IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Why can’t I breathe water even though a ﬁsh can
(sort of) breathe air?
When ﬁsh get stranded on the beach, they sometimes lie there gasping for 15 minutes or
even longer. But humans drown in three minutes. Why can ﬁsh breathe air a little bit, but
we can’t breathe water at all?
Both ﬁsh and humans breathe oxygen gas, but for ﬁsh the gas is dissolved in water. Water holds 20 times
less oxygen than air, so a ﬁsh’s gills—though not designed for it—can extract some oxygen from air. Humans
may one day be able to breathe liquid, though ….
Part of the reason life evolved to live on land (apart
from all the free real estate) is that the atmosphere
contains much higher concentrations of oxygen
than seawater—up to 30 times as much, depending
Land animals can be much more energetic than
sea life, because we can suck in so much oxygen for
our fast metabolisms. Our lungs have evolved from
gills to take in air.
But at the ﬁnal stage of oxygen extraction, we
actually dissolve the gas into a liquid (our blood).
The only big dierence between us and ﬁsh is that
the ﬁsh don’t need a clever system to dissolve the
oxygen into water. They just breathe the water.
But because seawater has such a low
concentration of oxygen, a ﬁsh’s gills need an
absolutely massive surface area. They are very
complex and ornate, with many branching
structures. Our lungs, on the other hand, don’t
need as much surface area because there’s so
much oxygen in the air.
This is why taking a ﬁsh out of water isn’t as
immediately fatal as dunking a human in the
deep end. The gills can extract oxygen out of
the air no problem—except there actually is a
Gills do not support themselves, they rely on
buoyancy in the water to stay open and spread
out and able to catch the most oxygen. When
you pull a ﬁsh out of the sea, its gills collapse
against themselves. There’s enough gill still
working to extract some oxygen, but not enough
to keep the ﬁsh alive. They suocate, quite
Human lungs don’t have as much internal surface area,
and they are designed to pump gas in and out. And since
water is so much thicker than air, we can’t pump it in and
out of our lungs fast enough. And since there’s so little ox-
ygen in water compared to what we’re used to, we run out
of oxygen much faster and drown in just a few minutes.
Actually ﬁsh can drown, too—if they get stuck or held
in a way that means they can’t open and close their gills,
they run out of oxygen. Or they might stray into a so-called
“anoxic” area of the ocean where there’s not much dis-
solved oxygen. They’d drown quickly there, too.
Having lungs full of a gas can be a disadvantage for hu-
mans when we want to mess around in areas of very high
or low pressure. Divers are limited in how deep they can
go because they need gas for their lungs.
But there is a liquid humans can (theoretically)
breathe. It’s a type of ﬂuorocarbon that’s very rich in
oxygen. As well as helping divers, it could be very useful
for patients with certain respiratory diseases—especially
children. Doctors could ﬁll the whole of the lung with
ﬂuid, or just the bottom 40 percent of the lungs. It could be
a real boon for premature babies whose lungs should still
be full of amniotic ﬂuid.
Astronauts might use liquid breathing one day, too. It
would allow them to accelerate at faster speeds without
getting injured by the gas compressing in their lungs,
because these liquids do not compress.
contains up to 30
times as much
oxygen as seawater,
Fish gills have