The heart and blood vessels: veins

The idea

Veins are blood vessels that carry blood back towards the heart. Blood in veins is at low pressure, so veins have a wide lumen and relatively thin walls containing less muscle and elastic tissue than arteries. Many veins, especially in the legs, contain valves formed from folds of the inner lining; these prevent backflow and keep blood moving towards the heart when skeletal muscles squeeze the veins during movement. The largest veins are the vena cavae: the superior vena cava returns deoxygenated blood from the head and arms, and the inferior vena cava returns deoxygenated blood from the lower body, both emptying into the right atrium. The pulmonary vein is the exception, carrying oxygenated blood from the lungs to the left atrium.

What earns the marks

• Veins carry blood towards the heart, not away from it; most carry deoxygenated blood, but the pulmonary vein carries oxygenated blood from the lungs to the left atrium.
• Vein walls are thinner and contain less muscle and elastic tissue than artery walls because blood in veins is at low pressure.
• The lumen of a vein is wide, which offers little resistance and allows a large volume of low-pressure blood to return to the heart.
• Valves in veins are folds of the inner lining that prevent backflow, ensuring blood keeps moving in one direction towards the heart.
• Skeletal muscles contracting during movement squeeze veins, and the valves then stop blood flowing backwards, helping return blood to the heart.
• The vena cavae return blood to the right atrium: the superior vena cava from the head and arms, the inferior vena cava from the lower body.

Common mistakes and how to put them right

• Saying all veins carry deoxygenated blood.

  Put it right by naming the pulmonary vein, which carries oxygenated blood from the lungs to the left atrium.

• Confusing the direction of flow and stating that veins carry blood away from the heart.

  Put it right by linking veins to return of blood to the heart and arteries to delivery away from the heart.

• Claiming that valves push blood along veins.

  Put it right by explaining that valves only prevent backflow while skeletal muscle action and the heart provide the driving force.

Exam tips

• When asked to compare vessels, structure the answer around direction of flow, wall thickness, lumen size, pressure and presence of valves.
• Link each structural feature to its function, for example wide lumen and low resistance, or valves and prevention of backflow.
• Use the names vena cava, pulmonary vein, right atrium and left atrium accurately when describing the route of blood back to the heart.