# 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

## Mark scheme (4 marks)

1. Semilunar valves are located at the base of the aorta and pulmonary artery (between ventricles and their respective arteries).
2. The valves consist of three cup-shaped (pocket-shaped) cusps / leaflets attached to the vessel wall.
3. When ventricular pressure exceeds arterial pressure, the cusps are pushed open (flat against the vessel wall), allowing blood to flow forward into the artery.
4. 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

- [semilunar valve](https://www.gradenine.co.uk/glossary/semilunar-valve)
- [backflow](https://www.gradenine.co.uk/glossary/backflow)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-fc16a771) · Published by Druglandscape Ltd.