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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