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Why do you sometimes shiver when you wee?

Asked by John Rae of Clapham, United Kingdom

Shiver wee? This is not to be confused with chivalry (the valorous qualities of a knight or gentleman) or a shivaree (a noisy mock serenade by friends of a newly wed couple).

This is a surprisingly commonly-asked Odd Body Question (OBQ), and no research has been done on this topic.

Low room temperature as covered parts of the body are exposed could be an obvious cause. More seriously, the shivering is an example of the human body's autonomic nervous system (ANS) at work.

We are not conscious of the ANS. It runs on automatic, hence its name, "autonomic", which literally means "self controlling, working independently". The urination reflex is relayed through the ANS. The reflex is directly related in strength to the amount of stretch of the bladder. Thus, the degree of shivering is generally related to how full the bladder is at the time of urination.

The ANS has two divisions. One is the parasympathetic nervous system (PNS) and the other is the sympathetic nervous system (SNS).

The SNS tends to keep the bladder relaxed and the urethral sphincter contracted. This is why one does not have an "accident" while one is concentrating on something else. It is true to say that the more "desperate" one becomes in response to a bulging bladder, the more the SNS acts to keep you dry.

The SNS response includes the release by the brain of chemicals doadrenal medulla catacholamines epinephrine, norepinephrine, and dopamine to bring about the necessary body reactions. When the opportunity arises to allow the parasympathetic side of the ANS to take over, the change in catacholamine production probably causes of the shivering.

Laboratory experiments which have not been undertaken would prove this beyond doubt.

In any case, at the moment of urination, there is a slight blood pressure rise and a momentary flushing or euphoria shortly after relaxing the urethral sphincter. Some find this feeling pleasurable. At such moments, some people say "ah". This same response in its most extreme forms causes fainting. All of this is the ANS doing its job.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au

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