WJEC GCSE PE Practice Test 2026 – Complete Exam Prep

Capillaries are the smallest blood vessels. Their tiny diameter, about 5–10 micrometres, means red blood cells often move through them in single file, and their walls are only one cell thick. This ultra-thin wall enables efficient diffusion of oxygen, carbon dioxide, nutrients, and wastes between the blood and surrounding tissues, which is exactly what happens at the site where tissues exchange substances with the blood. The other vessels are larger and have thicker walls: arteries, including the aorta, are muscular and high-pressure carriers; veins have larger lumens and thinner walls and return blood to the heart. So capillaries are the smallest.