Edexcel International A Level Biology

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1.7 Blood Vessels: Structure & Function

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Blood Vessels: Structure & Function

  • The body contains several different types of blood vessel
    • Arteries transport blood away from the heart, usually at high pressure, to the tissues
      • Remember; arteries carry blood away from the heart
    • Arterioles; arteries branch into narrower blood vessels called arterioles which transport blood into capillaries
    • Veins: transport blood to the heart, usually at low pressure
      • Remember; veins carry blood into the heart
    • Venules; these narrower blood vessels transport blood from the capillaries to the veins
    • Capillaries are microscopic blood vessels that carry blood to the cells
  • Blood flows through the lumen of a blood vessel; the size of the lumen varies depending on the type of blood vessel
    • Arteries have a narrow lumen and the veins a wider lumen
  • The walls of each type of blood vessel have a structure that relates to the function of the vessel

Blood vessels of the circulatory system_1

The blood vessels form a continuous network; the structure of each allows it to carry out its function


  • Artery walls consist of three layers
    • The endothelium, sometimes referred to as the tunica intima  
      • The endothelium is one cell thick and lines the lumen of all blood vessels.
      • It is very smooth and reduces friction for free blood flow
      • In arteries the endothelium is highly folded, enabling it to expand under high pressure
    • Smooth muscle and elastic tissue, sometimes referred to as the tunica media
      • This layer is thick in arteries
      • The layer of muscle cells strengthen the arteries so they can withstand high pressure
      • It also enables them to constrict and narrow the lumen for reduced blood flow
        • Contraction of the muscle causes constriction of the lumen
        • This is useful for diverting blood flow away from certain locations e.g. away from the digestive system during exercise
      • The elastic tissue helps to maintain blood pressure in the arteries
        • It stretches and recoils to even out any fluctuations in pressure
    • The outer wall, sometimes referred to as the tunica adventitia, or tunica externa 
      • Contains the structural protein collagen
      • Collagen is a strong protein that protects blood vessels from damage by over-stretching
  • Arteries have a narrow lumen which helps to maintain a high blood pressure
  • A pulse is present in arteries as they stretch to accommodate an increased volume of blood with each heart beat


  • Veins return blood to the heart
  • They receive blood that has passed through capillary networks, so the blood pressure is very low
  • Veins contain the same layers as arteries but in different proportions
    • The smooth muscle and elastic layer is much thinner in veins
      • There is no need for a thick muscular layer as veins don't have to withstand high pressure
  • The lumen of the vein is much wider than that of an artery
    • A larger lumen helps to ensure that blood returns to the heart at an adequate speed
    • A large lumen reduces friction between the blood and the endothelium of the vein
      • The rate of blood flow is slower in veins but a larger lumen means the volume of blood delivered per unit of time is equal to that of arteries
  • Veins contain valves
    • These prevent the backflow of blood, helping return blood to the heart
  • A pulse is absent in veins due to the increased distance from the heart

Artery and vein structure

Arteries have a thick layer of elastic and muscle tissue and a narrow lumen, while veins have a thin layer of elastic and muscle tissue and a wide lumen


  • Capillaries have thin walls which are permeable, allowing substances to leave the blood to reach the body’s tissues
  • They can form networks called capillary beds which are very important exchange surfaces within the circulatory system
    • large number of capillaries branch between cells
      • Substances can diffuse between the blood and cells quickly as there is a short diffusion distance
  • Capillaries have a lumen that is very narrow in diameter
    • Red blood cells need to pass through the capillaries in single file
    • This forces the blood to travel slowly which provides more time for diffusion to occur
  • The wall of the capillary is a single layer of endothelial cells
    • This layer also lines the lumen in arteries and veins
    • The wall is only one cell thick; this reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
    • The cells of the wall have gaps called pores which allow blood plasma to leak out and form tissue fluid
    • White blood cells can combat infection in affected tissues by squeezing through the pores in the capillary walls

Structure of a capillary

Capillaries have a narrow lumen and walls that are one cell thick

Exam Tip

You need to know about the structure and function of arteries, veins and capillaries; for “explain” questions remember to pair a description of a structural feature to an explanation of how it helps the blood vessel to function. For example, “Capillary walls are one-cell thick, which enables quick diffusion of substances due to the short diffusion distance.”

Remember that muscle tissue contracts and elastic tissue recoils.

Remember that capillary walls are one cell thick; they do not have cell walls.

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