Explain how the cardiac cycle produces the pressure changes that drive blood through the left side of the heart and into the aorta.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
During diastole the left ventricle relaxes, lowering its pressure below that of the left atrium; the mitral valve opens and blood flows into the ventricle.
When the ventricle contracts (systole) its pressure rises above atrial pressure, closing the mitral valve to stop backflow.
If ventricular pressure exceeds aortic pressure, the aortic valve opens and blood is ejected into the aorta.
At the end of systole ventricular pressure falls below aortic pressure, closing the aortic valve and preventing backflow into the ventricle.
When the ventricle contracts (systole) its pressure rises above atrial pressure, closing the mitral valve to stop backflow.
If ventricular pressure exceeds aortic pressure, the aortic valve opens and blood is ejected into the aorta.
At the end of systole ventricular pressure falls below aortic pressure, closing the aortic valve and preventing backflow into the ventricle.
Examiner tips
- Use the sequence of valve opening/closing and pressure relationships; mention diastole then systole.
- Show cause–effect: pressure change → valve action → blood flow direction.
- Keep answer concise, 4 points, no extra detail.
Common mistakes
- Confusing atrial and ventricular pressure directions; writing that the aortic valve opens during diastole.
- Omitting the role of the mitral valve closing during systole; or not mentioning the prevention of backflow.
Mark scheme (4 marks)
- During diastole, the relaxation of the left ventricle causes pressure to fall below that in the left atrium, so the atrioventricular (bicuspid/mitral) valve opens and blood flows from the atrium into the ventricle.
- Contraction of the left ventricle during systole raises ventricular pressure above atrial pressure, which closes the AV valve (preventing backflow into the atrium).
- When ventricular pressure exceeds aortic pressure, the semilunar (aortic) valve opens and blood is ejected into the aorta.
- At the end of systole, ventricular pressure falls below aortic pressure, causing the semilunar valve to close, preventing backflow of blood from the aorta into the ventricle.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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