What’s worse than needing a pee when you can’t get to a toilet? The sound of running water, that’s what. Be it from a tap or the rain outside, the soft trickle-trickle noise can seem like it’s mocking your bladder. Quite why the sound makes us even more desperate to pee is uncertain, but we can filter it down to a couple of likely reasons.
When you or I take the time to relieve ourselves, urine from our full bladder flows out through a narrow tube called the urethra. This stream is controlled by two sets of sphincters – rings of muscle – that can clamp down on the tube like taps. The first (internal) sphincter is at the top of the urethra, next to the bladder, and is controlled automatically by our nervous system. When our bladder is full, a message is sent to the internal sphincter, telling it to relax and open the floodgates. But to stop us wetting our seats on the bus, the second (external) sphincter is under our conscious control, allowing us to keep the tide at bay, overriding the body’s demand to urinate willy-nilly.
The part of our nervous system that automatically controls the internal sphincter is called the parasympathetic nervous system (PNS). The PNS tends to be most active at times of calm and rest, when it encourages the first tap (internal sphincter) to open. It is probable, therefore, that the calming sounds of running water relaxes us enough to cause the PNS to send more messages to the internal sphincter, telling it to be opened wide. It’s worth knowing that opposite to the PNS is the sympathetic nervous system (SNS). This separate nervous control system is behind the so-called ‘fight or flight’ response, when our heart rate increases, our skin goes pale and we get goose bumps in response to fear. It affects the contraction of the internal sphincter, telling it to stay shut – perfect for those moments when you have to run for the train (or when a sabre-tooth cat is nearby).
The second possible explanation for why running water sound makes us want to pee will be familiar to anyone who has heard of Pavlov’s dogs. In the 1890s, a Russian psychiatrist called Ivan Pavlov carried out famous experiments with dogs, showing how animals’ bodies unconsciously learn to anticipate something important. In his experiment, Pavlov rang a bell just before feeding his dogs some meat powder (I bet you’re drooling just thinking of it). He observed that, after repeating the ring-food routine, the dogs would begin to salivate as soon as the bell rang – even if he didn’t bring any food. The dogs had learned an automatic response, associating the sound of a bell with the arrival of food. (This is called ‘classical conditioning’.) In much the same way, we learn, through our experience, to associate the sound of water with urination and so, like a dog thinking of a bone, we find it difficult to think of little else when we hear that trickle.
By Michael McKenna