concept

Tidal ventilation

The way you and every mammal breathe: air rushes in and back out the same passage, like a tide. Simple and easy to run, but it carries two costs - a slug of stale air always left in the lung that can never be fully expelled, and a column of dead air sitting in the windpipe with every breath. In Pandora's dense atmosphere, stopping and reversing that heavy mass of gas each breath costs even more - which is why the highest-demand Pandoran animals seem to have abandoned it. The Na'vi keep it, like us.