The big idea: By the time blood reaches the veins its pressure is low, so veins need help to get it back to the heart: valves and muscles squeezing a flexible wall.
The points to remember
- Pocket valves let blood flow only towards the heart and prevent backflow.
- The wall is thin and flexible, so contracting skeletal muscles can compress it and push blood along.
- A wide lumen gives little resistance to the slow, low-pressure blood.
- Contraction of the muscles next to the vein is the main thing that moves blood along a leg vein.
Remember it as: Squeeze from the muscles, one-way valves: blood goes up.
Real example: varicose veins. When the valves in leg veins stop closing properly, blood flows back and pools, and the veins swell into the twisted, bulging varicose veins seen on some people's legs.
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Valves follow the pressure: A valve opens when the pressure behind it is higher, and shuts when the pressure beyond it is higher, so blood can only move one way.
Muscles contract
- Vein wall is squeezed
- Pressure inside rises
- Valve above opens: blood moves towards the heart
- Valve below shuts: no backflow
Muscles relax
- Wall springs back to shape
- Pressure inside falls
- Valve above shuts: no backflow
- Blood from below refills the vein
Two traps: Veins do not keep blood moving by elastic recoil: that is what arteries do.
Thin walls in veins are not for exchange (only capillaries exchange); they let muscles compress the vein.
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How it is asked: Expect a one-mark item on what moves blood along a vein or why vein walls are thin, or an 'explain how veins return blood' question.
IB-style questionExplain[2 marks]
Explain how blood in the veins of the leg returns to the heart even though its pressure is low.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.