OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What effect does the presence of valves in veins have on blood circulation?

1. They speed up blood flow

2. They maintain a consistent pressure

3. They prevent backflow of blood

4. They increase blood volume

The correct answer is: They prevent backflow of blood

The presence of valves in veins plays a crucial role in ensuring efficient blood circulation, particularly in returning blood to the heart from various parts of the body. Valves prevent backflow, which is essential for maintaining unidirectional flow toward the heart. This is particularly important in the lower extremities, where blood must travel against gravity. When muscles contract during activities such as walking or exercising, they squeeze the veins, and the valves open to allow blood to flow toward the heart. Once the muscles relax, the valves close to prevent any backward movement of blood. This mechanism helps maintain a steady return of blood to the heart, ensuring that the circulatory system operates efficiently and reducing the risk of blood pooling in the limbs. The other choices do not accurately describe the primary function of valves in veins, as they do not directly speed up blood flow, maintain pressure, or increase blood volume.