Question

explain why, when air is taken in and let out during breathing, the lungs always contain a residual volume of air.

Answer 1

This might be the reason:

Residual volume is necessary for the lungs to maintain a constant negative pleural pressure otherwise the lungs will collapse.

Also, even after we have expelled as much air as possible (expiratory reserve volume) gaseous exchange is still occurring by residual volume in the lungs. It also prevents the inner surfaces of the lungs sticking together.

Hope it helps,

Regards.