Windpipe and lungs in pink, air sacs in teal. Steps 1 and 3 happen at the same time — one breath of air is moving through the lungs and into the air sacs in back (1) at the same time as an earlier breath of air is moving out of the lungs and into the air sacs up front (3). Steps 2 and 4 happen at the same time as well — the air sacs in back are blowing air through the lungs (2) while the air sacs in front are blowing air out the windpipe (4).