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We breathe in and out between 15 and 25 times each minute, this automatic process is the means by which carbon dioxide is eliminated and oxygen taken into the body. The exchange of carbon dioxide and oxygen in animals demands three(3) conditions.

• Medium- a medium in which the gases can be present e.g. air or water
• Respiratory surface- a large, moist surface area where the gases are dissolved before diffusing into or diffusing out of the cells
• Transport system- a system to move the dissolved gases around the body to all the cells, and to collect gases which need to be eliminated.

In simple organisms the exchange of gases takes place through the skin and if the surface area/volume ratio is high no transport system is required. When the surface area/volume decreases a transport system becomes necessary. Thus special structures with a large area to absorb oxygen and eliminate carbon dioxide is needed. This transport system in humans is known as the respiratory system.