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

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How are capillaries designed to facilitate substance exchange?

  1. They are thick-walled

  2. They have thin, permeable walls and are numerous

  3. They are only present in organs

  4. They contain red blood cells

The correct answer is: They have thin, permeable walls and are numerous

Capillaries are uniquely structured to maximize the exchange of substances between the blood and surrounding tissues. Their thin, permeable walls allow for the easy diffusion of gases, nutrients, and waste products. Being just one cell thick facilitates this process, as it reduces the distance substances must travel to move in and out of the blood. Additionally, the numerous capillaries increase the total surface area available for exchange. This extensive network ensures that all cells in the body are in close proximity to blood, allowing for efficient delivery of oxygen and nutrients while simultaneously facilitating the removal of carbon dioxide and metabolic waste. The thickness of the walls in capillaries is minimal compared to other blood vessels, serving to enhance rather than hinder the process of exchange. By being numerous, they ensure that the overall efficiency of substance exchange is maximized throughout the body.

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