Three kinds of tube
Blood does not flow through one pipe. It goes through three kinds of tube.
- Blood leaves the heart in an artery with a thick wall.
- Arteries branch into capillaries with walls one cell thick.
- Capillaries join up as veins.
- A vein has a thin wall and a valve.
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Read the clue. Tap the tube it describes.
The rule
Blood leaves the heart in arteries, under high pressure. Artery walls are thick and elastic to stand it. Veins bring blood back. Many veins have valves that keep it moving one way. Capillaries are the tiniest tubes. Food, oxygen and wastes cross their one-cell wall.
Elastic walls stretch and spring back. A valve is a flap that lets blood pass one way.
Blood pressure is the push of blood on a vessel wall. Arteries carry blood at a much higher pressure than veins do. Pressure in veins is low. So veins need valves against backflow.
Valves in the heart also stop blood flowing back.
Watch out!
The vein from the lungs carries oxygen-rich blood. A vein is a tube that brings blood to the heart.