The bladder
Your bladder's task is to store and empty urine. The muscles in your pelvis are important for your ability to "hold water". The walls of your bladder are made up of several thin interwoven muscle layers. These muscles are relaxed while your bladder fills up, but you flex and contract them when you empty your bladder.
In the walls of your bladder you can find receptors that register the expansion of your bladder as it fills up. When your bladder contains about 200 ml, these receptors send signals to your brain that the bladder is starting to fill up. You feel the first urge to pee. When the bladder contains about 400 ml, most of us feel a strong urge and a marked need to empty the bladder.
The urethra
A woman's urethra is about 3-5 cm long. A man's urethra is much longer, 20-27 cm long. A man's spermatic duct also empties in to his urethra. This means that his semen passes through the urethra during ejaculation.
The urethral opening is called meatus.
The urethra's sphincter muscle
Immediately below the bladder, right by where the urethra begins, you find a sphincter muscle. This muscle is very important for your ability to hold water.
The purpose of your sphincter muscle is to squeeze the urethra shut as the bladder fills with urine and to relax when you want to empty your bladder. This requires good interplay between the bladder and the urethra.
Your ability to hold water is, among other things, a result of an equilibrium between high pressure in your urethra and low pressure in your bladder. When you empty your bladder, the pressure in the urethra decreases, while it increases in the bladder.