Figure 48.5 Fish Gills (A) Water flows unidirectionally over the gills of a fish, in through the mouth and out through the opercular opening. (B) Gill filaments have a large surface area and thin tissues. (C) Gill filaments are lined with lamellae, which are the gas exchange surfaces. Blood flows through the lamellae countercurrent (left to right in this drawing; small blue and red arrows) to the direction of the flow of water across the gills (right to left in this drawing; large blue arrows).