|Why do fishes die when they're removed from water?
I realize they require oxygen like other aerobes,
but (obviously) there's oxygen in the air. So what
is it about the water itself that keeps them alive?
|Question Date: 2014-04-28|
Great question. At first it doesn’t make sense.
Air has a lot more oxygen in it than water does.
A couple of key concepts are involved here.
One is surface area. A single cell can get all
the oxygen it needs just by having oxygen diffuse
in through the membrane over its surface. Larger
things, like fish, have to collect all the oxygen
in one area (gills for the fish) and transport it
to every cell (in blood for the fish). The more
surface area there is in the gills, the more
oxygen can be absorbed. Gills are made of many
thin, wrinkles and finger-like or hair-like
projections to increase surface area.
The next concept is density. Things that are less
dense float on things that are more dense. Water
is more dense than air, so many things that sink
in air can still float in water.
Now we’ll put these together. In water, the
projections on the gills float, so each is
surrounded with water from which it can absorb
oxygen. On land, these all collapse together,
blocking most of the surface area. So they don’t
work well and the fish suffocates.
Lungs increase surface area by having many tiny
pockets, which don’t collapse in air.
If these kinds of questions interest you, you
may want to explore physiology or fish biology.
Thanks for asking,
This actually has to do with one of the common
"themes" in biology: the relationship of structure
and function. The short answer is that gills
collapse. They need to have a large surface
exposed to water to collect enough oxygen to
transfer to the blood. When out of water, the
gills crumple and stick together as they
dry--think of wetting some tissue paper and
letting it dry. This makes all the surfaces stick
together, and reduces how much oxygen can be
I'm going to add a bit more to this because it's
one of the really, really cool things that you see
everywhere once you notice it. A main "driving
force" in biology is the transport of the
chemicals necessary for living. Tiny cells manage
this more easily because oxygen and other
necessary chemicals can move (or "diffuse")
through them easily, like food coloring through
water. Humans are much larger, and have to have
complex systems like the heart and the circulatory
system to make sure that the oxygen we need
outside of us makes it inside.
In biology there is always a competition for
the most efficiency. Look at plant leaves, for
instance. A tiny leaf will not absorb as much
sunlight (smaller surface), but too large of a
leaf will require a huge stem or trunk to hold it
up, and the sun will make it so hot that the water
will evaporate out of it. So it is the opposite
forces of cost (to build the structure) and
benefit (receiving the energy it provides) that
dictate if a leaf will be built. Scientists have
made models that use math to determine the "cost"
of building a leaf vs. the "benefit" of the
sunlight it receives, and have been able to
duplicate and predict exact leaf shapes based on
their math! It's really cool, and if you like math
it's even cooler.
To wrap this up, you can look anywhere in
nature and see where the effects of cost and
benefit of surface area changes happen. This is
why the brain has bumps all over it actually: to
maximize the surface area of the brain for the
most neuron connections, but at the same time to
keep the brain itself fairly small and
lightweight. It's why mitochondria have the
crumpled inner membrane (maximum surface in a
small space). It happens on every scale. Ever
since I learned this I see it literally everywhere
So basically, gills function by trying to have
a large surface in a small space, and drying them
out sticks the surfaces together and ruins it.
Many species of fish use a system of gills to
obtain their oxygen from the water. Gills are
designed with a high surface area of capillaries
that bring the blood close to the water to
exchange oxygen and carbon dioxide. As a
consequence of this, gills are extremely thin and
very delicate, like having thousands of pieces of
tissue paper very close to each other. These
gills operate by in-taking the water through the
mouth and having water flow through them at a
steady rate to keep the gills separated and
functioning, but when the gills are taken out of
the water, they effectively collapse on each
other, much like how wet tissues stick to each
other. As a result, a fish out of water cannot
absorb oxygen and will quickly asphyxiate. Some
species of amphibious fish do have modified gills
and are capable of surviving on land for up to a
few days, the mudskipper being one of them.
You're right, fish "breathe" by aerobic
respiration, but the way they use oxygen is
different than how we use oxygen. We inhale air,
use the oxygen and breathe out carbon dioxide.
Fish use gills to "breathe" by dissolving the
oxygen from the water into their bodies. Also, air
is way more concentrated with oxygen than water
is, so water is also better for fish in that way.
Fish need the water so they can dissolve the
oxygen to "breathe", whereas humans need air (and
the high oxygen concentration of the air) to
inhale their oxygen content.
Fish use gills to breath, and gills can only bring
in oxygen when moist, because the chemistry that
allows blood to carry oxygen happens in water. Out
of water, a fish's gills dry out, and then the
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