner wall | One cell thick |
| Lumen | Narrow | Wide | Very narrow |
| Valves | No | Yes | No |
| Function | Carry blood at high pressure to the organs | Carry blood back to the heart | Exchange substances with cells |
- Arteries have thick, muscular walls and elastic fibres. The wall can stand the high pressure and stretches and springs back with each heartbeat.
- Veins carry blood at low pressure. They have a wide lumen to let blood flow easily, and valves to stop the blood flowing backwards.
- Capillaries have walls only one cell thick, giving a short diffusion distance. They form a huge network so there is a large surface area for exchange, and they are narrow so they can reach every cell.
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Which part of the blood carries oxygen, and what is the name of the substance that binds to it?
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Red blood cells; haemoglobin.
What is the job of phagocytes?
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They engulf and digest pathogens.
Which blood vessels have valves?
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Veins.
Why are capillary walls only one cell thick?
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To give a short diffusion distance for exchange of substances.