Veins at Higher Level: The same statement as SL. HL papers ask why vein walls are thin and what drives blood along a vein, and link valves here to the valves of the heart.
Practise this as you read
- Identify the main driver of blood flow along a leg vein.
- Explain how veins are adapted for returning blood to the heart.
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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: fainting on parade. Soldiers who stand still for hours on parade sometimes faint: with the leg muscles not contracting, the muscle pump stops, blood pools in the leg veins and too little returns to the heart and brain.
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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.
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[3 marks]
Explain how veins are adapted for returning blood to the heart.
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See the answer broken into clear points, with guidance on how each point earns marks.