Mechanism of respiration in fish

• Most bony fish have a special covering that protects the gills called the operculum.
• As water carrying dissolved oxygen enters the mouth of the fish, the animal moves its jaw and operculum to pump water through the gills.
• As water passes over the gill filaments blood inside the capillaries picks up the dissolved oxygen.
• Blood flows opposite the flow of water over the filaments increasing the opportunity for absorption.
• At this point, the circulatory system transports the oxygen to all the tissues within the fish.
• Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place.
• The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.