Structure and function of arteries ,capillaries and veins

• Although diffusion is sufficiently quick to transport substances across cells, it is certainly not quick enough to move substances around the body
• Cardiovascular system


  The system responsible for moving substances around the body
• Components of the cardiovascular system

  • The heart
  • The blood
  • Blood vessels
• Blood circulation


  Blood circulates from the heart through the arteries to the capillaries to the veins and back to the heart
• Arteries


  • Outer layer of connective tissue containing elastic fibres
  • Middle layer containing smooth muscle with more elastic fibres
  • Elastic walls stretch and recoil to accommodate the surge of blood after each contraction of the heart
  • Wide lumen and no valves present
• Arteries lie deep within the body


  To protect them
• Vasoconstriction


  Narrowing of blood vessels so blood flow is reduced (pressure increased)
• Vasodilation


  Widening of blood vessels so blood flow increases (pressure reduced)
• Examples of vasoconstriction and vasodilation


  • Temperature falls, blood flow to surface of skin reduced to reduce heat loss
  • Temperature rises, blood flow to surface of skin increased to aid cooling
• Capillaries


  • Walls composed of only one layer of endothelial cells
  • Lumen is very narrow and can only allow transport of one cell at a time
• Veins


  • Outer layer of connective tissue containing elastic fibres but the muscular wall is much thinner than arteries
  • Contain valves to prevent backflow
• Veins don't need to accommodate high pressure so the muscular wall is thinner
• The purpose of valves in veins is to prevent backflow towards the heart
• The main difference between blood plasma and tissue fluid is that tissue fluid doesn't have any large molecules like blood cells or proteins
• Exchange of materials in a capillary bed

  1. Blood enters capillary network under high pressure

  2. High pressure causes pressure filtration, leaving small molecules like water and glucose to enter the tissue fluid

  3. Tissue fluid enters lymphatic vessels and is now called lymph

  4. Lymph is transported through the lymphatic system

  5. Much of the tissue fluid is reabsorbed into the capillaries by diffusion and osmosis

  6. Deoxygenated blood travels through the venule and is carried away at low pressure
• Flow of lymph through the lymphatic system

  1. Tissue fluid enters lymphatic capillaries

  2. Lymph travels through lymphatic vessels

  3. Lymph passes through lymph nodes/glands

  4. Lymph is returned to the bloodstream via the lymph duct and subclavian vein
• Factors assisting lymph flow

  • Muscular contraction provides pressure to aid flow
  • Valves in lymph vessels prevent backflow