Your turn
Each lab is one idea from the video. Change things and see what happens.
Each lab isolates one step of fish respiration. The models are simplified to show the idea.
Tap and slide, and see what happens.
Drag the magnifier through the water. How many loose oxygen molecules can you find among the water molecules?
Scan the water: the O₂ that fish use is free gas between H₂O molecules, and there isn’t much of it.
Move the magnifier. Can you find the little oxygen bits in the water?
Not to scale: real water has far more water molecules for each oxygen molecule.
Tap the fish. Follow the water: where does it go in, and where does it come out?
Trigger a buccal-opercular pump cycle and follow the unidirectional flow.
Tap the fish. Where does the water go?
Tap to zoom from the whole gill down to one tiny fold.
Step through the levels: arch, filament, secondary lamella, exchange barrier.
Tap to look closer and closer.
Switch the blood between the same direction and the opposite direction. How much oxygen does it pick up?
Compare co-current and countercurrent flow in a 12-segment exchanger with equal flows.
Simplified model (equal flows, 12 segments). Real gills take about 80%.
Warm the water and add algae. When does the fish run short of oxygen?
Oxygen solubility vs temperature (Benson–Krause), minus a simple algae-decay term. Find the hypoxic threshold.
Make the water warm or cold, and add green goo. Is the fish happy?
The curve is real (fresh water at sea level); the algae effect is a simple sketch.
Stop the shark, or lift the perch out of the water. Watch their oxygen.
Halt a ram ventilator; remove a bony fish from water and watch the lamellae collapse.
Stop the shark, or lift the fish out. What happens?
You breathe air. Fish live in water, but they need oxygen too! Water has tiny bits of oxygen mixed into it. Fish use those bits.
A fish opens its mouth and gulps water. Then it pushes the water back, over its gills. The water comes out under a little flap on its side. In at the mouth, out at the gills!
Gills are red and feathery. They are made of thin threads with lots of tiny folds. Blood flows inside the folds. The folds are so thin that oxygen slips right into the blood.
The water goes one way, and the blood goes the other way. That way the blood keeps meeting water with a bit more oxygen than it has, so it keeps picking up oxygen.
Most fish can't breathe out of water. Their soft gills flop flat and stick together. That's why most fish must stay in water with plenty of oxygen.
Fish breathe with gills. They take in water through the mouth and push it out over their gills, which pull out the oxygen that is dissolved in the water and pass it into the blood. At the same time, carbon dioxide, the waste gas, leaves the blood and goes out with the water.110 Gills work so well because they are made of huge numbers of thin, blood-filled folds, and because the blood inside them flows the opposite way to the water.2
So fish do breathe, much like you do. They need oxygen, they swap it for carbon dioxide, and their blood carries it around the body. They just get it from water instead of air.10
Fish breathe oxygen. A water molecule (H₂O) does contain an oxygen atom, but it's locked tightly to two hydrogen atoms, and fish can't use it.4 What they use is oxygen gas (O₂) that has mixed into the water from the air and from water plants, the same way sugar mixes into tea. Scientists call it dissolved oxygen.4
The catch is that there isn't much of it. The air around you is about 21 percent oxygen.1 A litre of fresh water holds only about 8 cubic centimetres of oxygen, compared with about 210 in a litre of air: roughly 25 to 30 times less.12 That's why a fish has to move a lot of water over its gills, and why its gills have to be very good at their job.
| Air | Water | |
|---|---|---|
| Oxygen per litre | ≈ 210 cm³ | ≈ 8 cm³ (fresh water) |
| Breathing organ | Lungs | Gills |
The water only goes one way: in at the mouth, out at the gills. That's different from your lungs, where air goes in and out through the same tubes.12 In most bony fish the gill cover also works as part of the pump, so the fish can breathe while it rests in one spot (a few fast swimmers, like many tunas, rely on swimming instead).8
Lift the gill cover of a fish and you'll see bright red, feathery gills. Most bony fish have four or five gills on each side of the head, each on a curved gill arch.1012 Zoom in and there are three levels:2
Each fold is full of tiny blood vessels called capillaries, which is why gills look red.1011 All that folding gives a small organ a very large surface.1 And the wall between the water and the blood is incredibly thin: in many fish only 0.0003 to 0.003 millimetres, about the same as the wall between air and blood in your lungs.2 Oxygen moves across simply because there's more of it in the water than in the blood. It always spreads from where there's more to where there's less, a process called diffusion.1
Here is the clever part. Inside each fold, the blood flows in the opposite direction to the water. Biologists call this countercurrent exchange.210
Imagine it the other way first. If blood and water flowed side by side in the same direction, oxygen would rush into the blood at the start, but soon both would hold the same amount and the swap would stop. With equal flows, that setup can never pass across more than about half.13
With opposite flows, the blood keeps meeting water that has a little more oxygen than it does, all the way along the fold, so oxygen keeps moving the whole time.12 In laboratory experiments, fish gills took about 80 percent of the oxygen out of the water in the normal direction, but only about 10 percent when scientists reversed the flow of water over the gills.2
same direction: ≤ ~50% vs opposite (real gills): ≈ 80%
The player above uses a simple model with equal flows to show this, so its numbers are rounded. Real fish vary.
Yes. Fish can suffocate in water that doesn't have enough dissolved oxygen, and two things often cause it:
When dissolved oxygen falls below about 2 milligrams per litre, the water is called hypoxic. Many animals that can't swim away may die, which is why these areas are nicknamed dead zones.47
| Water temperature | Oxygen in fresh water at 100% saturation (sea level) |
|---|---|
| 0 °C (32 °F) | ≈ 14.6 mg/L |
| 10 °C (50 °F) | ≈ 11.3 mg/L |
| 20 °C (68 °F) | ≈ 9.1 mg/L |
| 30 °C (86 °F) | ≈ 7.6 mg/L |
Values from the oxygen-solubility equations used by the U.S. Geological Survey's DOTABLES calculator.6 Real lakes and rivers often hold less than these saturation values (plants can sometimes push them a little higher on sunny days).
Air has far more oxygen than water, so you might expect a fish to breathe better out of water. It can't, because gills need water to hold their shape. In water, the thin folds float apart. In air, they collapse and stick together, so almost all of the surface is lost and the fish can't take in enough oxygen.1214 In a way, a fish out of water suffocates for the same reason you would underwater: its breathing organ is built for the wrong stuff.
Almost all fish have gills, but some have extra ways to breathe:
Gills aren't just for fish either: crabs, clams and many other water animals have them.111
Fish take water in through the mouth and push it out over their gills. The gills are made of thin, blood-filled folds that take the oxygen dissolved in the water into the blood and release carbon dioxide into the water. Blood flows the opposite way to the water, which lets gills take about 80 percent of the oxygen.
Oxygen. Fish can't use the oxygen atoms locked inside water molecules. They use oxygen gas that has dissolved into the water, called dissolved oxygen.
Yes. Warm water holds less oxygen than cold water, and rotting algae blooms use oxygen up. Below about 2 milligrams of oxygen per liter, water is called hypoxic, and many fish that can't swim away are in danger. Such areas are called dead zones.
Gills need water to hold their shape. In air, the thin gill folds collapse and stick together, so almost none of their surface is left to take in oxygen, even though air has much more oxygen than water.
Taking in oxygen and releasing carbon dioxide is called gas exchange or respiration, and in fish it happens at the gills. Pumping water over the gills is called ventilation; sharks that breathe by swimming with their mouths open use ram ventilation.
No. Bony fish pump water over their gills with their mouth and gill cover (operculum), so they can breathe while resting. Only some sharks, such as great white and mako sharks, must keep swimming.
A few can. Lungfish have lungs, betta fish and gouramis have a labyrinth organ for gulping air, and mudskippers take in oxygen through their wet skin. Most fish, though, can only breathe with gills in water.