Explain how the structural features of the atrioventricular (AV) valves prevent backflow of blood during ventricular systole.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. The AV valves consist of flap‑like cusps that are pushed shut when ventricular pressure rises above atrial pressure.
2. The cusps are attached to chordae tendineae, which in turn are anchored to papillary muscles.
3. During ventricular systole the papillary muscles contract, tightening the chordae tendineae and preventing the cusps from inverting or being pushed back into the atrium.
4. With the valve sealed, blood is forced through the semilunar valves into the aorta or pulmonary artery, preventing backflow into the atrium and maintaining unidirectional flow.
2. The cusps are attached to chordae tendineae, which in turn are anchored to papillary muscles.
3. During ventricular systole the papillary muscles contract, tightening the chordae tendineae and preventing the cusps from inverting or being pushed back into the atrium.
4. With the valve sealed, blood is forced through the semilunar valves into the aorta or pulmonary artery, preventing backflow into the atrium and maintaining unidirectional flow.
Examiner tips
- Use the exact terms ‘cusps’, ‘chordae tendineae’, and ‘papillary muscles’. Show the sequence: pressure rise → cusps close → papillary contraction → chordae tension. Explain the functional consequence (no backflow).
Common mistakes
- Confusing atrial contraction with ventricular contraction. Omitting the role of chordae tendineae. Using vague language like ‘valve closes’ without linking to pressure changes.
Mark scheme (4 marks)
- AV valves have flap-like cusps / leaflets that are pushed closed when ventricular pressure exceeds atrial pressure
- Chordae tendineae (heart strings) attach the cusps to the papillary muscles and prevent the cusps inverting / being pushed back into the atrium
- Papillary muscles contract during ventricular systole, tensioning the chordae tendineae to hold the valve closed under pressure
- The sealed, closed valve ensures blood is directed through the semilunar valves into the aorta / pulmonary artery rather than back into the atrium, maintaining unidirectional flow
Key terms in this question
atrioventricular (AV) valve · ventricular systole
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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