Blood Vessels (WJEC GCSE Biology)

• There are three main types of blood vessels:
  • Arteries
  • Veins
  • Capillaries
• 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 (opening in the middle that the blood flows through)
  • 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

A cross-section of an artery diagram

A cross section showing the structure of an artery

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 (opening in the middle that the blood flows through)
  • 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

A cross-section of a vein diagram

A cross section showing the structure of an artery

Comparing arteries and veins

• When comparing arteries and veins there are two main differences in their structures:
  • Thickness of the walls: arteries have thicker walls with more muscle and elastic layers. They have to stretch and recoil to withstand the high pressure of blood coming directly from the heart
  • Size of the lumen: veins have a wider lumen. The narrow lumen of the artery helps to maintain the high pressure of the blood, which is not necessary in the veins

Comparison of artery and vein structure diagram

Comparing the structure of arteries and veins including the thickness of the walls and the size of the lumen

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
  • The 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

The structure of a capillary diagram

The structure of a capillary is linked closely with its important role as an exchange surface