MadSci Network: Medicine Query:

Hi Andrew Thanks for your question, it has got to be one of the most interesting and amusing I�ve received. I�m not really a fluid dynamicist, so other experts may be able to give a more accurate and detailed answer, but I have run my answer past a friend who has knowledge of fluid dynamics, and he agrees with me, so I hope I�ll be able to give you a layman�s explanation of what is going on (I must note that my explanation is not based upon fluid dynamics). But firstly two apologies: One apology I have to make is to any females reading this � I am assuming the person urinating is male, so you may wish to make adjustments for your own physiology. The second apology is for my poor attempts at humour � forgive me, I�m only a scientist. You ask why, when urinating, the urine seems to take a spiral or helical path on it�s way to the toilet (assuming that�s where we�re aiming :-), rather than a uniform path with a disc-shaped cross-section. The first thing to note is that the opening of the urethra (the tube that carries urine from the bladder to the outside world) at the tip of the penis is not circular; it is more of a slit. So we immediately have a situation where the disc-shaped cross-section model is not valid. But why does the flow of urine appear to twist on its way down? There are a number of effects going on, and I�ll try to explain them. Firstly, I will assume that the urine is a liquid made up of discrete particles. Because urine is a liquid, there is moderate attraction between the particles in the liquid. This attraction is a result of gravitational attraction (small particles such as molecules or atoms are attracted to each other in exactly the same way as planets are - even urine can be attractive :-) and electrostatic attraction (some of the particles in the urine may not be electrostatically neutral, and opposing electric charges attract one another). In a gas, the energy of the particles is such that the force of the attraction between particles can be easily broken and they can roam freely; in a solid the attraction between particles is so strong that the particles are fixed into a lattice and cannot easily move. Something in-between the two is true for a liquid, so we can think of any two particles in a liquid as being connected by an imaginary elastic band. When we urinate, the urine does not flow at equal speed across the urethra � some parts of the liquid may flow faster than others. So when the urine exits the body, parts of it are travelling faster than others. Image if we were to fire two metal balls in the same direction at the same time, but one of them was set off travelling slightly slower than the other. If you took a side-on view, you would see two similar, but different arcs that the two balls would take. Now imagine that they are joined by an elastic band � the paths that the balls take would still be two arcs, but they would be slightly distorted. Now imagine thousands or millions of these balls, fired in a stream, with differing speeds, many of them connected together by elastic bands of differing strength. The result would look similar to what happens when you urinate. Another reason that the path of the urine seems to spiral is that not every drop (particle) of urine leaves the body in a perfectly straight direction � some are ejected moving slightly to the right, or slightly to the left, and likewise in the vertical directions. Because of the attraction mentioned earlier, the path of the urine is changed from that which we would expect. Yet another reason is that air resistance will contribute to the path that urine takes. For particles of similar size, shape and density, the change in speed due to air resistance is proportional to the velocity that the object is travelling at � so particles (droplets) of urine will experience a different force due to air resistance depending on how fast they are travelling. Particles in the stream can be decelerated by other particles in the stream. So, in essence, the path that the urine takes does not actually spiral, but is fairly complex (some would say chaotic), and it is our brain trying to make sense of the shape it sees that makes us think the path is a spiral. To answer the question about why the stream splits into two, I have two ideas: (Warning � Personal information about a scientist�s toilet habits coming up!) - firstly, I�ve noticed that if I urinate with my foreskin over the head of my penis, sometimes the flow of urine seems to come more from outsides of the opening than the inside, so the stream forms a sort of �V-shape� � splitting into two. Secondly, another explanation is that of air resistance and the attraction between the particles in the stream. It may be possible that due to the factors discussed earlier, it is easier for the stream of urine to split into two than to stay as one stream. I disagree with your statement that this isn�t real science. For me, science is about asking the question �Why?� about any area of life � whether that be �Why is the sky blue?� or �Why does my urine seem to rotate as I pee?� � and then looking for the answer in a rigorous way. I hope this has answered your questions, if you are interested in the areas of science mentioned in this answer then I suggest you look at general textbooks on the following subjects: Newtonian/Classical Mechanics, Electrostatics, Fluid Dynamics. A standard college physics textbook should discuss the above subjects. However I would be surprised if there is an example of urinating in the textbooks � perhaps someone will write such an example one day :-) Chris Rose Imaging Science and Biomedical Engineering University of Manchester, UK

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