Most fish primarily breathe using their gills but also take a look at the sections on intestine and swimbladder.

As the fish opens its mouth to inhale, its gill covers close and the fish sucks water into its pharynx (the cavity behind the nose and mouth). When the mouth closes, water is forced past the gill arches. It is here that a gas exchange takes place via small blood vessels in the lamellae (gill filament) - the leaf like structures that form the gills. Oxygen is taken up by the lamellae and distributed through the body by the bloodstream. Simultaneously, carbon dioxide is removed by the lamellae and discarded through the gills in to the water.

So why can't a fish breathe on land? Well some fish can however, when a fish is out of water its gills collapse, reducing the area of the respiratory surface, which then become dry stopping the diffusion of oxygen into the blood. In effect the fish suffocates.

Extracting oxygen from water is more difficult and requires a greater expenditure of energy than extracting oxygen from air. Water is a thousand times more dense, has 50 times more resistance to flow than air and contains only 3% as much oxygen as an equal volume of air. Therefore the gills have evolved to become very efficient systems for extracting oxygen from water. Some fish can extract as much as 80% of the oxygen contained in the water via the gills, whereas humans can extract only around 25% of the oxygen from the air taken into the lungs.

Several factors make gills very efficient:

• They have a huge surface area for gaseous exchanger. Each lamellae has many folds in its surface, giving it a rough appearance and thus maximising the surface area along a given length of filament. The surface area of the gills can be 10 to 60 times more than that of the whole body surface of the fish
• Fish use countercurrent circulation in the gill, the blood in the lamellae folds travels forward, in the opposite direction to the water flow, so that a constant imbalance is maintained between the lower amount of oxygen in the blood and the higher amount in the water, ensuring passage of oxygen to the blood. If the blood were to flow in the same direction as the water, oxygenated blood at the rear of the gills would be traveling with deoxygenated water and not only could not extract oxygen from the water but would even lose oxygen to it
• Fish have short diffusion distance for the oxygen into the bloodstream. Water is separated from the bloodstream by a membrane in the gills measuring less than 3 micron thick.
• Gills have minimal dead space. The folds of the lamellae are close enough together so that most of the water passing between them is involved in the gas-exchange process.
• Unlike humans, fish have a one way breathing system. Water flows continuously in one direction over the gills, extracting oxygen and dispelling carbon dioxide in one uninterrupted movement. Humans use a two-way flow of air in and out of lungs.