To further expand, there can be times where we desperately need to urinate, and the volume of urine varies massively each time.
What causes this, and why is the need not consistently triggered at the same volume?
In: Biology
Also important to remember that the bladder is in a space with other things that have constantly changing volumes (eg. your intestines while eating, or the uterus while pregnant). Think of it as like a balloon.. If you fill it up without any external pressure, the balloon will fill evenly and the stretching forces on the balloon walls will be equally distributed to all sides. Now if you forcibly compress part of the balloon without changing the volume, the air will move to the uncompressed areas and cause increased stretch on that portion of the balloon.
If you follow me so far the next part should be fairly simple to understand. The micturition, or urinary reflex is triggered by stretch receptors within the walls of the bladder. Once that stretch threshold is crossed a signal is involuntarily sent to your brain and your detrusor muscle contracts, causing bladder contraction and the sensation to urinate among other things. The important point here is that external factors can also add into bladder pressure so urine volume isn’t the only variable that can trigger this reflex (back to our balloon analogy that you can connect with any process that increases intra-abdominal pressure).
The bladder does it as a safety precaution so too much bacteria isn’t building up in the bladder or urea, which is what causes infections. The bladder can fill up with air or be affected by blood chemistry levels or have pressure from other bodily functions or your posture. Also keep in mind that when people hold their pee for a long time, it causes urinary retention which makes people only pee a little bit at a time and they think their bladder is being emptied completely.
It’s far from at the same volume each time. You have issues of fullness from food, irritation from activities such as biking, psychological stress, blood chemistry levels, inflammatory issues, etc. Also, when you are sitting or lying down, the trigger will usually be at a higher volume level.
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