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Why Do Some of Us Shiver When We Pee?
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Almost everyone will be familiar with the comical sight of a baby who suddenly elicits a violent shudder: It's a pretty reliable indicator that the infant needs a diaper change. That's because peeing is oddly associated with shivering — a strange phenomenon that persists even into adulthood. But what's going on inside our bodies to generate this unusual response to a basic, daily function?

The truth is that we don't really know. There's no peer-reviewed research on the subject to shed light on the precise biological underpinnings of this phenomenon. But from what scientists do know about the bladder and its relationship with the nervous system, they've pieced together some possible explanations for why we shiver when we pee.

These center on two main ideas: It's caused either by the sensation of the drop in temperature as the warm pee leaves your body or by a confusion between signals in the autonomic nervous system (ANS). [Why Does Asparagus Make Your Pee Smell Funny?]

The first idea is founded on the common-sense fact that we typically shiver when we feel a sudden chill. As far as peeing is concerned, the logic goes that when we expose our nether regions (an obvious necessity for peeing) to cool air, and then simultaneously void the body of warm liquid, it creates an internal temperature imbalance — a chill — that triggers an uncontrollable shiver.

But some scientists aren't convinced by this idea, including Dr. Simon Fulford, a consultant urologist at the James Cook University Hospital in the United Kingdom. He prefers the alternative theory, which digs deeper into the nervous system for clues.

The process of urination is overseen by the ANS, the control center that orchestrates many automatic bodily functions, such as temperature and the beating of a heart, Fulford said. Obviously, urination isn't entirely automatic because we do have voluntary control over when we pee. But before that crucial decision point, urination is largely governed by two parts of the ANS, called the parasympathetic nervous system (PNS), and the sympathetic nervous system (SNS).

When the bladder reaches fullness, tiny stretch receptors in its muscular wall detect the motion of the bladder stretching and activate a set of nerves in the spinal cord called the sacral nerves. In turn, these spring the PNS into action, which causes the muscular bladder wall to contract, preparing it to push urine out of the body. This autonomic process works like an on-off switch, suppressing the instructive nerve reflexes while the bladder is still filling up, but "stimulating those reflexes to act when the bladder is full," Fulford told Live Science.

An odd quirk of this arrangement is that when urine leaves the body, blood pressure drops. "There does seem to be good evidence that blood pressure rises slightly with a full bladder, and that this drops on voiding, or soon after," Fulford said.

What happens next is difficult to untangle, biologically speaking. But it seems that this sudden dip in blood pressure spurs a reaction from the sympathetic nervous system, a part of the ANS that is involved in the body's fight-or-flight response. The SNS regulates many factors, including blood pressureas part of this reaction. Experts already know that when the SNS detects low blood pressure, it releases a series of neurotransmitters called catecholamines, which among their many functions, will carefully restore blood pressure to its former balance across the body. When it comes to urination, it's possible that this sudden surge of catecholamines causes the pee twitch. [Why Do People 'Twitch' When Falling Asleep?]

But why? For reasons that aren't fully understood, the interaction between the two nervous system components — the release of urine, fine-tuned by the PNS, and the surge in catecholamines, orchestrated by the SNS — may be causing mixed signals in the nervous system. That seems to trigger a glitch in the system that makes us shudder involuntarily.

Fulford says a similar phenomenon called autonomic dysreflexia sometimes occurs in patients with a spinal cord injury. This happens when a stimulus, like a full bladder, occurs below the site of the spinal injury, resulting "in an excessive autonomic nervous system response that causes the blood pressure to climb rapidly, the pulse rate to drop and patients to flush and sweat," he explained. This incongruous reaction echoes the weird shivers that we get when we pee.

Another clue is that men seem to experience this phenomenon more than women do, which might be explained by the fact that men usually stand when they urinate — possibly intensifying the dip in blood pressure that's thought to precede the shudder.

Whatever the cause, this bodily oddity shouldn't be a cause for concern. "There's not been any substantial research on this subject, but it's a normal bodily function and nothing to worry about," Dr. Grant Stewart, an academic urological surgeon at Cambridge University in England and chair of The Urology Foundation's Science and Education Committee in the United Kingdom, told Live Science.

Original story on Live Science.