Fish Respiratory Structure and Function

The respiratory system of fish begins with the intake of oxygen through the gills. The gills are located on the sides of the head. Gill filaments are feathery structures that make up the gills. They provide a large surface area for gas exchange.The filaments are arranged in rows in the gill arches. Each filament contains lamellae.
Lamellae are discs that contain capillaries. Capillaries, similar in humans, are the site of exchange of oxygen into the blood stream. In fish the blood enters and leaves the gills through these small blood vessels.

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 it's jaw and operculum in order to pump water through the gills.
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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.