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  • Blood
    • Circulatory Systems
      • Open
        • blood is not always in the blood vessels, it can circulate through the body cavity
          • some organs and tissues are bathed in the blood
        • action of body muscles can help to circulate the blood or a muscle acts as a pump
          • muscle can be a long, muscular tube that blood enters through pores called ostia
            • heart pumps blood towards head by peristalsis
              • at the forward end of the heart the blood pours out into the body cavity
        • works because insects such are small enough that the blood does not have to travel far
          • insects do not  rely on the blood to transport oxygen and carbon dioxide, they have a separate transport system for this
      • Closed
        • blood is always in the blood vessels
          • Tissue fluid bathes the tissues and cells
          • enables the heart to pump the blood at a high pressure so that it flows more quickly
            • can deliver oxygen and nutrients more quickly and remove CO2 and other wastes more quickly
        • rely on the blood to transport O2 and CO2 so the pressure needs to be high to maintain speed
    • Vessels
      • Arteries - away from the heart
        • blood is at a high pressure so the artery wall must be able to withstand the pressure
        • narrow lumen to maintain pressure
        • wall contains elastic tissue that allows the walls to stretch and then recoil as the heart pumps - felt as a pulse when arteries are close to skin
          • recoil maintains high pressure whilst the heart relaxes
        • thick wall containing collagen (fibrous protein) to give strength - withstand pressure
        • wall contains smooth muscle that can contract and constrict the artery
          • constriction narrows lumen - used to limit blood flow to certain organs/tissues so it can go else where
        • endothelium is folded and can unfold when the artery stretches
      • Veins - into the heart
        • blood is at a low pressure
          • wide lumen to ease passage of blood
        • walls have thin layers of collagen, smooth muscle and elastic tissue
          • do not need to stretch and recoil and are not actively constricted to reduce blood flow
        • contain valves to stop blood flowing in the wrong direction and into the heart
          • as the walls are thin the vein can be flattened by the action of the surrounding skeletal muscle
            • pressure is applied to the blood, forcing it along in a direction dictated by the valves
      • Capillaries
        • narrow lumen - same width as 1 red blood cell(7 micro metres)    so they are squeezed as they pass through them
          • helps them to give up their oxygen as they are pressed close to the wall, reducing diffusion distance
        • 1 squamous cell thick to reduce diffusion distance of materials between blood and tissue fluid


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