Blood Vessels

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  • Blood Vessels
    • Open circulatory systems
      • The blood isn't always in the vessels.
      • The blood fluid circulates through the body cavity, so the tissue and cells of the animal are bathed directly in blood
      • EG insects
      • In insects there is a muscular pumping organ much like a heart. Blood from the body enters the heart through pores called ostia
      • Why don't all animals have an open system?
        • An open system works for insects because they are small. The blood doesn't have to travel far.
          • Also, they don't have to rely on blood to transport oxygen and carbon dioxide, have a separate system for this.
        • In a open circulatory system, the blood remains at a low pressure, and the flow is slow
          • Wouldn't be a sufficient supply of nutrients
    • Closed circulatory system
      • Blood always remains in blood vessels
      • Tissue fluid bathes the tissues and cells. This enables the heart to pump the blood at a higher pressure, so it flows more quickly -  can deliver oxygen and nutrients more quickly and remove carbon dioxide and other wastes more quickly
      • EG Fish
      • There must be exchange surfaces at the gills and at the body tissues, to allow materials to be exchanged between blood and tissue fluid
    • Blood flows through a series of vessels. Each is adapted to its particular role in relation to its distance from the heart.
      • All types of blood vessels have an inner layer or lining, made of a single layer of cells called the endothelium. This is a thin layer that is particularly smooth to reduce friction with the flowing blood
    • Arteries
      • Carry blood away from the heart. The blood is at a high pressure, so the artery wall must be able to withstand that pressure
      • Lumen -  small to maintain pressure
      • Wall -  thick  to give strength to withstand pressure.
      • Elastic tissue in wall - allows wall to stretch and recoil
      • Smooth muscle in wall - can constrict the artery, narrows the lumen
      • Endothelium - folded and can unfold when the artery stretches
    • Veins
      • Carry blood back to the heart. Has a low pressure and the walls don't need to be thick
      • Lumen - large, to ease the flow of blood.
      • Walls - thinner layers of collagen, smooth muscle and elastic tissue.
        • They don't need to stretch and recoil, not actively constricted to reduce blood flow
      • Contain valves - to help blood flow back to the heart and to prevent it flowing in the opposite direction.
    • Capillaries
      • Very thin walls, allow exchange of materials between the blood and cells of tissue via the tissue fluid
      • Lumen -very narrow (diameter same as a RBC) ensures the RBC are squeezed as they pass along the capillary wall reducing the diffusion path

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