In mammals - and that includes us humans - the lungs make up 7% of the total body volume. This allows us to inhale oxygen and exhale carbon dioxide, thanks to the pulmonary alveoli.
But bird lungs are unrivaled. They make up 15% of the total body volume. Gas does not mix between in- and ex- halation.
If we humans have a unidirectional respiratory system like a bird's, how would it affect the following?