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

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Why are capillaries described as being one cell thick?

To prevent blood loss
To increase the rate of diffusion
To maintain high blood pressure
To enable blood to flow quickly

The correct answer is: To increase the rate of diffusion

Capillaries are described as being one cell thick primarily to increase the rate of diffusion. This thin structure allows for more efficient exchange of materials such as oxygen, carbon dioxide, nutrients, and waste products between the blood and surrounding tissues. The single layer of endothelial cells minimizes the distance over which diffusion must occur, facilitating quicker transfer of substances. This design is crucial because capillaries are the sites of exchange in the circulatory system, and maximizing the surface area-to-volume ratio enhances the efficacy of this process. Other considerations, such as blood loss, high blood pressure, or the speed of blood flow, are not the primary reasons for the thin structure of capillaries. The focus is instead on optimizing diffusion, which is essential for maintaining cellular respiration and overall tissue health.