The purpose of residual air in the lungs is to

Residual air is the air left over in the lungs after you exhale. Now the purpose of residual air in the lungs is to prevent the lungs from collapsing. If no air will remain, the lungs would completely deflate and the tissue will stick together and case the lungs to collapse. In addition, the residual air keeps the lungs ventilated between breathes. 

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