Blood Vessels: Structure & Function (Edexcel A (SNAB) A Level Biology)

• 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: 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

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

Arteries

• 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 heartbeat

Veins

• 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

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 

• 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
  • A 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

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