Lesson 2.7 · AQA Combined Science

Blood vessels

Arteries, veins and capillaries

Name a blood vessel by its direction or function. An artery carries blood away from the heart. A vein carries blood towards the heart. Capillaries exchange substances with tissues.

Structure fits function

Vessel	Adaptations
Artery	Thick muscular, elastic walls withstand high pressure and stretch/recoil. Relatively narrow lumen.
Vein	Lower pressure; thinner walls and wider lumen. Valves prevent backflow.
Capillary	Very narrow; wall only one cell thick, giving a short diffusion path. Networks reach cells.

The lumen is the space inside a vessel where blood flows. Diagrams compare structure and are not drawn to the same scale.

Do not use oxygen to name vessels

Most arteries carry oxygenated blood, but the pulmonary artery carries blood low in oxygen to the lungs. The pulmonary vein carries oxygenated blood to the heart.

Exchange and flow

Oxygen and glucose move from blood to respiring cells. Carbon dioxide moves from cells into blood. Thin capillary walls and concentration gradients help exchange.

Calculate blood flow

Rate of blood flow = volume ÷ time. If 120 cm³ flows in 2 minutes, the rate is 60 cm³/min. Keep units consistent. To compare fairly, use values measured over the same time or calculate a rate.

Remember

Artery = away. Vein = towards. Capillaries = exchange through thin walls.