Question about the urge to urinate; or, gotta pee gotta pee gotta pee ... I can wait.

Physiology question.

If you’re like me, you pee sometimes.

Also if you’re like me, sometimes you really have to pee, because for whatever reason you didn’t space out your bathroom trips.

Often, when this happens, feelings progress like this:
[ul]
[li]I have to pee[/li][li]I really have to pee[/li][li]Where’s a bathroom where’s a bathroom …[/li][li]OK, I could pee, but it’s suddenly not urgent[/li][/ul]
In other words, the urge to urinate gets more and more intense, but if I don’t actually go (good Kegel work, I guess), the urge backs off for a while. I still need to pee, and have no trouble once I reach a restroom, but I’m no longer in a panic.

Of course, if I still don’t go, the powerful urge eventually comes back.

But my question is this: Why does the urge recede for a while? What’s going on physiologically to make the intensity rise, then fall, then rise again, even though the cause (un-peed urine) has not gone away?

I cannot for the life of me find a citation (at least, in 2 minutes on Google), but I’ve read that when there is an applicable amount of urine accumulated, the bladder periodically contracts for a few minutes (which intensifies the full-bladder sensation), and then relaxes again, on approximately a (iirc) 20 minute cycle. Presumably this is to encourage the owner to find a quiet spot and discharge, without actually causing them to wet themselves. So “gotta pee” is the contraction phase, and “I can wait” is the relaxation phase.

There’s the other reaction - I can hold it, I can hold it, hey! I’m close to a bathroom and now I really really need to go. Psychology?

That’s basically it, as I understand it. There are two different non-voiding contractions of the bladder. One is controlled by the normal filling of the bladder, peristaltic waves which push the urine from the top of the bladder down the walls to the bottom of the bladder. Those appear to be controlled by electrical impulses from the ureters. It’s what makes urine fill the bottom of the bladder first, even if you’re lying down.

The other seems to be (but this is still something of a medical mystery) a nerve signal between the spinal cord and the dome of the bladder that’s triggered when the dome stretches because the bladder is full. This is the “urgency” signal, and the one that people with the right part of the spine injured don’t feel. I don’t know if there’s a 20 minute window for these signals, but that seems about right in my experience. If you have little “spurts” of incontinence, that’s these contractions briefly overcoming the ability of the sphincter to stay closed. This is the “find a bathroom!” signal. And yes, it will go away after a bit. It’s a warning alarm, but it has a snooze button.

Both of these are different from the actual micturation or voiding contractions, the ones that happen when the pressure behind the urethra overcomes the contractile force of the sphincter. This one is controlled by contractions of the detrusor muscles. Healthy adults have the ability to control the contraction of the detrusor muscles. Once they are contracted, you’re peeing.

Lots more detail on the subject here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1472861/

Richard Feynman, among his many investigations, demonstrated to his dorm mates that one can pee while standing on one’s head – thus proving that peeing isn’t simply gravity driven. He tells this story in one of the early chapters of Surely You’re Joking.

For me, the sudden urge to pee was an early sign of diabetes. However, I never had the experience of having the sensation abate after I ignored it for a while.

From what I’ve read, it’s that you have two sets of muscles–one you consciously control, and one you don’t. And when you’re getting ready to go, you switch the using the conscious ones, which makes you more aware of what’s going on.

I was amazed that he apparently never got that he was pranked. You can easily demonstrate this by just angling up a bit so that the stream is obviously higher than your bladder. A 17-year old kid full of beer could presumably really aim high.

He was pranked by his frat-mates into doing something stupid.

Does the filling of the bladder ever cause it to squash the urethra, such that it is most difficult to start the flow when you most need to?

I don’t think so. The most common cause of retention is acute prostatitis in men, and prolapse in women. You can also get it from bladder stones or tumors. Some medications and post operative retention are also possibilities. But I’ve never heard of retention from a full bladder.

But that would only demonstrate that the flow of urine through the penile part of the urethra isn’t gravity-driven. It would not refute the hypothesis that gravity is what gets urine from the bladder down to the membranous urethra at the base of the penis.

To disprove that hypothesis conclusively, you do have to show that urination can happen with the bladder lower than the base of the penis: i.e., with urine going upwards through the entire urethra. E.g., by peeing while standing on your head.

If someone has an enlarged prostate, a full bladder can cause the lower part of the prostate to press on the urethra and make it difficult to get the flow started. I would assume that a similar mechanism might operate in cases of acute prostatitis.

Exactly. Both cases have an enlarged prostate, not just a full bladder. (Although actual retention with BPH is rarer than with acute prostatitis, as i understand it. Hesitation and incomplete voiding are the more common issues with BPH.)

Actually, I just noticed that I worded that wrongly. The full bladder presses on the upper part, not the lower part, of the prostate, causing the urethra to close off.