Gas exchangeGas exchange
takes place at a respiratory surface - a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is simply the cell membrane
, but for large organisms it is part of specialised organs like lungs
. This name can cause problems - in biology the word "respiration
" can mean cellular respiration
(ATP generation inside cells), however sometimes (such as here) it can also refer to breathing (which is how the word is most often used by non-biologists).
Gases cross the respiratory surface by diffusion, so from Fick's law we can predict that respiratory surfaces must have:
- a large surface area
- a thin permeable surface
- a moist exchange surface
Many also have a mechanism to maximise the diffusion gradient by replenishing the source and/or sink.
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