Explain how the cardiac cycle produces the pressure changes that drive blood through the heart and into the aorta.

IB DP Biology Standard Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Atrial systole raises atrial pressure, forcing blood through the AV valves into the ventricles.
2. Ventricular systole raises ventricular pressure above aortic/pulmonary pressure, opening the semilunar valves and ejecting blood into the aorta.
3. During ventricular diastole ventricular pressure falls below aortic pressure, closing the semilunar valves and preventing backflow.
4. The AV valves close when ventricular pressure exceeds atrial pressure, preventing backflow into the atria.

Examiner tips

Use the four key events in order; link pressure changes to valve movement.
Show the cause–effect relationship between pressure and valve opening/closing.
Include both atrial and ventricular phases to cover all required points.

Common mistakes

Forgetting to mention AV valve closure during ventricular systole.
Confusing the timing of atrial systole with ventricular diastole.
Omitting the pressure comparison that triggers semilunar valve opening.

Mark scheme (4 marks)

Contraction of the atria (atrial systole) raises pressure in the atria, forcing blood through the atrioventricular valves into the ventricles.
Contraction of the ventricles (ventricular systole) greatly increases ventricular pressure, which exceeds aortic/pulmonary pressure, causing the semilunar valves to open.
During ventricular diastole (relaxation), ventricular pressure falls below aortic pressure, causing the semilunar valves to close, preventing backflow into the ventricles.
The atrioventricular (bicuspid/tricuspid) valves close during ventricular systole when ventricular pressure exceeds atrial pressure, preventing backflow into the atria.

Key terms in this question

cardiac cycle