Which of the following is the first step in human respiration?

Respiratory is the lungs, or "Breathing system". The first step that you take in the system is breathing in air, or inhaling V. I assume you need more than just this so i will tell you the other steps. The second step is when the exchange of gas occurs, Oxygen enters, makes the blood cells release carbon dioxide(CO2). The third step is when the carbon dioxide(CO2) is released out of the lungs and enters the earths plants.  :) 

 Hope this helps!

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