How Do We Pee and Why Can’t We Do It Any Time?

Erica H. asks: How do we pee in terms of the nuts and bolts of it? Why can we only pee sometimes and not just anytime there is liquid in our bladders? What’s the trigger and how exactly is the pee expelled once the floodgates do open? Is it just gravity or do our stomach muscles squeeze our bladders? Inquiring minds want to know!

the_BladderAn average healthy adult responds to nature’s call anywhere between 4 and 10 times each day. The need to take a leak begins when pressure in the bladder rises, stretching the bladder wall which is made up of bands of smooth muscle (detrusor) that are filled with beta adrenergic receptors that send and receive messages between the muscle’s cells and the nervous system.

While the bladder is initially stretching as it fills via the ureters propelling urine from the kidneys to your bladder, messages are sent from the sympathetic nervous system (the fight or flight part of the autonomic nervous system) via the hypogastric nerve to the detrusor muscles telling them to suppress the urge to contract. At this point, you experience no urge to pee and even if you wanted to, (unless you’re experiencing some sort of renal system issue) you typically wouldn’t be able to without doing something to squeeze the bladder manually (pushing on it directly, using your abdominal muscles, etc.). This is due to an internal urethral sphincter keeping things closed tight along with the lack of squeezing by the detrusor, both controlled by your autonomic nervous system.

As the urine in the bladder reaches approximately one quarter to half full, however, 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.

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.

However, the longer you continue to hold it, the less effective this is and the more intense and frequent this micturition wave will be, until it reaches a level where the detrusor contracts for a long time. This is the point when you really, really need to go. But if you’re on a road trip and your husband just refuses to stop and you don’t have some form of travel urinal, you can still generally (until things become critical) tell your external urethral sphincter it’s really not a good time and consciously make sure it stays closed. (Though this system will continue to remind you on occasion that your bladder can only expand so much.)

As alluded to, all the time you’re searching for a suitable spot, your brain is working with other muscles helping to prevent an accident. An internal automatically controlled sphincter surrounds the neck of the bladder (where it meets the urethra), and the voluntarily controlled muscles of your pelvic floor form the external sphincter that supports the muscles of the urethra itself. Working together, these help you hold it in if you’re so inclined.

Also, if your renal system is working properly, a sort of 1-way valve at each of the two connection points of the ureters and the bladder makes sure urine isn’t squeezed back up the ureters into your kidneys (via the detrusor and ureter muscles creating a 1-2 cm compression point, squeezing that section of your ureters shut at appropriate times).

When you are finally ready to relieve yourself, if your detrusor isn’t already bearing down, you’ll typically increase the pressure in your bladder via your abdominal muscles. This triggers an unsuppressed micturition reflex, at which point your automatically controlled internal sphincter is relaxed, as is the voluntary external urethral sphincter (now that the higher centers of your brain aren’t telling it to stay squeezed shut), and your detrusor muscles give a good, long squeeze. The result is the majority of the contents of your bladder being squeezed out, with typically around 50 or so milliliters remaining. Ah, sweet relief.

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

Bonus Facts:

  • Urine is made in the kidneys (two fist sized organs on either side of your backbone, just below the rib cage). Adult kidneys process up to 150 quarts of blood each day to produce anywhere from 1 to 2 quarts of urine. As mentioned, the urine is propelled from the kidneys to the bladder via the ureters, which are essentially thin tubes of muscle.
  • You can monitor your health by observing the color of your pee. If it’s pale or transparent yellow, you’re good-to-go, and if it’s dark yellow, you just need to take a little water. If it’s honey-colored you’re a bit dehydrated, and if it’s the color of maple syrup you probably have a problem, which could include severe dehydration and even liver disease. Pink pee could indicate anything from kidney disease to prostate problems or a urinary tract infection, although if you’ve eaten beets or blueberries, it could just be that. Orange is also troubling, but could be anything from food dye to a bile duct condition. Blue or green urine typically comes from food dye or medication, but there is a rare genetic condition that can cause this color. If your urine is completely clear, you’re probably drinking a lot of water and in the extreme case should maybe cut back because it’s possible (though rare) to kill yourself via drinking too much water (with a variety of ways this could kill you).
  • For instance, in 2003, doctors reported the death of a 64 year old woman who had consumed anywhere between 30 and 40 glasses of water over an evening, all the while hysterically exclaiming that she needed more. She ultimately died in her sleep that night. After an autopsy was performed, the cause of death was determined to be “hyponatraemia as a result of acute water intoxication.”
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  • Thanks for this informative article. I wish to ask, however, that the title and some portions of the text be edited to remove the inappropriate word, “pee.” I am astounded to see that word on a page that was (presumably) prepared by adults, because it is a word that, throughout my life, has been used only by little children.

    By the time tots have reached puberty, they should have been taught to use the word, “urinate,” a word that we adults should always use (in speech and writing). That is the word that questioner Erica should have used and that answerer Melissa should have used. Amazingly, the words, “urinate” and “urination,” are completely absent from this article.

    The title and text of the article should be changed to reflect the level of maturity expected by readers here. No one could reply that the use of “pee” is OK, because this article was written for pre-teens, because the explanation and various other terms used in this article are too complex for 99% of pre-teens to understand. This article is for adults and adolescents, so it ought not to include any “baby language,” such as “pee.”

    Adults and adolescents, when they use terms like “pee” and “poo[p]” appear to be at least extremely immature — and probably even vulgar to some readers.

    • I believe the usage of the term pee is much more common among teens and adults than you have experienced. I do not remember a single road trip with family or friends where someone asked the driver “Please find a rest stop so I can urinate.” instead they would invariably say “I gotta pee.” with a strained look on his/her face. Is the first sentence I presented more mature than the second? I don’t really think so, just more clinical. Also, I think that anyone who would find the use of the terms pee and/or poop in an article about the biology and physics of waste production and expulsion in the human body to be vulgar, would probably be far too sensitive to click on the article in the first place.

    • Get a life,

      Having a pee is in general usuage nowadays. Sounds much better than urination.

      I cannot believe in this day and age anyone can object.

  • They were just answering the question with the same terminology as it was asked. Seeing that this is an informative, but typically light-hearted website, the use of “pee” is okay. We don’t need, and most probably don’t want, this to turn into the rigid text of a medical journal.

  • I’ve always preferred the onomatopoetic term, “Whiz.” As in, “I’ve gotta take a whiz.” Dunno if it’s more “adult” but it has flair…

  • Ugh, there’s always one. Anyway, I’m off to see a man about a big brown horse..

  • I would love to see a pol, with the actual number, or the terms we commonly use for pee and poo, oops I mean urine and feces.

  • In my pre-plumbing early life, my grandparents said, “I’m goin’ to see Aunt Nancy.” One set of grandparents, from boot heel Missouri, pronounced it ain’t, the other set from southern Indiana pronounced it the same as the small insect, as I always have. I don’t remember that we made a distinction as to what went on in the outhouse, but my father-in-law, who happened to be a year older than one of my grandparents, used to use the expression “make water.” The first time I remember hearing “pee or poo” was twenty years ago shortly after the birth of one of my grandchildren. We’d changed his diaper, and the RN on duty, speaking with an accent I didn’t recognize, asked whether he’d peed or pooed. While it was clear enough what she meant, I thought at the time it must be an ethnic expression. Of course, it was not.

    As Grampa above commented, it’d be interesting to see a list of euphemisms we use for excretion. I can think of several more I’ve heard, but I won’t bore you any more.