Explain one-way flow from the action of valves
Use pressure on either side of a valve to predict its movement.
Section B9.1 · Circulatory systems
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall pressure differences and one-way flow.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a functioning one-way valve prevent?
What you will learn
- Forward pressure opens the route
- Use the direction shown, not a memorised picture
Forward pressure opens the route
A one-way valve allows flow in its permitted direction when pressure on the incoming side exceeds pressure on the outgoing side. The valve flaps move with the pressure difference. The pump supplies the energy for moving blood.
If the pressure difference reverses, blood begins to push against the flaps in the opposite direction and they close. Closing helps prevent backflow; it does not mean that blood can never move anywhere in the system while that particular valve is shut.
Use the direction shown, not a memorised picture
For a model valve that permits A → B, compare the pressure at A with pressure at B. A pressure of 12 units at A and 7 at B favours opening; A at 5 and B at 9 favours closure. These supplied numbers are arbitrary teaching units, not a clinical blood-pressure reading.
The relative pressure is the useful evidence. A valve drawn upright or upside down still permits its labelled direction. Never decide from the page position alone, and do not treat a pressure difference as proof that a damaged valve closes normally.
Sources for this lesson
Pause and explain
For a valve permitting A → B, which pressure comparison favours closure?
Worked example
A functioning valve permits A → B. A is 12 pressure units and B is 7. Predict its state and flow.
Show the worked solution
- Compare incoming and outgoing pressure: 12 > 7.
- The difference pushes the valve open in its permitted direction.
Answer It opens and blood can flow A → B; the supplied difference is 5 pressure units.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For A 12 and B 7, calculate A minus B and state the permitted flow.
Give me a hint
Subtract and retain the arrow A → B.
Compare my reasoning
- 12 − 7 = 5 units.
- The positive incoming-side difference favours opening A → B.
5 units; opening favours A → B.
Look for these in your work
- Correct subtraction
- Connect pressure with permitted direction
A is 5 and B is 9. Predict the action of a functioning A → B valve.
Give me a hint
Higher pressure is now on the outgoing side.
Compare my reasoning
- B pressure exceeds A pressure.
- Reverse pressure presses the flaps closed.
The valve closes, preventing B → A backflow.
Look for these in your work
- Compare both pressures
- Describe closure and prevented backflow
A diagram is rotated 180 degrees but its labels and pressures remain unchanged. Should your predicted valve action change?
Give me a hint
Use the labelled incoming/outgoing sides.
Compare my reasoning
- Rotation changes page position, not the valve’s permitted direction.
- The same pressure difference produces the same predicted action.
No; follow A → B and compare the same pressures, regardless of page orientation.
Look for these in your work
- Use labels rather than page position
- Retain the pressure-based explanation
Common mistakes
- Choosing flow from the top of the page.
- Treating the valve as the source of pressure.
What can you explain now?
For a valve permitting A → B, which pressure comparison favours closure?
Compare with the explanation
A 5; B 9
Reverse pressure closes the flaps.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain why valves open and close without calling them a pump.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.