No, it squeezes the drainage channel (urethra).

You don't empty completely, so you always have a partially full bladder.

There is limited capacity because the bladder is already nearly full.

See above.

You are assuming the completing a cycle of urination has emptied the bladder.
This is not the case. If the bladder were being squeezed as you first stated,
this might be the result, but since the channel is being constricted so that
it takes more pressure to get the urine out, the bladder can't supply that
extra pressure and urine is retained in the bladder.

Hope that this helps.

Gene

Here's the way I understand it:  the enlarged prostate squeezes the urethra,
restricting flow from the bladder so during urination, the bladder becomes
"tired" before completely emptying.  This residual means it takes less time
for the bladder to refill to the point where it feels full again, i.e., one
feels the need to pee.  In some cases, the prostate protrudes into the
bladder, further reducing its capacity.  In some cases, this causes
extremely slow and perhaps painful flow if one waits too long.  In my
particular case, my flow is ok (not strong but steady) when I pee before the
urge gets too strong.  If I wait too long, then the stream slows down
considerably and just dribbles out.

Trouble is, with bph you don't empty the bladder completely.

The bladder walls are muscle, that has to contract to squeeze out the
contents. If there's an obstruction, the muscle has to work harder,
and gets thicker, like any exercised muscle.

Now that it's thicker, as it tries to contract it cuts off its own
blood supply and runs out of energy. So it stops squeezing before it
is empty.

So you have urine left in there, and as soon as you've added a bit
more you automatically get the feeling you need to pee again.

If you've got 500 ml in the bladder and, with no bph, you can get rid
of it in one go, brilliant. If your bladder has to quit after pushing
out 100ml, how many times will you get up that night?

Hope that helps!