Fish are large = small surface area to volume ratio.
Body surface is not engough to supply and remove respiratory gases.
Structure of gills
Gills = located within the body of the fish behind head, made up of gill filaments.
Gill filaments = stacked up in a pile, at right angles to filaments are gill lamellae (increase the surface area of gills).
Water takin in through the mouth, forced over the gills and out through an opening on each side of the body.
Flow of water over gill lamellae and flow of blood = opposite directions = countercurrent flow.