Everyone is familiar with the feeling of a full bladder. They usually know, too, that when you feel that urge to urinate, it’s because the ‘bladder is full’. But what exactly is the bladder and how does it get full? What happens then?
The bladder is filled by the kidneys, small organs that lie ‘upstream’ from it. They filter the blood, help regulate electrolyte levels in the body, and perform other vital functions. One of those functions is very basic: to control the amount of fluid in the body. It does it by excreting excess fluid through the ureters down into the bladder.
While the kidneys filter about 42 gallons (160 liters) of blood per day, they produce only about a liter or two per day of urine. About 5% of that fluid is dissolved material such as urea, small amounts of protein and other substances. The remaining 95% is ordinary water.
Where the kidneys are, well, kidney-shaped and filled with neprhons (small filtering sacs) the bladder itself is hollow and more balloon shaped. It sits inside the pelvic region directly behind the pubic symphysis or pubic bone and stores the urine the kidneys excrete. As it swells with fluid, the bladder becomes rounder. At its peak, the average adult’s bladder will hold about 2 cups (about 1/2 liter), but the urge to urinate normally begins at about 1 cup (250-300 ml).
What happens next?
As the physical volume of the bladder increases, it presses on nearby nerves. Before long, those nerves (called ‘stretch receptors’) send signals to the brain that tell us ‘urinate’. That doesn’t happen automatically in the case of most people (fortunately), of course. The bladder is one of the few organs over which we have a large degree of conscious control.
Counteracting the urge created by those nerve signals, we can (up to a point) use the urethral sphincter muscles to ‘hold back’, for some people up to as long as five hours.
Then, whether through loss of control or by an act of will, we squeeze the bladder muscles to push urine through a tube called the urethra. But we have to do more than push. We have to relax those ring-like sphincter muscles to allow the urine to flow. It is poor control of the latter that produces incontinence.
The urethra, as well as the ureters and the bladder itself, are lined with a type of mucous that prevents fluid movement through their material. The bladder muscles are able to operate as they do because of their three layers: the mucosa, submucosa, and detrusor. The middle layer contains blood vessels that feed the cells of the organ. The outer layer performs the ‘heavy lifting’ in urination. When you ‘press down’ to urinate, it is this muscle you are chiefly contracting.
That sense of relief you feel is the result of those same nerves now signalling the brain, ‘done’.