The urinary bladder is a musculomembranous sac located in the lower portion of
the abdominal cavity behind the symphysis pubis. The organ is highly specialized to
store urine until the urine 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. See figure 1-6.
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 causes 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 of time, 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 has been
reached. At that limit, the urine must be voided.
The urethra is a tube that conducts urine from the urinary bladder to the outside
of the body. This structure begins at the anterior base of the urinary bladder.
a. Male-Female Differences. The female urethra is short and opens directly to
the outside. The male urethra, however, is much longer and has two curvatures. The
male urethra is divided into three sections: prostatic, membranous, and penile. The
prostatic portion enters the prostate gland. The membranous portion enters the
peritoneum and the penile portion forms in the shaft of the penis.
b. Urethral Sphincters. The urethral sphincters are two muscular structures
that prevent urine from leaving the urinary bladder. Each urethral sphincter is a circular
mass of muscle tissue. Relaxation of the sphincters allows urine to be forced through
Remember the difference between the ureter and the urethra. The ureter is a
tube draining urine from the kidneys to the urinary bladder. The urethra is a
tube draining urine from the urinary bladder to the outside.