LUNG as an organ of exchange

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  • Created by: Steff06
  • Created on: 08-04-16 10:59
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  • Lungs as an exchange organ
    • Trachea -> Bronchi -> Bronchioles -> Alveoli.
    • Gaseous exchange is the movement of gases by DIFFUSION between an organism and its environment across a barrier such as the alveolus wall.
    • Gases pass both ways through the thin walls of the alveoli. OXYGEN passes from the air in the alveoli to the blood in the capillaries. CARBON DIOXIDE passes from the blood to the air in the alveoli.
    • The lungs are adapted for exchange by:
      • LARGE SURFACE AREA which provides more space for molecules to pass through.
      • BARRIER PERMEABLE TO OXYGEN + CO2 which readily allow diffusion of oxygen and CO2.
      • THIN BARRIER TO REDUCE DIFFUSION DISTANCE: Adaptions include alveolus wall is ONE CELL THICK, capillary wall is ONE CELL THICK, walls consist of SQUAMOUS CELLS,
        • Capillaries are in CLOSE CONTACT with alveolus walls, capillaries are so NARROW that red blood cells are squeezed against the wall making them closer to air in alveoli, total barrier of diffusion is only 2 FLATTENED CELLS THICK.
      • MAINTAINING THE DIFFUSION GRADIENT: For diffusion to be rapid, a steep diffusion gradient is needed. So a HIGH concentration of molecules on the supply side of the exchange surface and a LOW concentration on the demand side.
        • To maintain a steep conc gradient, a FRESH SUPPLY of molecules on one side is needed to keep the conc high and a way of removing molecules from the other side is needed to keep the conc low.
        • Achieved by the action of the blood TRANSPORT SYSTEM and the VENTILATION MOVEMENTS
          • Blood brings CO2 from the tissues to the LUNGS which ensures the conc of CO2 in the BLOOD is HIGHER than that in the air of the ALVEOLI. Also carries OXYGEN AWAY from the lungs to ensure the conc of oxgen in the blood is kept LOWER than the conc in the air inside the alveoli.
            • Breathing movements of lungs ventilate the lungs and REPLACE the used air with FRESH air. Brings more oxygen into lungs+ensures conc of oxygen in air of alveolus remains higher than conc in the blood. Ventilation removes air containing CO2 from the alveoli which ensures conc of CO2 in alveoli remains lower than that in the blood.
              • CONSTANT supply of gas to 1 side of exchange surface+removal from other side ensures diffusion and exchange can continue.
          • Heart pumps blood along PULMONARY ARTERY to the lungs. In the lungs, artery divides up to form finer vessels which carry blood into tiny capillaries.
      • The lungs must produce a SURFACTANT to reduce COHESIVE FORCES between water molecules. Without the surfactant, the alveolus would collapse due to cohesive forces between water molecules lining the air sac.


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