Urinary incontinence is a problem where the process of urination, also called micturition, happens involuntarily, meaning that a person might urinate without intending to.
Urinary incontinence is particularly problematic because it affects a person’s personal hygiene as well as their social life in a way that can be very limiting.
Normally, urine is held in the bladder, which receives urine from two ureters coming down from the kidneys and then that urine leaves the bladder through the urethra. As urine flows from the kidney, through the ureters and into the bladder, the bladder starts to expand into the abdomen.
The bladder is able to expand and contract because it’s wrapped in a muscular layer, called the detrusor muscle, and within that, lining the bladder itself is a layer of transitional epithelium containing “umbrella cells”. These umbrella cells get their name because they physically stretch out as the bladder fills, just like an umbrella opening up in slow-motion. In a grown adult, the bladder can expand to hold about 750ml, slightly less in women than men because the uterus takes up space which crowds out the bladder a little bit.
Alright, so when the urine is collecting in the bladder, there are basically two “doors” that are shut, holding that urine in. The first door is the internal sphincter muscle, which is made of smooth muscle and is under involuntary control, meaning that it opens and closes automatically. Typically, that internal sphincter muscle opens up when the bladder is about half full.