Exchanging substances

    Cards (5)

    • Multicellular Organisms Need Exchange Surfaces
      • In single-celled organisms, gases and dissolved substances can diffuse in (or out of) the cell across the cell membrane because they have a large surface area compared to their volume, so substances can be exchanged across the membrane to supply the volume of the cell
      • Multicellular organisms have a smaller surface area compared to their volume, so they need some sort of exchange surface for efficient diffusion
    • Exchange surfaces
      • They have a thin membrane so substances only have a short distance to diffuse
      • They have a large surface area so lots of a substance can diffuse at once
      • In animals, they have lots of blood vessels to get stuff into and out of the blood quickly
      • In animals, gas exchange surfaces (e.g. alveoli) are often ventilated too - air moves in and out
    • Gas Exchange Happens in the Lungs
      The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it
    • Gas exchange in the lungs
      1. The lungs contain millions of little air sacs called alveoli where gas exchange takes place
      2. The alveoli have an enormous surface area (about 75 m² in humans)
      3. The alveoli have a moist lining for dissolving gases
      4. The alveoli have very thin walls
      5. The alveoli have a good blood supply
    • Humans need alveoli for gas exchange
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