The heart and blood vessels: The body contains three different types of blood vessel:

The idea

The circulatory system is a closed network of tubes carrying blood away from and back to the heart. Three vessel types are distinguished by structure and function. Arteries carry blood away from the heart at high pressure; their walls contain thick elastic and muscular tissue, and a narrow lumen maintains pressure. Capillaries are one cell thick, forming extensive networks in tissues so substances diffuse over a short distance. Veins return blood to the heart at low pressure; they have thinner walls, a wide lumen and valves preventing backflow.

For example: The aorta is an artery, the pulmonary vein is a vein, and capillary networks surround alveoli and villi.

What earns the marks

• Names the three vessel types as arteries, veins and capillaries.
• States that arteries carry blood away from the heart and veins carry blood towards it.
• Links thick elastic and muscular artery walls and a narrow lumen to high pressure and maintaining flow.
• Describes capillaries as one cell thick with a large surface area for diffusion.
• Describes veins as having a wide lumen and valves that prevent backflow at low pressure.
• Relates vessel structure to function, for example capillary walls for gas exchange in alveoli.

Common mistakes and how to put them right

• Saying arteries always carry oxygenated blood.

  Put it right: The pulmonary artery carries deoxygenated blood to the lungs, so direction relative to the heart defines an artery.

• Confusing the lumen with the wall.

  Put it right: The lumen is the central channel, while the wall contains muscle, elastic tissue and, in arteries, a thick outer layer.

• Stating that veins have no muscle or elastic tissue.

  Put it right: Veins have thinner muscle and elastic layers than arteries, plus valves.

Exam tips

• Use the words 'away from' and 'towards' the heart when defining arteries and veins, as direction is the key distinction.
• When asked to explain an adaptation, name the feature and then state the benefit, for example a one-cell-thick capillary wall shortens the diffusion distance.
• Sketch a simple table comparing wall thickness, lumen width and valves across the three vessel types to organise comparisons.