Define gas exchange and distinguish between a respiratory medium and a respiratory surface

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GAS EXCHANGE: the intake of Oxygen and the expulsion of Carbon dioxide. In the capillaries of the alveoli, oxygen travels due to the concentration gradient into those capillaries. Carbon dioxide is then expelled from those capillaries.  

RESPIRATORY MEDIUM: is air for animals and water for fish. It is basically the substance through with living animals receive their oxygen.  

RESPIRATORY SURFACE: is where the gaseous exchange takes place. The oxygen and carbon dioxide move across the respiratory surface entirely by diffusion.

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