The ureter then enters the pelvic region. The urine moves along the ureters drop by
drop, pushed by the wave like muscular contractions (peristalsis) of the tubular wall. In
the pelvis, the two ureters enter the posterior inferior corners of the urinary bladder.
8-6. THE URINARY BLADDER
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.
8-7. THE URETHRA
The urethra is the single tubular structure that connects the urinary bladder to the
a. Sexual Dimorphism. Relatively short and straight, the female urethra opens
directly to the outside. However, the male urethra is incorporated into the penis. Since
the male urethra has two more-or-less right-angle turns, one permanent and one
flexible, the male is more difficult to catheterize than the female.
b. Urethral Sphincters. The urethral sphincters are two muscular structures
which 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