How do fish breathe underwater?

Have you ever pondered how fish pull oxygen from water, where oxygen is scarce in comparison to air? This is a basic question that demonstrates a beautiful and practical biological process. Unlike humans who have lungs, fish have developed an entirely unique respiratory system — gills — which allows them to live in water.

Gills: Nature’s Under-Water Lung

The basis of fish respiration is their gills. All fish have multiple sets of gill arches (usually 5 sets) situated near their pharynx (throat). On top of each set of arches lies a protective shield called the operculum. Many people notice that if you look at the sides of a fish, the operculum appears to pulse as it “breaths.” Each set of gill arches also contain hundreds of thin, fleshy, comb-like projections referred to as gill filaments.

Surface Area Enhancement Using Filaments & Lamellae

These filamentous projections themselves are more than just simple pieces of flesh. Each filament includes many tiny, finger-shaped extensions called lamellae. As such, we may think of these lamellae as the micro-fingers of a glove. And as you might expect, lamellae greatly increase the total surface area of a fish for gas exchange. For instance, the surface area provided by a fish’s gills can be 10 to 60 times larger than the outside surface area of its body. As such, this extremely large surface area is necessary to draw adequate amounts of oxygen from water, which contains significantly fewer oxygen molecules than does air — roughly 20 to 30 times less depending upon the water temperature and/or salinity level.

Mechanisms of Water Circulation: Ventilation

In order for a fish’s gills to work properly, a steady stream of water must flow past them. Fish make use of a two-step suction-based pump system to accomplish this, known as buccal pumping (or ram ventilation for some fast-moving fish).

Stages in Buccal Pumping: An Active Process

Active pumping is used by most fish to push water over their gills. There are two distinct steps involved:

The fish will partially open its buccal cavity (mouth), expand its pharyngeal region (throat), create a partial vacuum (negative pressure) which pulls water into the fish.
Afterward, the fish will close its mouth, reduce the size of the buccal cavity, and simultaneously lift the operculum, pushing water out over the gills. This produces a one-way flow of water, ensuring that new, well-oxygenated water continually comes into contact with the gill surfaces.

Because both stages of active pumping occur in coordination with each other, there is always a one-way flow of water. Thus, oxygenated and deoxygenated water do not mix.

Passive Ram Ventilation Used by Fast Swimming Fish

Many fast-swimming fish, including some types of tuna and sharks, use an alternate approach to obtain water flow over their gills. These fish utilize ram ventilation. Instead of using muscle power to pump water over their gills, fast-swimming fish merely swim with their mouths open and allow water to flow past their gills due to their speed. Ram ventilation is an extremely efficient way for fish to obtain oxygen since they are able to maintain relatively high speeds while obtaining their needed oxygen supply. However, if these fish slow down long enough to prevent water flowing rapidly past their gills, they can become oxygen-deprived (suffocate).

Countercurrent Exchange Systems: An Elegant Solution

While having a large surface area for gas exchange and providing a rapid flow of water over the gills greatly increases the ability of a fish to extract oxygen from water, it still remains difficult to accomplish. Therefore, fish have developed an elegant solution called the countercurrent exchange system.

Functionality of Countercurrent Exchange

To visualize how this system works, consider two flows going in opposite directions. Inside a fish’s gills, blood moves through the lamellae in one direction while water moves through the lamellae in the opposing direction.

When entering the lamellae, water that is rich in oxygen encounters blood that has lost most of its oxygen and is just beginning to acquire oxygen.

Oxygen diffuses from the water into the blood due to differences in oxygen concentrations.

As water leaves the lamellae and continues downstream, it has removed considerable amounts of oxygen. Yet as it proceeds further downstream toward the end of the lamella, it continues to encounter blood that now has decreasing amounts of oxygen than does the water. Therefore, the concentration gradient for oxygen decreases continuously along the length of each lamella.

Due to this arrangement of flows in opposite directions, nearly all of the oxygen from the water that passes over a fish’s gills can be extracted. Researchers estimate that as much as 80% or even 90% of the oxygen available in the water that passes over a fish’s gills can be utilized. Although concurrent flows would only yield approximately 50%, this countercurrent flow provides substantially higher efficiencies in gas extraction. Not surprisingly, scientists such as Knut Schmidt-Nielsen, who investigated aspects of various animals’ physiology, have consistently emphasized the incredible efficiency demonstrated by this system.

Transportation of Oxygen Throughout a Fish’s Body

After oxygen diffuses into a fish’s blood inside the lamellae during gas exchange, it is necessary for this oxygen to be distributed throughout the rest of the fish’s body. Like in humans, fish transport oxygen throughout their bodies utilizing proteins specifically designed for this purpose. These are called hemoglobin and exist within red blood cells. Hemoglobins in fish are specially designed to operate efficiently under diverse conditions (various temperatures and/or oxygen levels) found in their aquatic environment. High affinities for binding with oxygen enable this oxygen absorbed at the gills to reach tissues needing it for aerobic respiration.

Alternative/Accessorial Methods for Obtaining Oxygen

Although gills provide virtually all of a fish’s respiratory needs, some species have developed additional or supplementary ways to obtain oxygen in low-oxygen environments.

Supplementary Air-Breathing Structures

Some species commonly referred to as air-breathing fish have developed supplementary air-breathing systems. Catfish and lungfish are examples. Some catfish and lungfish can take in air from the surface and transfer oxygen via altered parts of their stomachs/intestines or modified sacs functioning similarly to primitive lungs. The African lungfish (Protopterus annectens) can survive prolonged periods without water when burrowed underground during dry spells by using its lungs to breathe air.

Skin-Based Gas Exchanges

Small fishes (especially larvae) and other fish exhibiting high surface area/volume ratios may be able to obtain minor amounts of oxygen from their skin through a process called cutaneous respiration. Cutaneous respiration serves primarily as an auxiliary source of oxygen for gill-based respiration and almost never serves as the major respiratory method for adult fishes.