How Do Fish Breathe Underwater?
– Oxygen Dissolved in Water Is Only 1/30 of What Air Holds
Fish need oxygen to live, just as we do. But what they take in is not air. It is oxygen dissolved in water, and a litre of water holds only about 1/30 as much as a litre of air. A fish's gills are a remarkably well-made device for not letting any of that thin supply slip away.
A child stands in front of an aquarium tank and asks, "Why aren't the fish in pain, staying in the water all the time?"
Look closely and you will see the fish opening and closing their mouths. Each time, the flap-like part at the side of the head opens and closes a little too.
It is easy to answer, "They breathe with their gills." But what do gills actually pull out of the water, and how? And why, when air has so much more oxygen, do fish stop breathing once they are out on land?
Only two things decide how a fish breathes
Oxygen does dissolve in water. But the amount is only about 1/30 of what the same volume of air holds. Water is also about 800 times heavier than air, so moving it takes effort. A fish breathes in something heavy that has little oxygen in it.
Gills are stacks of thousands of thin plates, giving a wide surface to touch the water. And by running water and blood in opposite directions, they take in most of the oxygen in the water. That is a far higher share than human lungs take from air.
Let's go through them in order, starting with how much oxygen there really is in water.
Oxygen dissolved in water is far less than you might think
Leave a glass of water standing and oxygen from the air slowly dissolves into it. The water of ponds, rivers and the sea holds oxygen dissolved in this way, and fish live on it.
Look at Figure 1. A litre of air holds about 280 milligrams of oxygen. A litre of water at 20°C can hold, at most, only about 9 milligrams, even when it is as full as it can get. Try the slider to change the water temperature. The warmer the water, the less oxygen it can hold.
It is not only the amount that is small. Oxygen spreads through water about 10,000 times more slowly than through air. In still water, once the oxygen right next to the gill is used up, the next supply takes a long time to arrive.
So fish keep sending fresh water over the gills. Water goes in through the mouth and out through the gap under the gill cover. It flows one way and never back and forth, because swinging heavy water to and fro would waste too much effort.
Some fish, such as tuna and bonito, swim constantly with their mouths open and let that motion push water through the gills. These fish are said to be unable to breathe well if they stop swimming.
Gills run water and blood in opposite directions
Look closely at a gill and you will see thin filaments lined up like the teeth of a comb. Each one is covered with even thinner plates, and each plate has fine blood vessels running through it. Between the water and the blood there is just one membrane, much thinner than paper.
What matters is the direction of flow. Look at Figure 2. On the left, water and blood flow the same way. Oxygen moves across briskly at first, but stops once both sides reach the same level. The blood can pick up only about half of the oxygen in the water.
On the right is how real gills work. Water and blood flow in opposite directions. Everywhere along the way, the blood meets water that is just slightly richer in oxygen than itself. So it keeps taking up oxygen right to the end, and at the exit it comes close to the level of the incoming water.
Thanks to this trick, fish are said to take most of the oxygen out of the water passing over their gills. Human lungs use only about a quarter of the oxygen in the air we breathe in before we breathe it out. Fish, living where oxygen is scarce, are far better at extracting it.
In water, the thin gill plates stand apart from one another and expose a wide surface. Out of water, they stick together. Almost none of the surface can touch oxygen, and it also dries out. Even with plenty of oxygen all around, the window to receive it has shut.
In hot summer weather, water can hold less oxygen to begin with. At night, water plants and tiny algae stop photosynthesis and only use up oxygen. So just before dawn is often the time of day when oxygen is lowest. Fish gather at the surface, where the water touches air and holds a little more oxygen. If you keep fish in a tank, it matters in summer not to let the water get too warm, not to switch off the air pump, and not to put in too many fish.
Summary
Fish breathe by taking in oxygen dissolved in water through their gills. Water holds only about 1/30 as much oxygen as air, and it is heavy and hard to move. Fish keep water flowing one way and run it opposite to the blood, and in this way they take in most of that thin oxygen.
Fish are not free from the strain.
They are simply shaped so that thin oxygen doesn't slip away.
Why we humans feel distress so quickly when we hold our breath is explained in "Why can't you hold your breath for even a minute?" Gills also control the salt balance of the body, not just breathing. See also how sea fish and river fish differ in the way they drink water.
- In front of a goldfish or tropical fish tank, count how many times the mouth opens in one minute. Notice that the gill cover moves slightly out of step with the mouth.
- If the tank has an air pump, see whether the fish prefer to stay near or far from the bubbles.
- Compare the number of mouth movements of the same fish on a hot day and a cool day. When the water is warmer, the count usually goes up.
Warm water holds less oxygen, and the fish's body also works harder and needs more of it. Just by counting, you can check the story of Figure 1 with your own eyes.
For those who want to know more – terms, formulas and links to textbooksEach part is labelled with its level, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school basic biology and chemistry
- High school+Advanced high school material, or a textbook sidebar
- UniversityUniversity-level material (animal physiology, mass transfer) not taught in high school
- ResearchTopics that researchers are still investigating, not yet settled enough to be taught as established
Junior highTerms: this phenomenon has names
- Gill breathing: breathing in which oxygen dissolved in water is taken into the blood through the gills.
- Dissolved oxygen: oxygen dissolved in water. It is used as a measure of how clean the water is and how well living things can live in it.
- Secondary lamellae: the thin plates lined up on the surface of the gill filaments. Oxygen is handed over here.
Junior highHigh schoolCheck it with a formula: how much must be moved to get 100 milligrams of oxygen?
Suppose a fish and a person each take 100 milligrams of oxygen into the body. Let's calculate how much water or air has to be passed through the gills or lungs.
| In symbols | V ＝ Q ÷ ( C × E ) |
| In words | Amount of water (air) passed through ＝ oxygen wanted ÷ (oxygen per litre × fraction that can be extracted) |
| Where it comes from | Conservation of oxygen. The oxygen that enters the body is exactly the oxygen lost from the water or air passed through. |
| Symbol | Meaning and unit |
| V | Amount of water (air) passed through the gills (lungs); unit: litres |
| Q | Amount of oxygen to take into the body; unit: milligrams |
| C | Oxygen in 1 litre of water (air); unit: milligrams per litre |
| E | Fraction of the oxygen passed through that can be extracted (a number from 0 to 1) |
| Oxygen wanted, Q | 100 milligrams |
| Oxygen in 1 litre of water at 20°C | about 9 milligrams |
| Fraction gills can extract | about 0.8 (said to be around 80%) |
| Oxygen in 1 litre of air | about 280 milligrams |
| Fraction human lungs can extract | about 0.25 (roughly a quarter) |
| Weight of 1 litre | water 1000 grams, air 1.2 grams |
| Oxygen obtained from 1 litre of water | 9 × 0.8 ＝ 7.2 |
| Water the fish passes through (litres) | 100 ÷ 7.2 ≒ 13.9 |
| Oxygen obtained from 1 litre of air | 280 × 0.25 ＝ 70 |
| Air the person passes through (litres) | 100 ÷ 70 ≒ 1.43 |
| Weight of water the fish moves (grams) | 13.9 × 1000 ＝ 13900 |
| Weight of air the person moves (grams) | 1.43 × 1.2 ≒ 1.72 |
| How many times heavier | 13900 ÷ 1.72 ≒ 8081 |
To get the same amount of oxygen, a fish passes about 14 litres of water, a bucket and a half, over its gills. A person passes about 1.4 litres of air through the lungs, so by volume the difference is about tenfold. By weight, the fish moves about 8000 times as much as a person to breathe. Gills take such a high share of the oxygen not out of luxury, but because otherwise they could not keep up.
High schoolHigh school+How gases dissolve, and how blood picks up oxygen
High schoolThe amount of gas that dissolves in a liquid is proportional to the pressure (partial pressure) of that gas. This is called Henry's law. Also, the hotter the liquid, the less gas it can hold. This is why the water bar in Figure 1 gets shorter when you raise the temperature.
High school+In blood, oxygen is carried by haemoglobin. As the oxygen dissociation curve taught in basic biology shows, it binds oxygen where oxygen is plentiful and releases it where oxygen is scarce. Fish haemoglobin is also said to differ in its properties depending on the species and the temperature of the water they live in.
UniversityCountercurrent exchange and the speed of diffusion
The mechanism on the right of Figure 2 is called countercurrent exchange. When two flows run in opposite directions, a difference in concentration is kept up everywhere, so the transfer carries on to the very end. The amount of oxygen crossing a membrane follows Fick's law of diffusion: it is proportional to the area and the concentration difference, and inversely proportional to the thickness of the membrane. Gills, with their wide surface and thin membrane, are shaped to fit this law. The same countercurrent idea also appears in the exchange of heat and substances in bird legs and in the kidney.
📖 Derivations and further reading: Wikipedia (English), "Countercurrent exchange" / Wikipedia (Japanese), "Henry's law"
ResearchWhat is still not well understood
- Do fish get smaller as the ocean warms? One idea is that as a body grows larger, the gill area fails to keep up with body weight. This leads to a prediction that fish will become smaller in warm, oxygen-poor seas, but opinions are divided.
- Balancing "breathing" and "salt control" in the gills. Gills are where oxygen comes in, and also where salt moves in and out. A wider, thinner surface lets salt and water pass more easily too. How fish manage this trade-off is still being studied.
- The path toward breathing air. Some fish, such as lungfish, use both gills and lungs. How low oxygen in water pushed the evolution of air breathing is still being researched.
In other words, this article too is "an explanation within what is known today." In particular, ideas about ocean warming and fish may change with future observations.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, Field 2: "Structure and function of animal bodies" | Gill and lung breathing; oxygen amounts in Figure 1 |
| High school | Chemistry: "Solubility of gases"; Basic Biology: "Haemoglobin" | Henry's law; water temperature and oxygen |
| High school+ | Biology: "Oxygen dissociation curve" | How blood picks up oxygen |
| University | Animal physiology; mass transfer | Countercurrent exchange in Figure 2; Fick's law of diffusion |
| Research | Marine ecology; evolutionary physiology | Warming and fish size; evolution of air breathing |
| – | Links to daily life | Caring for summer tanks and ponds; handling caught fish |
- Wikipedia (Japanese), "Gill" (ウィキペディア「えら」)
- Wikipedia (Japanese), "Dissolved oxygen" (ウィキペディア「溶存酸素量」)
- Wikipedia (English), "Countercurrent exchange"
- Knut Schmidt-Nielsen, Animal Physiology: Adaptation and Environment (Japanese edition: 動物生理学 ― 環境への適応, University of Tokyo Press 東京大学出版会)
- Daniel Pauly, Gasping Fish and Panting Squids: Oxygen, Temperature and the Growth of Water-Breathing Animals (International Ecology Institute, 2010)
* This article is a general-audience science explainer. The numbers given are rough estimates to help you understand the mechanism. The amount of oxygen and the fraction that can be extracted change with the species of fish, water temperature, salinity and more. If you have trouble keeping animals, please consult a pet shop specialist or a vet.