concept

Countercurrent exchange

The cleverest trick life found for keeping the pressure slope from ever closing. In a fish gill, water flows over the lamellae one way while the blood inside flows the opposite way. So at every point along the frond the blood meets water fresher than itself - the oxygen slope is held the whole length instead of dying at the halfway mark. A countercurrent gill can strip 80-90% of the oxygen from the water; let blood and water flow the same direction and they quickly equalise at a mediocre level and exchange stops.