• 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