The Surprisingly Complicated Science of Having a Wee.

Georgina Watson

Aug 16

5 min read

You feel the urge. You find a loo. You sit down, have a wee, wash your hands, and then carry on with your day.

Simple, right?

Not quite.

Having a wee (or, to give it its medical name, micturition) is actually a remarkably sophisticated biological process. Every time you go to the toilet, your kidneys, bladder, spinal cord, brain, pelvic floor, sphincters and an intricate network of nerves have to communicate with one another in precisely the right order.

Most of us never give any of this a second thought, and that's rather the point: when everything works properly, having a wee is so effortless that it's easy to forget just how much is going on behind the scenes.

First things first: where does wee actually come from?

Before your bladder gets involved, your kidneys are already hard at work.

Your kidneys continually filter your blood, removing waste products and excess water while carefully regulating things such as salts and fluid balance. The resulting urine travels from each kidney down a narrow muscular tube called a ureter and into the bladder.

The bladder's job is then deceptively simple: store the urine until you're somewhere appropriate to get rid of it.

But achieving that requires some rather impressive engineering.

The bladder is a hollow, muscular organ capable of expanding as it fills. Its wall contains a layer of smooth muscle called the detrusor muscle. During the storage phase, this muscle needs to remain relatively relaxed so that the bladder can accommodate increasing amounts of urine without immediately trying to empty itself.

Meanwhile, muscles around the bladder outlet and urethra help keep the exit closed.

In other words, your bladder has to perform two almost opposite jobs:

Hold on.

And, when the time is right:

Let go.

The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) provides a useful overview of how these different parts of the urinary tract work together.

So how do you know when you need a wee?

This is where things get particularly interesting.

Your bladder isn't simply a bag that suddenly announces, FULL!.

As urine gradually accumulates, the bladder wall stretches. Sensory, or afferent, nerves carry information from the bladder towards the spinal cord and brain, allowing the nervous system to monitor what is happening.

The lining of the bladder, known as the urothelium, also appears to be much more biologically active than was once thought. Rather than simply providing a waterproof barrier between urine and the tissues beneath it, research suggests that the urothelium participates in signalling and interacts with sensory nerves and other cells within the bladder wall.

Researchers have described this interconnected system as a kind of “sensory web” involved in detecting and communicating what is happening inside the bladder.

You can read more about this fascinating area of research in reviews of urothelial signalling and bladder sensation and the neural control of the lower urinary tract.

Your bladder and brain are having a conversation

As the bladder fills, sensory information travels upwards through the nervous system.

The brain then has to interpret those messages.

Eventually, that collection of nerve signals becomes something conscious:

I need a wee.

The International Continence Society describes normal bladder-filling sensation as an awareness of the bladder filling with increasing sensation until there is a strong desire to urinate.

Importantly, this usually isn't an all-or-nothing process.

You might first become vaguely aware that your bladder contains urine. Later, you realise that you'll probably need the loo before too long. Eventually, that message becomes considerably more persuasive.

And yet, under normal circumstances, you can still decide not to go immediately.

That's a surprisingly important ability.

Your brain gets a vote

Imagine you're sitting in a meeting.

Your bladder begins sending increasingly enthusiastic messages that it would quite like to be emptied.

Your brain considers the situation.

Is there a toilet nearby?

Can I leave?

Can I wait another twenty minutes?

In healthy bladder control, higher areas of the brain can help suppress the voiding reflex until urination is socially appropriate.

This means that having a wee isn't simply an automatic reflex. In adults, it involves a remarkable mixture of automatic nervous-system activity and conscious control.

When you finally reach a toilet and decide that, yes, now is an acceptable time, the nervous system changes mode.

And... release

Emptying the bladder requires several events to happen in coordination.

The muscles keeping the urethra closed need to relax.

The pelvic floor relaxes appropriately.

At the same time, parasympathetic nerve activity helps trigger contraction of the detrusor muscle in the bladder wall.

Pressure inside the bladder rises and urine flows through the urethra.

Research into the neural control of urination shows that these storage and emptying phases depend upon coordinated pathways involving the bladder, spinal cord and brainstem, including an area commonly known as the pontine micturition centre.

Essentially, the nervous system has to perform a beautifully timed switch:

Storage mode → emptying mode.

The bladder contracts.

The outlet relaxes.

Wee happens.

And then the whole system resets.

All of that... just to have a wee?

Yes.

And that's why problems with urination aren't necessarily just problems with the bladder itself.

Normal bladder function depends upon the urinary tract, muscles and nervous system working together.

If the signalling between them is disrupted, bladder function can change dramatically.

Depending on what is affected, somebody might experience:

an overwhelming or sudden urge to urinate
needing to urinate unusually frequently
difficulty sensing that the bladder is filling
difficulty starting urination
incomplete bladder emptying
urinary retention
leakage or incontinence
bladder pain or abnormal sensations.

The International Continence Society even formally recognises reduced and absent bladder filling sensation. In some people, the normal sensation of filling occurs much later or is less intense; in others, the usual sensation of bladder filling and desire to urinate may be absent altogether. You can read the ICS definitions here.

Likewise, the NIDDK explains that damage or disruption to the nervous system can interfere with the messages travelling between the bladder and brain.

That matters because symptoms involving urination can sometimes provide important clues about what is happening elsewhere in the body.

Perhaps having a wee isn't so simple after all

For something we do several times a day, urination is remarkably easy to take for granted.

Behind every ordinary trip to the loo is a constant conversation between organs, muscles, sensory nerves, the spinal cord and the brain.

Your kidneys make urine.

Your bladder stores it.

Sensory pathways monitor the bladder as it fills.

Your brain interprets those signals.

Your nervous system keeps everything safely in storage mode until you decide the time is right.

Then, within seconds, muscles and nerves change their behaviour in a coordinated sequence that allows the bladder to empty.

And most of the time, we don't notice any of it.

Perhaps that's one reason urinary problems can be so difficult to appreciate until something goes wrong. A process that feels incredibly simple actually depends upon an extraordinarily complicated system.

The image below is a helpful way to explain the above process to younger children:

At Oui, we think understanding our bodies is one way of making them easier to talk about.

Because whether you call it urination, passing water, peeing or simply having a wee, it's a normal bodily function, and problems with it deserve to be talked about without embarrassment.

Oui. Let's talk about wee.

Sources & further reading

This article is intended for general information and awareness and is not a substitute for individual medical advice. If you notice a new or persistent change in your urinary symptoms, speak to an appropriate healthcare professional.