Anyone who has ever used the bathroom, which I suspect is just about every one, is likely familiar with the bladder. But what exactly is the structure of the bladder and how does it work?
The bladder can be thought of as a collapsible bag. It is located directly behind the pubic bone, known to anatomists as the symphysis pubis. It also lies below a layer of tissue known as the parietal peritoneum, which covers only its top surface. Three different layers of smooth muscle make up the walls of the bladder. Collectively these are known as the detrusor muscle. The lining of the bladder is comprised of a mucus membrane which are formed in to rugae. Rugae is simply a fancy term for deep folds.
Because of the stretchiness of the bladder walls, the bladder is capable of considerable distention, although it's capacity can vary greatly from person to person. The average person will be able to hold about 250ml of urine before they begin to feel the need to empty it.
There are three openings in the bottom of the bladder. Two are from the ureters - one coming from each kidney. The third opening leads to the urethra, which is the tube allowing urine to exit. The ureter openings lie at the back corners of the floor while the urethral opening is located in the lower front corner.
The bladder has two primary functions. The first is to serve as a reservoir for urine. Without the bladder, you would just have a constant trickle of urine coming out all the time as your kidneys are constantly processing new urine. Needless to say, that would be a touch - inconvenient. The second function is to expel urine with the help of the urethra.
When the bladder fills up and becomes distended the urine stimulates stretch receptors in the bladder wall. These receptors initiate a reflex contraction of the muscles in the bladder wall, but also simultaneous relaxation of the internal sphincter. The internal sphincter is a "valve" of sorts which is normally closed and keeps the urine in the bladder until it is time to let it out. This is quickly followed by relaxation of the external sphincter (yes, there are two sphincters) and thus emptying of the bladder.
A specific type of nerve fiber, known as a parasympathetic fiber, transmits a signal that causes contractions of the bladder and the relaxations of the internal sphincter. However, the external sphincter is controlled voluntarily, a process which is learned and aids in the prevention of urination when it's not desired.
This voluntary control of urination is only possible if the nerves supplying the bladder muscles and urethra as well as the spinal cord and brain are all intact and functioning properly. Injury to any of these parts of the nervous system can result in involuntary emptying of the bladder. This is known as incontinence.
Some people are unable to urinate even when the bladder contains a large volume of urine. This is known as urinary retention. There are many causes of retention, but it can be common after certain surgical operations or even childbirth. The "fix" for retention involves catheterization, where a rubber tube is inserted through the urethra and in to the bladder to allow the urine to drain.
So there you are, a basic introduction to the bladder. It may not be the most complicated organ in the body, but it sure is useful.