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