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

What is the primary function of capillaries in the circulatory system?

1. To carry blood away from the heart

2. To transport blood back to the heart

3. To exchange materials at the tissues

4. To branch into arterioles

The correct answer is: To exchange materials at the tissues

The primary function of capillaries in the circulatory system is to exchange materials at the tissues. Capillaries are tiny, thin-walled blood vessels that connect arteries and veins. Their structure allows for efficient diffusion of substances such as oxygen, carbon dioxide, nutrients, and waste products between the blood and surrounding tissues. The walls of capillaries are only one cell thick, which facilitates this exchange process. While arteries carry blood away from the heart and veins transport blood back to the heart, these blood vessels are primarily involved in the movement of blood rather than in the exchange of materials. The branching into arterioles is a characteristic of arteries, as they move from larger vessels into smaller ones leading to capillary beds, but this function is more about blood flow distribution than material exchange. Hence, the option focusing on the exchange of materials reflects the crucial role capillaries play in supporting cellular metabolism and overall tissue health.