Hello, my name is Wilco Verberk, I’m a researcher at the Radboud University in the Netherlands. and I’m interested in how
animals obtain oxygen underwater and how this challenge
breathing underwater changes as animals grow. so if animals do not obtain enough oxygen they suffocate, however at the same time oxygen can be
poisonous and having too much oxygen can be toxic. Therefore animals have to balance the
risks of suffocation and poisoning and this requires
animals to match their oxygen uptake to their oxygen requirements. So how does this balancing
act change as animals grow? One of the key things that changes is
the surface area to volume ratio. Small objects generally have
a small surface area compared to large objects and this is because volume increases more than surface area with increasing size. Volume increases cubically whereas
surface area increases only quadratically. Hence, large objects have a smaller surface area relative to their volume. This principle explains why larger
animals have relatively smaller respiratory surfaces
with which to take up oxygen. So in growing animals the increase in
respiratory surfaces does not keep pace with the increase in mass of tissue
that which actually demands the oxygen. This means that the excess capacity for
oxygen uptake is smaller and larger animals. and hence they have less energy available
to fuel their activity, their growth and their reproduction. And this suggests that small animals actually better off. They should have more excess capacity. But there is a catch, and this has to do
with the effects of viscosity. Water is viscous, like honey, but a bit less so. And especially small animals are effected by this viscosity. So to illustrate that imagine
waterfleas swimming in water and they experience more or less the same viscosity, as if I was swimming in honey,
which would require a lot of honey obviously. But the effect of viscosity is also
that it creates a thin layer of water that clings to the respiratory surfaces and
this thin layer of water is called a boundary layer. And it impedes the diffusion of oxygen
and it also can become depleted of oxygen. So animals really need to refresh this boundary layer and to do that they have to move water across the respiratory surfaces and
that is especially easy for larger animals, but progressively more difficult smaller
animals. So, moving dense, viscuous water takes a lot of effort, especially since large quantities
of water need to be moved and this is because water water contains
about 20 to 30 times less oxygen than air. and this is also what makes breathing
underwater so much of a challenge. For example in fish, the mere act of breathing takes about ten
times more energy than it does in us humans. The high cost of breathing is also
the reason why many aquatic animals still use cutaneous breathing,
even when they are large and when they have gills.
For example, in resting adult fish, cutaneous breathing may amount up to a
third of the total oxygen uptake. So, in short: water breathers expend great efforts
to move the dense viscuous water and they rely on cutaneous breathing. And those two factors combined, make it
very difficult for them to regulate their oxygen uptake. So as a result,
when the oxygen levels in the water change they get into difficulties in
balancing the risks of suffocation and risks of poisoning that I
talked about earlier. And especially small animals have a very
low capacity to regulate oxygen uptake as they rely specially on cutaneous oxygen uptake. And because they face this problem of
viscosity in refreshing the boundary layers. In contrast large animals can more easily
increase ventilation rates and hence overcome the difficulties with differences in environmental oxygen availability. So, in conclusion, the challenge of breathing underwater changes as animals grow. small animals face this problem of viscosity,
preventing them from maximizing their oxygen uptake. While larger animals have a lower
capacity for oxygen uptake because they have relatively small
surface areas to respire across. And that is why this breathing is so interesting
as it changes from small to large animals. So thank you for listening and if you want to know more have a look at my website or check out the links listed in
the description of this video Goodbye!

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