Students should be able to explain how the structure of these vessels…

The idea

Each blood vessel has a structure adapted to its function. Arteries carry blood away from the heart at high pressure, so they have thick muscular and elastic walls; elastic tissue stretches and recoils to smooth pressure surges. Veins carry blood at lower pressure, so their walls are thinner and they contain valves to prevent backflow; skeletal muscles help squeeze veins during movement. Capillaries are one cell thick, giving a short diffusion distance, and are numerous to provide a large surface area for exchange. Their narrow diameter slows blood flow, allowing more time for diffusion. For example, capillary networks in muscle deliver oxygen and glucose and remove carbon dioxide. Explaining these links requires stating the feature, its function and the advantage.

What earns the marks

• Arteries have thick elastic and muscular walls that withstand high pressure and maintain blood flow away from the heart.
• Arteries have a narrow lumen relative to wall thickness, helping to maintain pressure.
• Veins have thin walls and a wide lumen because blood pressure is lower.
• Veins contain valves that prevent backflow, ensuring blood returns to the heart.
• Capillaries have walls one cell thick, reducing diffusion distance for exchange of substances.
• Capillaries form extensive networks, increasing surface area for exchange between blood and tissues.
• Capillaries have a narrow diameter, slowing blood flow and allowing more time for exchange.

Common mistakes and how to put them right

• Describing a feature without linking it to a function.

  Put it right: State both the structural feature and the advantage it provides.

• Claiming capillaries have valves.

  Put it right: Valves are found in veins, while capillaries rely on their thin walls and slow flow for exchange.

• Saying arteries have valves to maintain pressure.

  Put it right: Arteries have elastic tissue that recoils to smooth pressure surges, and valves are not normally present in arteries.

Exam tips

• Use the structure–function–advantage chain in your answer to ensure full explanation.
• Compare vessels directly when the question asks for differences, for example thicker walls in arteries than in veins.
• Refer to diffusion distance, surface area and concentration gradient when explaining capillary exchange.