C h a p t e r 2 7
Disorders of the Bladder and Lower Urinary Tract
657
Bladder Structure
The bladder, also known as the
urinary vesicle
, is com-
posed of four layers: an outer serosal layer covers the
upper surface of the bladder and is continuous with the
peritoneum; beneath it is an external network of smooth
muscle fibers called the
detrusor muscle;
this is followed
by a submucosal layer formed largely of connective and
elastic tissue; and an innermost layer of transitional epi-
thelium, often referred to as the
urothelium
.
1,3
Stretching
of the bladder wall as a result of distention with urine
is accomplished by flattening of its numerous mucosal
folds and thick transitional epithelium (see Fig. 27-1).
The urothelium is essentially impermeable to urine
solutes and water. The tonicity and composition of the
urine often is quite different from that of the blood, and
the urothelium acts as an effective barrier to prevent the
passage of water and other urine elements between the
bladder and the blood.
4
The surface of the urothelium
is covered with a mucin layer that is thought to act as a
nonspecific anti-adherence factor and as a defense mech-
anism against infection. The urothelium not only acts as
barrier between the bladder contents and the underlying
bladder tissues but also as a sensory organ by transmit-
ting physical and chemical information to afferent sen-
sory neurons in the underlying smooth muscle fibers.
4
It has been suggested that conditions such as overac-
tive bladder and interstitial cystitis are associated with
alterations in the function of this sensory system.
The detrusor muscle is the muscle of micturition or
urination. When it contracts, urine is expelled from
the bladder.
1,2
Muscles in the bladder neck, sometimes
referred to as the
internal urethral sphincter,
are a
continuation of the detrusor muscle. They run down
obliquely behind the proximal urethra, forming the pos-
terior urethra in males and the entire urethra in females.
When the bladder is relaxed, these circular muscle fibers
are closed and act as a sphincter. When the detrusor
muscle contracts, the sphincter is pulled open as the
shape of the bladder changes as urine is forced into the
urethra.
Another muscle important to bladder function is the
external sphincter,
a circular muscle composed of stri-
ated skeletal muscle fibers that surrounds the urethra
distal to the base of the bladder
1,2
(see Fig. 27-1). The
external sphincter operates as a reserve mechanism to
stop micturition when it is occurring and to maintain
continence in the face of unusually high bladder pres-
sure. The skeletal muscle of the pelvic floor also contrib-
utes to the support of the bladder and the maintenance
of continence.
Neural Control of Bladder Function
To maintain continence, or retention of urine, the blad-
der must function as a low-pressure storage system, with
the pressure in the bladder being lower than that in the
urethra. To ensure that this condition is met, the increase
in intravesical pressure that accompanies bladder filling
is almost imperceptible.
5,6
Abnormal sustained eleva-
tions in intravesical pressures often are associated with
vesicoureteral reflux (i.e., backflow of urine from the
bladder into the ureter) and the development of ureteral
dilation (see Chapter 25, Fig. 25-7). Although the pres-
sure in the bladder is maintained at low levels, sphincter
pressures remains high, preventing loss of urine as the
bladder fills.
Micturition, or urination, involves the activity of
both sensory and motor neurons. When the bladder is
distended to 150 to 250 mL in the adult, the first sen-
sation of fullness is transmitted to the spinal cord and
then to the cerebral cortex, and at 350 to 450 mL there
is a definite sense of bladder fullness.
5
During the act
of micturition, the detrusor muscle of the bladder fun-
dus and bladder neck contract down on the urine and
the ureteral orifices are forced shut. The bladder neck
is widened and shortened, and the external sphincter
relaxes as urine moves out of the bladder. Descent of the
diaphragm and contraction of the abdominal muscles
raise intra-abdominal pressure and aid in the expulsion
of urine from the bladder.
Normal bladder function requires coordinated inter-
actions between the sensory and motor components
of both the autonomic nervous system (ANS), which
controls involuntary smooth muscle activity, and the
somatic nervous system, which controls voluntary skel-
etal muscle activity.
5–7
The motor components of the
neural reflex that causes bladder emptying is controlled
by the parasympathetic division of the ANS, and the
relaxation and storage functions of the bladder is con-
trolled by the sympathetic division. The somatic nervous
system innervates the skeletal muscles of the external
Epithelium when
bladder is empty
Epithelium when
bladder is full
Detrusor
muscle
Ureters
Trigone
Internal sphincter
External sphincter
FIGURE 27-1.
Diagram of the bladder, showing the detrusor
muscle, ureters, trigone area, and urethral orifice. Note the
folding of the epithelial (urothelial) cells when the bladder is
empty and flattening of the cells when the bladder is full and
the wall is stretched.
