The urinary bladder is an organ that is highly specialized to store urine until it is eliminated from the body.
a. Trigone. The base of the urinary bladder is known as the trigone because of its triangular shape. The trigone is fairly solid and nonstretchable.
b. Stretchable Wall. The rest of the wall of the urinary bladder is very stretchable and forms a spherical sac when filled.
c. Transitional Epithelial Lining. The mucosal lining of the urinary bladder is made up of a unique epithelium, called the transitional epithelium.
(1) Voiding reflex. The transitional epithelium has the capacity to stretch to a certain degree. At the limit of its stretchability, it causes a message to be sent to the spinal cord about the fullness of the urinary bladder. This initiates the voiding reflex, which would cause the urine to pass out of the body.
(2) Increments of stretching and reorganization. Often, however, it is not convenient to void (empty the bladder). Thus, after a short period, the transitional epithelium can reorganize itself and undergo another increment of stretching. Soon, however, the fullness message is somewhat more urgent. There can be several increments of stretching until the limit of the urinary bladder is finally reached. At that limit, the urine must be voided.
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