The Science Of Peeing: How Do We Pee And Why Can’t We Do It Any Time We Want?

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Everyone pees about four to ten times a day, but very few of us know the science of peeing. If you feel like you don’t know enough about your urine, here are a few golden facts you should definitely know!

When do we pee?

The need to pee begins when pressure in the bladder rises. This pressure stretches the bladder wall, which is made up of smooth muscle that is filled with beta-adrenergic receptors that send and receive messages between the muscle’s cells and the nervous system.

How does this process develop?

The urine coming from our kidneys is pushed into the bladder, while the abdominal nerves allow the muscles in the bladder wall to loosen and thus enlarge the bladder. At this stage, it is not possible to pee as we can’t exert pressure or use our abdominal muscles. In order to be able to pee, we need to activate the muscles that are controlled by our autonomic nervous system, which is only possible if the bladder is full to a certain level.

What certain level?

As the urine in the bladder reaches approximately one quarter to half full, the stretch receptors in the detrusor as well as other receptors in and around the opening to the urethra (the tube that runs from the bladder to the outside world) signal back to the sacral region of the spinal cord triggering the micturition reflex, as well as letting your brain (and you) know it’s time to pee.

To pee or not to pee, that’s the question.

The parasympathetic system kicks in and signals the detrusor muscle to now alternate between contracting and relaxing, in a process known as the micturition wave. If you choose to simply hold it, your brain sends impulses down the spinal cord inhibiting the micturition reflex, particularly making sure your external urethral sphincter is ignoring a secondary action of the micturition reflex which is telling your voluntary external sphincter it should go ahead and relax and let the urine flow. These impulses from the brain, thus, make sure this sphincter stays clamped shut until you decide it’s time to pee.

The brain and muscles should work together for this seemingly simple task.

If we have to hold our pee for a while, our brain will work to keep our muscles as tight as possible. In this process, our pelvic floor muscles get rigid and this allows us to keep the pelvis tight for as long as possible.

And finally...

When we’re ready to pee, with the help of our abdominal muscles, we increase the pressure in our bladder so that we can pee. This triggers an unsuppressed micturition reflex, at which point our automatically controlled internal sphincter is relaxed, as is the voluntary external urethral sphincter (now that the higher centers of our brain aren’t telling it to stay squeezed shut), and our detrusor muscles give a good, long squeeze. The result is the majority of the contents of our bladder being squeezed out, with typically around 50 or so milliliters remaining.

Oh yeah, sweet relief...

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