Blood Vessels: Structure & Function (Edexcel IGCSE Biology)

Blood Vessels

• There are three main types of blood vessel:

  • Arteries

  • Veins

  • Capillaries

• Smaller vessels that branch off from arteries are called  arterioles (small arteries) and those that branch into veins are called venules (small veins)

• Each vessel has a particular function and is specifically adapted to carry out that function efficiently

Arteries

• Key features:

  • Carry blood at high pressure away from the heart

  • Carry oxygenated blood (except the pulmonary artery)

  • Have thick muscular walls containing elastic fibres

  • Have a narrow lumen

  • Blood flows through at a fast speed

• The structure of an artery is adapted to its function in the following ways

  • Thick muscular walls containing elastic fibres withstand the high pressure of blood and maintain the blood pressure as it recoils after the blood has passed through

  • A narrow lumen also helps to maintain high pressure

Veins

• Key features:

  • Carry blood at low pressure towards the heart

  • Carry deoxygenated blood (other than the pulmonary vein)

  • Have thin walls

  • Have a large lumen

  • Contain valves

  • Blood flows through at a slow speed

• The structure of a vein is adapted to its function in the following ways:

  • A large lumen reduces resistance to blood flow under low pressure

  • Valves prevent the backflow of blood as it is under low pressure

Comparing the structure of arteries and veins

Capillaries

• Key features:

  • Carry blood at low pressure within tissues

  • Carry both oxygenated and deoxygenated blood

  • Have walls that are one cell thick

  • Have ‘leaky’ walls

  • Speed of blood flow is slow

• The structure of a capillary is adapted to its function in the following ways:

  • Capillaries have walls that are one cell thick (short diffusion distance) so substances can easily diffuse in and out of them

  • The ‘leaky’ walls allow blood plasma to leak out and form tissue fluid surrounding cells

Structure of a capillary