Explain the cardiac cycle
Relate pressure changes during systole and diastole to atrioventricular and semilunar valve movement.
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What causes the atrioventricular valves to close during ventricular systole?
What you will learn
- Describe atrial and ventricular systole and diastole
- Explain valve opening and closure using pressure differences
Build the explanation
During atrial systole, atrial contraction raises atrial pressure above ventricular pressure; atrioventricular valves are open and blood enters ventricles. During ventricular systole, ventricular pressure rises above atrial pressure and closes the atrioventricular valves. When ventricular pressure exceeds pressure in the aorta and pulmonary artery, semilunar valves open and blood is ejected. During ventricular diastole, ventricular pressure falls; semilunar valves close as arterial pressure exceeds ventricular pressure. When ventricular pressure falls below atrial pressure, atrioventricular valves open and filling resumes.
Apply it carefully
Valves move passively in response to pressure differences; they do not open because an electrical signal pulls them. 'Systole' means contraction and 'diastole' means relaxation. State which chamber pressure is higher than which neighbouring pressure at each valve event. The cycle is continuous; simplified diagrams often separate phases for learning.
Pause and explain
When do semilunar valves open?
Worked example
Why does the left atrioventricular valve close at the start of ventricular systole?
Show the worked solution
- Ventricular contraction raises pressure.
- Ventricular pressure becomes greater than atrial pressure.
Answer The left ventricular pressure rises above left atrial pressure, forcing the atrioventricular valve closed and preventing backflow into the atrium.
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.
Explain the sequence that opens the aortic semilunar valve.
Give me a hint
Compare ventricular and aortic pressure.
Compare my reasoning
- Ventricular systole raises left ventricular pressure.
- When it exceeds aortic pressure, the aortic valve opens and blood is ejected.
During left ventricular systole, ventricular pressure rises. When it becomes greater than aortic pressure, the semilunar valve opens and blood is ejected into the aorta.
Look for these in your work
- Systole raises pressure
- Pressure comparison correct
- Opening and ejection linked
During ventricular diastole, aortic pressure is greater than left ventricular pressure. Predict the semilunar valve state and explain why.
Give me a hint
A higher pressure on the artery side pushes the valve closed.
Compare my reasoning
- The aortic semilunar valve closes.
- This prevents blood flowing backwards into the ventricle.
The aortic semilunar valve closes because aortic pressure exceeds ventricular pressure, preventing backflow from the artery into the ventricle.
Look for these in your work
- Valve state correct
- Pressure cause stated
- Backflow prevented
Put these events in order: AV valves open; ventricular pressure rises; AV valves close; semilunar valves open.
Give me a hint
Start with filling, then pressure reversal at the start of ventricular contraction.
Compare my reasoning
- AV valves open during filling.
- Ventricular pressure rises, closes AV valves, then exceeds arterial pressure so semilunar valves open.
AV valves open during ventricular filling → ventricular pressure rises during systole → AV valves close when ventricular pressure exceeds atrial pressure → semilunar valves open when ventricular pressure exceeds arterial pressure.
Look for these in your work
- All events ordered
- Both pressure thresholds stated
Common mistakes
- Saying valves open due to contraction rather than a pressure difference
- Swapping systole and diastole
- Omitting the arterial pressure threshold for semilunar opening
What can you explain now?
What causes the atrioventricular valves to close during ventricular systole?
Compare with the explanation
Ventricular pressure exceeds atrial pressure
The pressure reversal pushes the valves closed.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What pressure relationships close AV valves, open semilunar valves and then close semilunar valves?
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.