In the hearts of fish, blood flows in one direction only, and in order to get the most amount of oxygen, the water must flow past gills in the opposite direction.
(blood; flows; in one direction only; L:In the hearts of fish) (the most amount of oxygen; must flow; past gills; in the opposite direction)
blood the most amount of oxygen
flows must flow
in one direction only L:In the hearts of fish
past gills in the opposite direction