Gill

Gills are highly vascularised organs that perform gas exchange between the blood and the surrounding water. In bony fishes there are four pairs of gill arches, each bearing two rows of gill filaments. Each filament has secondary lamellae across which gas exchange occurs. A countercurrent exchange system (blood flowing opposite to water flow) maximises oxygen uptake efficiency to 70–80%, far exceeding the 25% efficiency of lungs. Gills also play major roles in osmoregulation, acid-base balance, and nitrogenous waste excretion.

Fish with high oxygen demands (e.g. tunas) have a much higher gill surface area per unit body mass than sedentary bottom-dwelling species.