How do Fish Breathe?

Most fish breathe with gills, arched structures on either side of the body that contains features known as filaments and lamellae. Water, which contains oxygen, flows in through the fish's open mouth and then out across the gills. Rakers filter out the particles as the water flows through.

Blood coursing through the filaments and lamellae delivers oxygen into the bloodstream. The filaments and lamellae also dump carbon dioxide from the body into gills, and the water carries it out of the body. Many species, including sharks and bony fishes, have a pumping mechanism that aids respiration.

A fish out of water suffocates because its gill structures collapse and the oxygen exchange cannot continue.