Explain how the structure of a semilunar valve prevents the backflow of blood in the mammalian heart.

IB DP Biology Higher 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)

Semilunar valves sit at the base of the aorta and pulmonary artery, between the ventricles and their arteries. Each valve is made of three cup‑shaped cusps or leaflets that are attached to the vessel wall. When ventricular pressure rises above arterial pressure, the cusps are forced open and lie flat against the vessel wall, allowing blood to flow forward into the artery. When arterial pressure rises above ventricular pressure during diastole, the cup‑shaped cusps are filled with blood and close, preventing any backflow into the ventricles.

Examiner tips

Mention the valve location and the three cusps
Explain the opening mechanism when ventricular pressure is higher
Explain the closing mechanism when arterial pressure is higher
Use terms such as ‘cup‑shaped’, ‘leaflets’, ‘ventricular pressure’, ‘arterial pressure’

Common mistakes

Confusing semilunar valves with atrioventricular valves
Describing the valves as flat rather than cup‑shaped
Failing to mention the valve location at the base of the arteries

Mark scheme (4 marks)

Semilunar valves are located at the base of the aorta and pulmonary artery (between ventricles and their respective arteries).
The valves consist of three cup-shaped (pocket-shaped) cusps / leaflets attached to the vessel wall.
When ventricular pressure exceeds arterial pressure, the cusps are pushed open (flat against the vessel wall), allowing blood to flow forward into the artery.
When arterial pressure exceeds ventricular pressure (during diastole), blood fills the cup-shaped cusps, causing them to close and prevent backflow into the ventricles.

Key terms in this question

semilunar valve · backflow