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The lower urinary system is comprised of the bladder with the bladder muscles (detrusor muscle) and the urethra with the sphincter muscle.
Bladder
The bladder's task is to store and empty urine. The muscles in the pelvis are important for the ability to hold water. The bladder are made up of several thin interwoven muscle layers. These muscles are relaxed while the bladder fills up, and flex and contract to empty the bladder.
The bladder walls have receptors that register the expansion of the bladder as it fills. When the bladder contains about 200ml, these receptors send signals to the brain that the bladder is starting to fill up and the urge to urinate is felt. When the bladder contains about 400ml, most people feel a strong urge and a marked need to empty the bladder.
Urethra
A woman's urethra is about 3-5 cm long. A man's urethra is much longer at 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.
Urethra's Sphincter Muscle
Immediately below the bladder, where the urethra begins, is the sphincter muscle. This muscle is very important for the ability to hold water, it is the 'gateway' to the bladder.
The purpose of the sphincter muscle is to squeeze the urethra shut as the bladder fills with urine and to relax when emptying the bladder. This requires good interplay between the bladder and the urethra.
The ability to hold water is, among other things, the result of an equilibrium between high pressure in your urethra and low pressure in your bladder. When the bladder is emptied, the pressure in the urethra decreases, while it increases in the bladder.
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