Blood Vessels (OCR Gateway GCSE Biology: Combined Science)

• There are three main types of blood vessel:
  • Arteries- carry blood away from the heart
  • Veins- carry blood towards the heart
  • Capillaries- involved in the exchange of gas of materials with tissues
• Each vessel has a particular function and is specifically adapted to carry out that function efficiently
• There are also smaller vessels, arterioles that branch off from arteries and venules that branch into veins

Arteries carry blood under pressure

• Key features:
  • Carry blood at high pressure away from the heart
  • Carry oxygenated blood (except the pulmonary artery)
  • Have a narrow lumen
  • Have thick muscular walls compared to the size of the lumen
  • The strong muscular walls contain elastic fibres to allow them to stretch and spring back
  • Blood flows through at a fast speed
• The structure of an artery is adapted to its function in the following ways
  • Thick muscular walls 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 take blood back to the heart

• Key features:
  • Carry blood at low pressure towards the heart
  • Have thin walls as the blood is at a lower pressure
  • Have a larger lumen than arteries
  • 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 are small

• Key features:
  • Very small - too small to be seen with the naked eye
  • Carry blood at low pressure within tissues
  • Have permeable walls that are one cell thick
  • Supply oxygenated blood and nutrients to tissues
  • Take away waste and deoxygenated blood
  • 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 walls are ‘leaky’ allowing blood plasma to leak out and form tissue fluid surrounding cells

Structure of a capillary

A network of small capillaries allows for efficient exchange of materials in tissues