How Do Fish Breathe Underwater? Inside the Remarkable Gill

We take breathing for granted: open your mouth, fill your lungs with air, and oxygen floods into your blood. Fish live surrounded by water, yet they too need oxygen, and they get it without ever taking a breath of air. Their secret is one of nature’s most efficient organs: the gill.

The oxygen problem

Water does contain oxygen, but not as part of the water molecule itself. The oxygen fish use is dissolved oxygen, oxygen gas from the air that has mixed into the water. There is not much of it. A litre of air contains about 280 milligrams of oxygen, while a litre of cool fresh water holds only around 10 milligrams, roughly 30 times less.

Water is also about 800 times denser than air, so moving it over a breathing surface takes much more effort. To survive, fish need an organ that squeezes as much oxygen as possible out of every mouthful of water.

How gills work

Most bony fish have four pairs of gill arches on each side of the head, protected by a hard flap called the operculum. Each arch carries two rows of thin filaments, and each filament is covered with tiny folds called lamellae. The lamellae have walls only a few cells thick, packed with capillaries, the smallest blood vessels.

Breathing happens like this:

1. The fish opens its mouth and lowers the floor of its mouth, drawing water in while the opercula stay closed.
2. It closes its mouth and raises the floor, pushing water back over the gills.
3. Oxygen diffuses from the water, through the thin lamellae, into the blood. Carbon dioxide moves the other way, into the water.
4. The water leaves through the gill openings behind the head.

Unlike our lungs, where air goes in and comes out the same way, water flows through the gills in one direction only. Together, all the lamellae give a fish a huge surface area for gas exchange, in some fast-swimming species more than ten times the area of their body surface.

The countercurrent trick

The real genius of gills is the direction of flow. In the lamellae, blood flows in the opposite direction to the water. This is called countercurrent exchange.

Why does it matter? As blood flows through a lamella, it keeps meeting water that has slightly more oxygen than itself, so oxygen keeps diffusing into the blood along the whole length. If blood and water flowed the same way, they would soon reach the same oxygen level and exchange would stop halfway. Thanks to countercurrent flow, fish can extract up to about 80% of the oxygen in the water passing over their gills. Human lungs, by comparison, take only about a quarter of the oxygen from each breath of air.

More than breathing

Gills are multi-purpose organs. As physiologist David Evans and colleagues described in a major review, they also:

• balance salt and water: saltwater fish pump out excess salt through special gill cells, while freshwater fish absorb salt;
• remove waste: most of a fish’s nitrogen waste leaves as ammonia through the gills, not in urine;
• control acidity in the blood.

Unusual breathers

• Ram ventilators. Some sharks, tuna and mackerel swim with their mouths open, letting their forward motion drive water over the gills. Some must keep moving to breathe properly.
• Air-breathing fish. Lungfish have lungs as well as gills; African lungfish can survive dry seasons buried in mud, breathing air for months. Bettas and gouramis have a labyrinth organ that lets them gulp air at the surface, useful in warm, oxygen-poor water.
• Not fish at all. Whales and dolphins live in water but are mammals with lungs, and must surface to breathe.

Why fish suffocate out of water

Air has far more oxygen than water, so why does a fish die on land? Out of water, the delicate filaments and lamellae collapse and stick together, like wet hair clumping when lifted out of a bath. The surface area for absorbing oxygen shrinks dramatically, and the gills dry out. The fish suffocates surrounded by oxygen.

Fish can also suffocate in water. Warm water holds less oxygen, and pollution or algal blooms can use it up, which is why fish kills sometimes happen in lakes and ponds during hot summers.

In short

• Fish breathe with gills, thin filaments packed with blood vessels.
• Water holds about 30 times less oxygen than air, so gills must be very efficient.
• Water flows one way in through the mouth and out through the gill openings.
• Countercurrent flow lets fish extract up to about 80% of the oxygen.
• Gills also control salt balance and remove ammonia.
• Out of water, gills collapse, so fish suffocate despite the oxygen in the air.