An 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:

• Why Asparagus Makes Pee Smell
• Why Do Vitamins Make Urine Bright Yellow?
• How Do Astronauts Go to the Bathroom in Space?
• How One Man’s Attempt to Create the Philosopher’s Stone Out of Human Urine Led to the First Element Discovered Since Ancient Times
• Why Do We Still Have Pubic and Armpit Hair?

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.”