concept

Cross-current exchange

The gas-exchange arrangement in a bird's lung, where blood crosses the air stream at an angle rather than meeting it head-on. Not as perfect as the fish gill's countercurrent, but because the airflow runs one way continuously, the blood meets fresh air at many points along its path - more than enough to leave the mammalian tidal lung gasping. It is almost certainly the arrangement Pandora's giant flyers use.