The urinary bladder functions as a temporary reservoir for the storage of urine.
When empty, the bladder is located below the peritoneal membrane behind the symphysis pubis. When full, it rises into the abdominal cavity.
There are four layers that make up the wall of the bladder:
- The innermost layer is a mucous membrane, containing several thicknesses of transitional epithelium, which extends to the ureters and urethra as well.
- The second layer is called the submucosa, consisting of connective tissue that contains many elastic fibers.
- The third layer is composed of involuntary smooth muscle called the detrusor muscle.
- The outermost layer of the upper bladder is the serosa and the lower portion is covered by connective tissue.
The bladder wall is arranged in folds called rugae, which allow the bladder to stretch as it fills.
The urge to urinate begins when the bladder has accumulated about 200 ml of urine. At about 300 ml, the urge becomes more uncomfortable. A moderately full bladder contains about 500 ml of urine (about 1 pint).
An overextended bladder may contain more than a liter of urine (about 1 quart). With over distention, the urge to void may be lost.
The floor of the bladder contains a triangular area called the trigone that is formed by three points: the entrance points of the two ureters and the exit point of the urethra. This is the area where the most infections take place.
The exit of the urinary bladder contains a sphincter muscle called the internal sphincter, composed of smooth muscle that contracts involuntarily to prevent emptying prematurely.
Below this sphincter, and surrounding the upper region of the urethra, is the external sphincter, composed of skeletal muscle that is voluntarily controlled. Contraction of this sphincter allows the resistance of the urge to urinate.