How Do Fish Breathe Underwater?
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Fish extract dissolved oxygen from the water using gills. Water contains far less oxygen than air, so the system has to be remarkably efficient, and the way it achieves that efficiency is one of the neatest pieces of engineering in biology.
How do fish breathe when water holds so little oxygen?
Air is about twenty one percent oxygen. Water holds dissolved oxygen at a concentration roughly thirty times lower, and it is also far denser and more viscous, so moving it over a breathing surface costs more energy.
Fish solve this with enormous surface area and a very efficient extraction method. Gills are made of filaments covered in tiny folds called lamellae, packed with blood vessels, giving a huge area for gas exchange in a small space.
The countercurrent trick
This is the key mechanism. Water flows across the gill in one direction while blood flows through it in the opposite direction.
Because of this arrangement, blood always meets water that is slightly richer in oxygen than the blood itself, all the way along the gill. The gradient never reverses, so oxygen keeps transferring the whole way across. Fish can extract up to about eighty percent of the available oxygen, whereas human lungs manage roughly a quarter of what is in inhaled air.
Getting water moving
Fish use two main strategies. Most pump water actively by opening the mouth, closing it and forcing water over the gills and out through the gill covers, which lets them breathe while stationary.
Fast swimmers such as tuna and many sharks use ram ventilation instead, simply holding their mouths open and letting forward motion push water through. Some species that rely entirely on this must keep swimming to breathe, which is the origin of the belief that all sharks must never stop moving. Many bottom dwelling sharks pump water and rest quite happily.
Fish that breathe air
A number of fish supplement or replace gill breathing:
- •Lungfish, which have true lungs and can survive dry seasons buried in mud
- •Labyrinth fish such as bettas and gouramis, using a folded air breathing organ
- •Catfish species that absorb oxygen through the gut
- •Mudskippers, which breathe partly through moist skin while out of water
Why gills fail in air
A fish out of water suffocates despite being surrounded by oxygen. Gill filaments are supported by water, and in air they collapse and stick together, destroying the surface area the whole system depends on.
They also dry out quickly, and gas exchange requires a moist surface. This is why fish kept out of water die of what is effectively suffocation rather than dehydration, and why warm or polluted water can be lethal too, since warm water holds less dissolved oxygen.
Why warm water is dangerous for fish
Oxygen is less soluble in warm water than in cold, so a rise in temperature reduces the oxygen available at exactly the moment fish need more of it, since their metabolism speeds up as they warm.
This double squeeze is why fish kills often follow heatwaves, and why cold water species such as trout and salmon are particularly vulnerable to warming rivers. Nutrient pollution compounds it: algal blooms grow, then die and decompose, and the decomposition consumes oxygen, creating dead zones where fish cannot survive.
The takeaway
Fish breathe by passing water over gills with a huge surface area, using a countercurrent flow that keeps oxygen transferring the whole way across, which lets them extract far more of the available oxygen than lungs manage from air.