Blood Vessels (OCR Gateway GCSE Biology: Combined Science)

Revision Note

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  • 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 arteries and veins, IGCSE & GCSE Biology revision notes

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, IGCSE & GCSE Biology revision notes

Structure of a capillary

The blood vessel network, IGCSE & GCSE Biology revision notes

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

Exam Tip

The circulatory system can be viewed as a network of cylindrical tubes. The velocity of blood flow varies inversely with the total cross-sectional area of the blood vessels. As the total cross-sectional area of the vessels increases, the velocity of flow decreases. The slow flow in capillaries is because of the high total cross-sectional area of capillaries and not because capillaries are narrow. The collective cross-sectional area of all the capillaries in the human body is about a 1000 fold greater than the aorta. The slow flow in capillaries favours gas exchange and nutrient supply to cells.

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.