What is a normal bladder?
The bladder is a muscular organ in the pelvis which is neurologically connected to the brain, the spinal cord, and the pelvic floor muscles. It stores urine produced by the kidneys until you are ready to eliminate it, and although we take it for granted, it is fascinating that human beings have evolved to be continent of urine.
The bladder is constantly filling with urine, even when you are not drinking fluids. As blood filters through the kidneys, the kidneys are constantly producing urine to eliminate waste from the bloodstream. At a minimum, your body should produce about 30mL of urine an hour. You get hydration from drinking fluids and from the water content in food, especially fruits and vegetables which have a lot of water. If you are well hydrated, you will probably make more like 100mL of urine an hour. One way of telling how well hydrated you are is by the color of your urine. If it is a clear straw color, you are well hydrated. If it is more concentrated it will look dark yellow or amber. It may also have a stronger smell. As the bladder fills, it stretches and sends signals to your brain telling your brain how full it is. The brain is constantly inhibiting the bladder, so that the bladder does not contract and empty at an inappropriate time (like before you have reached the toilet!). The bladder should signal a stronger urge to urinate when it fills to 400-600mL. A normal bladder capacity is about 500mL. A can of soda or beer has about 300mL, so that gives you some idea of the volume. When your bladder is very full, it can hold up to a liter of urine. It’s wise not to let it get to that point!
It is normal for a person to urinate once every 3-4 hours during the day. When the bladder starts to signal that it is full the brain determines whether it is an appropriate time to urinate. If not, the brain continues to inhibit the bladder, and often the urge to urinate will pass. If the urgency is strong and it is an appropriate time to use the bathroom, as you get near the toilet, the brain stops inhibiting the bladder and the bladder enters an emptying phase. During the emptying phase, the pelvic floor muscles relax and the bladder neck (where the urine comes out) descends and opens. The bladder neck also contains an internal sphincter which needs to relax and open in order for urine to come out. Once the bladder outlet (the pelvic floor and internal sphincter) are relaxed, the bladder muscle contracts and empties the urine out. Urinating seems like such a simple thing, but it is incredible how much complex neurologic signaling needs occur simultaneously for us to be continent.