| Why
do I want to Pee when I Jump into the Pool?
Yun
Hwang
Come on, admit
it. When you jump into the pool, after what seems like
a very short time, you feel like going to the toilet .
No, the time is not passing by faster because you are
having fun, and yes, it has only been a little while since
you last went to the toilet. This urge seems like a personal
issue and a trivial one at that – you get out of the pool
and go to the toilet, end of story. As a matter of fact,
the urge to go is so important that NASA and the US Air
Force have spent millions of dollars on researching exactly
why we want to go to the toilet when we are in water.
It would seem
at first glance that water immersion and urinating do
not have much in common. After all, NASA is all about
firing things into space. And space is up there, nearly
empty, and extremely dry, while water is down here, certainly
not empty, and relatively speaking, very wet –.
One
unique and physically important feature of space is the
absence of gravity; one experience that is not readily
available on earth. The best earth-bound simulation of
the zero-g environment is inside a ‘vomit rocket’ – a
plane that literally drops out of the sky for a few minutes,
simulating free-fall. The problem with the vomit rocket,
apart from people spewing vomit everywhere in the plane,
is that it only lasts for a short time. Ultimately, there
is a limit as to how long a plane can fall before it hits
the ground.
The
simulation of a zero-g environment under water is an approach
used by NASA to investigate the effects on astronauts.
The
second best approximation to a zero-g environment is water
immersion. While the effects of gravity have not been
‘cancelled out’ as in the vomit rocket, water provides
buoyancy that opposes gravity and minimizes its effects.
One major advantage of immersing astronauts in water is
that it is very cost-effective (a deep pool with water
is a lot cheaper than fitting out an aeroplane). Additionally,
astronauts can be submerged in water for hours at a time,
during which they practice the tasks that they will be
required to carry out during extravehicular activity (EVA)
in space.
Because water
immersion is the closest long-term simulation of zero-g,
NASA employs water immersion to study the effects of zero-g
on the human body. One of the short-term effects of being
in water is the increase in the subjects’ urine volume
to more than twice the usual amount. It turns out that
the sudden desire to go to the toilet after jumping into
the pool, though not entirely normal for the average person,
does have a very good physiological basis.
Normally, there
is gravity pulling things down to earth. This pull includes
blood, which consequently pools in the legs. In water,
the effect of the gravity is reduced, so less blood pools
in the legs. Since there has been no change in the total
blood volume, the blood that would have pooled in the
legs has to end up somewhere else. It turns out that that
‘somewhere else’ is the torso, increasing the local blood
volume in the upper body.
In the blood
vessels, especially in the upper body, there are pressure
and volume receptors that control the fluid balance of
the body. This is a critical function, since about 70%
of the human body by mass consists of water. When these
detectors note the increase in blood volume, regardless
of whether this is due to immersion in water, being in
the zero-g environment of space, or drinking lots of water,
they trigger the same response – the desire to urinate
in order to get rid of the excess fluid.
Your backyard
pool and NASA – who would have thought they would be associated
with each other???
Further Reading
http://quest.arc.nasa.gov/neuron/teachers/stellar/Neutral.html
http://www.dsls.usra.edu/meetings/howi/
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