B
bladder A muscular, saclike structure in the
lower pelvis that serves as a reservoir for the URINE
the KIDNEYSproduce. In women the bladder is in
front of and slightly below the UTERUS. During
PREGNANCYthe expanding uterus limits the blad-
der’s ability to expand, accounting for the URINARY
FREQUENCYcommon in pregnancy’s last trimester.
In men the bladder is in front of the RECTUM, with
the PROSTATE GLANDencircling the first segment of
the URETHRAas it exits the bladder. Swelling of the
prostate gland, such as typically occurs with
advancing age, as in BENIGN PROSTATIC HYPERPLASIA
(BPH), can constrict the flow of urine through the
urethra in a man, accounting for symptoms such
as urinary frequency and dribbling.
Three layers of tissue form the bladder. The
outermost and innermost layers are membranous,
the outer being a continuation of the peritoneum
that lines the abdominal cavity and the inner
being mucous-secreting epithelium. The bladder’s
middle layer is smooth MUSCLEcalled the detrusor
muscle that itself has three layers, the fibers of
each running differently. The outer muscle fibers
run longitudinally (lengthwise), the middle mus-
cle fibers form patterns of circles that ultimately
culminate in the sphincter muscle that encloses
the bladder’s neck, and the inner muscle fibers
run laterally (crosswise). Together these muscle
layers allow the bladder to expand to accommo-
date the urine draining from the kidneys and also
to contract, in coordination with relaxation of the
urethral sphincter, to expel urine from the bladder
through the urethra.
The ureters drain urine from the kidneys into
the bladder; the urethra drains urine from the
bladder to the outside of the body. One URETER, a
narrow tubelike structure, drops from each kidney
and enters the back wall of the bladder near its
midline. Urine drips continuously from the ureters
into the bladder. The urethra, a somewhat muscu-
lar tube, carries urine from the bladder to outside
the body. When empty the bladder is about the
size of a large lemon; when filled to its capacity of
about 500 milliliters (32 to 34 ounces) the bladder
can reach the size of a small cantaloupe. As the
bladder expands it extends upward into the
abdominal cavity.
URINATION, the process of expelling urine from
the bladder (also called micturition), is an involun-
tary function that becomes an action of learned
control. NEURONsensors in the muscle fibers of the
bladder wall send NERVEsignals to the sacral portion
of the SPINAL CORD. This activates the micturition
REFLEX, which sends nerve signals via the spinal
cord to micturition centers in the BRAIN. These cen-
ters activate nerve impulses that cause the urethral
sphincter to relax and the detrusor muscle to begin
a series of wavelike contractions. These involuntary
actions create the urge to urinate, experienced as a
sensation of pressure.
Learning to control the pubococcygeal muscle,
which forms the floor of the pelvis, serves to over-
ride the micturition reflex for a period of time.
Relaxing the pubococcygeal muscle and contract-
ing the abdominal muscles synchronize with the
involuntary responses of the micturition reflex,
and urination occurs. Most children acquire the
developmental ability, a blend of conscious effort
and neuromuscular maturity, to learn to control
urination (commonly called bladder control)
between the ages of three and five. With advanced
age this control may diminish, a consequence of a
weakened urethral sphincter, neurologic condi-
tions, and other factors.
For further discussion of the bladder within the
context of the urinary system’s structure and178