A trickle of urine is constantly passing from the kidneys, where it is produced, to the bladder down the ureters (thin tubes of muscle that carry urine from the kidneys to the bladder).
The bladder is a hollow, balloon-shaped muscle (also known as the detrusor muscle) that is responsible for storing urine until you find an appropriate time and place to urinate. It swells into a round shape when it is full and gets smaller as it empties. A healthy bladder can hold up to 300 - 500ml of urine comfortably. The urethra is the tube that allows urine to pass from the bladder out of the body.
You make different amounts of urine depending on how much you drink, eat and sweat. How often you need to urinate depends on how quickly the kidneys produce the urine that fills the bladder. Although a person does not control kidney function, a person does control when the healthy bladder empties.
A number of muscles work together like a dam, keeping urine in the bladder between trips to the bathroom. The opening and closing of the urethra is controlled with the help of sphincter muscles. The bladder is supported by the pelvic floor muscles (described as being shaped like a hammock or sling), which help maintain continence.
Nerves in the bladder tell you when it is time to empty your bladder. As your bladder fills and reaches its limit, nerve signals are sent to the brain and your urge to urinate intensifies. When you go to the toilet to pass urine, the brain signals the muscular bladder wall to tighten, squeezing urine out of the bladder. At the same time, the brain signals the sphincter and pelvic floor muscles to relax. It is normal to empty your bladder eight times in a 24-hour period.2