What's the mechanism of urinating?

Question

I googled and read about the kidneys, bladder, and urethra but no source explains the dynamic process.

From my own personal experience, the process has four stages:

1. The muscles are in normal mode. Then, a feeling comes that you need to empty the bladder.

2. The muscles are relaxed so the flow starts.

3. After the last drop, the muscles are contracted and this time a short flow happens (much fewer amount of urine comes out).

4. Muscles go back to normal mode.

I would like to ask specifically about stage 3. Why does the last amount of urine come out this way? Why doesn't it simply go out with the rest of the urine? What's the responsible muscle?

Answer 1

The act of urination goes like this (Visible Body):

Micturition, or urination, is the act of emptying the bladder. When the bladder is full of urine, stretch receptors in the bladder wall trigger the micturition reflex. The detrusor muscle that surrounds the bladder contracts. The internal urethral sphincter relaxes, allowing for urine to pass out of the bladder into the urethra. Both of these reactions are involuntary. The external urethral sphincter is voluntary. It must be relaxed for urine to flow through the urethra and outside the body.

Picture: Bladder anatomy (source: Wikipedia, creative commons license)

So, to urinate, normally, all you need to do is to relax the external urethral sphincter. Sometimes, urination can spontaneously stop even if there is still some urine in the bladder; a common cause is enlarged prostate - it often enlarges with age. In this case, pressing on the prostate can remove the remaining urine. Also, it is possible to close the external urethral sphincter more consciously and thus prevent the leak of the remaining urine. The problem with the leak at the end may be relying on the external urethral sphincter to close by itself quickly, while it may need some time to do so if you don't think on it...

Answer 2

There's no extra muscle involved - it's like squeezing an almost-empty toothpaste tube to get the last bits of toothpaste out.