Download The Urinary Ureters, Bladder and Urethra

The Urinary Ureters, Bladder and Urethra
The Ureters
The ureters are a pair of thin, muscular tubes that transport urine from
the kidneys to the bladder. Beginning at the level of the second lumbar
vertebra, the location of the ureters is retroperitoneal. Each ureter runs
inferiorly and enters the posterolateral wall of the urinary bladder. This
angle of entry is important, because it helps prevent urine from flowing
back into the ureters when the bladder fills with urine. In addition,
accumulating urine increases the internal pressure of the bladder, and
this pressure compresses and seals the distal portion of the ureters.
There are three layers in the ureter wall. The innermost mucosa lining
contains transitional epithelium capable of stretching but is
impermeable to urine. The ability to stretch allows the ureter wall to
accommodate changing volumes of urine. The middle muscularis layer
is composed of two layers of smooth muscle: an inner longitudinal layer
and outer circular layer. In the lower third of the ureter, the muscularis
has a third outer layer of longitudinal muscle fibers. The muscularis
layer is responsible for the peristaltic contractions needed to move urine
through the ureters and into the bladder. The external layer of the ureter
wall, the adventitia, is made of fibrous connective tissue and helps
anchor the ureter to the abdominal wall.
When urine enters and distends the ureters, stretch receptors are
stimulated. Reflexive action results in the contraction of the muscularis
and, movement of the urine into the bladder. The power and frequency
of peristalsis is directly related to the rate of urine formation. Although
the ureters are innervated by both sympathetic and parasympathetic
fibers, the nervous system does not appear to have major involvement in
the transport of urine in these organs.
The Urinary Bladder
The urinary bladder is a hollow, collapsible muscular sac that serves
as a temporary storage facility for urine. It is located in the pelvic cavity,
just posterior to the pubic symphysis. In females, the bladder lies
anterior to the vagina and inferior to the uterus. In males, it is
immediately anterior to the rectum.Peritoneal folds hold the bladder
in place.
The bladder can hold up to about a liter of urine, although this amount
varies from person to person. Despite its capacity to enlarge, an overfull
bladder can burst but it is more likely that excess urine will leak out of
the urethra. When empty, the bladder collapses into a pyramidal shape.
When a small amount of urine accumulates, it is spherical. When a
larger volume of urine accumulates, the bladder becomes pear-shaped
and ascends in the abdominal cavity. There are three openings in the
bladder: two for the ureters and one for the urethra. These openings
frame a triangular region at the base of the bladder called the trigone.
The bladder wall is made up of a mucosa with transitional epithelium, a
submucosa, a thick muscularis called the detrusor muscle, and a
fibrous adventitia. The adventitia is on the inferior surface only. In
contrast, the peritoneum covers the superior surface. The detrusor
muscle is composed of inner and outer layers of longitudinal smooth
muscle fibers and an intermediate layer of circular muscle fibers.
The Urethra
The urethra is a small muscular tube that transports urine from the
bladder out of the body. The urethra is five times longer in males (8
inches, 20 cm) than in females (1.6 inches, 4 cm). In males, the urethra
is also part of the reproductive system, providing a passageway for
semen as well as urine. The course of the urethra also differs between
the sexes. In females, fibrous connective tissue binds it to the anterior
vaginal wall, and its external urethral orifice (external opening) is
located anterior to the vaginal opening and posterior to the clitoris. In
males, the urethra is divided into three regions. The prostate surrounds
the prostatic urethra. The membranous urethra passes through the
urogenital diaphragm. The spongy urethraruns through the penis
and ends at the external urethral orifice.
At the bladder-urethra junction, the circular fibers of the bladder's
detrusor muscle form the internal urethral sphincter. When urine
is not draining from the bladder, this involuntary sphincter closes off
the urethra to prevent the leakage of urine when you are not voiding. At
the point where the urethra passes through the urogenital diaphragm,
the external urethral sphincter, a skeletal muscle, surrounds it. The
external sphincter is voluntarily controlled and is kept contracted until
voiding.
EXAMPLE
Urinary Tract Infections
Urinary tract infections (UTIs) are the most common type of bacterial infection.
Women are predisposed to UTIs because their urethras are shorter than those of
men. Moreover, the urethra's external opening in women is closer to the anus
than it is in men. Over 50 percent of women will have a UTI at some point during
their lifetime. Fecal bacteria such as Escherichia coli (E. coli) can easily travel up
the urethra. This is why women should never wipe the anus in a forward direction
after defecation. However, most UTIs in women occur as a result of sexual
activity. During intercourse, bacteria from the external genital area and the
vagina can be pushed up the short urethra toward the bladder. The use of
spermicides actually increases the risk of UTIs, because spermicides kill bacteria
that would otherwise help destroy infectious fecal bacteria in the vagina. Drinking
plenty of water and urinating immediately after sexual activity can help prevent
UTIs by flushing bacteria out of the urethra. Infection of the urethra (urethritis) can
easily spread to the urinary bladder (cystitis) and sometimes to the kidneys
(pyelitis or pyelonephritis). Symptoms of a UTI include pain during urination
(dysuria), frequent urination or an urgent need to urinate, cloudiness or blood in
the urine (hematuria), urine with a strong odor, nausea, and fever. Fortunately,
most UTIs respond to antibiotics. Analgesics may also be prescribed to reduce
discomfort. Unfortunately, having a UTI increases the chances of having
subsequent UTIs. UTIs are also common in infants, particularly in uncircumcised
male infants.