Gas Exchange in Animals

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Gas exchange - process through which oxygen reaches cells and carbon dioxide is removed from them.

Gas exchange surfaces must have:

  • a thin membrane
  • large surface area 
  • be moist
  • permeable 

Fick's Law: surface area of membrane x difference in concentration across membrane all/ thickness of membrane

Large surface to volume ratio needed as more gas can diffuse due to a grater surface


  • thin
  • large surface area to volume ratio
  • moist
  • diffuse across whole surface
  • short diffusion paths
  • obtains dissolved oxygen from water


  • diffusion happens across whole body
  • flattered shape = increased surface area to volume ration and shorter diffusion paths
  • oxygen from water


  • short diffusion pathway
  • secrets mucus - moist
  • low metabolic rate - reduces demand for oxygen
  • rudimentary blood system, has blood vessels underneath surface of membrane, has blood pigments


counter current - as blood flows through gill capillaries it encounters water with a higher concentration of oxygen. Therefore the gradient is maintained and oxygen can diffuse into the blood across the whole gill surface

gills provide : 

  • specialised area
  • large surface area
  • network of blood capillaries


  • gills covered by flap called…


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