MadSci Network: Medicine

Re: The helix form of excreted fluids.

Date: Wed Feb 28 14:33:42 2001
Posted By: Chris Rose, Grad student, PhD, Medical Image Processing (Mammography), Manchester University
Area of science: Medicine
ID: 982756284.Me

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 

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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