Gas exchange

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  • Created by: wilby99
  • Created on: 10-02-16 14:08

specialist exchange surfaces

Single celled organisms

  • substances can diffuse directly into or out of the organism across the cell surface membrane
  • quick diffusion rate due to reletively highh surface area:volume ration.

Multicellular organisms

  • diffusion is slow because
    • some cells are deep within the body 
    • low surface area:volume ratios
    • higher metabolic rate means they use up oxygen and glucose more quickly
  • this is why specialised exchange surfaces are needed
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special features of exchange surfaces

Large surface area

  • root hair cells


  • some are only one cell thick
    • Alvioli

Good blood supply and/or ventilation

  • increases efficiency
    • alveoli - surrounded by capillaries
    • Gills - contain a large network of blood vessels
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maths stuff

For the volume

Width x depth x hight

for the surface area

number of sides x width of sides x length of sides 

                                            CYLINDRICAL MEASUMENTS

for the volume

V = π r 2 h

for the surface area

A = 2 π r h + 2 π r 2

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Mammalian gas exchange

O2 is inspired and then CO2 is expired

The lungs

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gas exchange in insects

  • Insects have system of treacheal tubes throught their bodies
  • spiricles open into the sides of these allowing air to enter
  • the smallest of these tubes are called tracheoles 
    • they are in contact with the tissues
  • when muscles are active, fluid is drawn into the tissue
  • the fluid is then released when the muscle is at rest removing waste CO2
  • tracheoles provide a large surface area
  • some small insects can rely on diffusion to get gases in and out of tissues
  • larger insects ventilate their tracheal system with rythemic body movements
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Gas exchange in fish

  • Because water contains less oxygen than air, diffusion of gases is slower


  • made up of numerous folds giving a large surface area for diffusion 
  • located in the pharynx
  • supported by a bony gill arch
  • along each arch is a double row of gill lamellea 


  • out of water the lamelea stick together ad collapse meaning the fish suffocates
  • there is a counter current flow of blood around the lamellea which means that gas exchange is always happening

Ventilation of the gills:

  • to take in water:
    • the operculum closes the opening at the back of the pharynx
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Gas exchange in fish continued

    • the floor of the mouth cavity is lowered
    • the volume inside increases causing the pressure to fall
    • water rushes into the mouth
  • For water to pass out:
    • the mouth closes
    • the floor of the mouth cavity raises 
    • the volume inside decreases and so the pressure increases forcing water back over the gills
    • the operculum opens and water flows out

The counter current system:

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