The body contains three different types of blood vessel: • arteries •…

The idea

The circulatory system is a closed network of tubes through which blood is pumped by the heart. There are three types of blood vessel: arteries, veins and capillaries. Arteries carry blood away from the heart; their walls are thick and elastic to withstand high pressure. Veins carry blood back to the heart; they have thinner walls and valves to prevent backflow. Capillaries are microscopic vessels with walls one cell thick that link arteries and veins, allowing exchange of substances with tissues. For example, in the lungs, capillaries surround alveoli for gas exchange. Recognising each vessel's direction of flow and structural features is essential before explaining how structure relates to function.

What earns the marks

• Arteries carry blood away from the heart, while veins carry blood towards the heart.
• Capillaries connect the smallest arteries (arterioles) to the smallest veins (venules) and form networks in tissues.
• Arteries have thick, muscular and elastic walls to withstand and maintain high blood pressure.
• Veins have relatively thin walls and contain valves that prevent the backflow of blood.
• Capillary walls are one cell thick, providing a short diffusion distance for exchange of substances.
• The three vessel types form a continuous closed system: heart → arteries → capillaries → veins → heart.

Common mistakes and how to put them right

• Thinking arteries always carry oxygenated blood and veins always carry deoxygenated blood.

  Put it right: The correct rule is that arteries carry blood away from the heart and veins carry blood towards it, regardless of oxygen content.

• Confusing the direction of blood flow in capillaries.

  Put it right: Capillaries link arterioles to venules, so blood flows from arteries through capillaries into veins.

• Stating that veins have valves to push blood along.

  Put it right: Valves prevent backflow, while movement of blood in veins is assisted by skeletal muscle contraction.

Exam tips

• When asked to identify a vessel, use the direction of blood flow relative to the heart as the deciding feature, not oxygen concentration.
• Link each structural feature to its function in the same sentence, for example thick elastic walls withstand high pressure.
• Use comparative language such as thicker, thinner, narrower or wider when distinguishing vessel types.