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U N I T 7
Kidney and Urinary Tract Function
Mechanisms of Renal Damage
The destructive effects of urinary obstruction on kidney
structures are determined by the degree (i.e., partial ver-
sus complete, unilateral versus bilateral) and the dura-
tion of the obstruction. The two most damaging effects
of urinary obstruction are stasis of urine, which predis-
poses to infection and stone formation, and progressive
dilation of the renal collecting ducts and renal tubular
structures, which causes destruction and atrophy of
renal tissue.
A common complication of urinary tract obstruction
is infection. Stagnation of urine predisposes to infection,
which may spread throughout the urinary tract. When
present, urinary calculi serve as foreign bodies and con-
tribute to the infection. Once established, the infection
is difficult to treat. It often is caused by urea-splitting
organisms (e.g.,
Proteus,
staphylococci) that increase
ammonia production and cause the urine to become
alkaline.
27
Calcium salts precipitate more readily in stag-
nant alkaline urine; thus, urinary tract obstructions also
predispose to stone formation.
In situations of severe partial or complete obstruc-
tion, the impediment to the outflow of urine causes
dilation of the renal pelvis and calyces associated with
progressive atrophy of the kidney.
4,5
Even with com-
plete obstruction, glomerular filtration continues for
some time. Because of the continued filtration, the caly-
ces and pelvis of the affected kidney become dilated,
often markedly so. The high pressure in the renal pel-
vis is transmitted back through the collecting ducts of
the kidney, compressing renal vasculature and causing
renal atrophy. Initially, the functional alterations are
largely tubular, manifested primarily by impaired urine-
concentrating ability. Only later does the GFR begin to
diminish.
Hydronephrosis
Hydronephrosis
refers to urine-filled dilation of the renal
pelvis and calyces associated with progressive atrophy
of the kidney due to obstruction of urine outflow.
4,5,27
The degree of hydronephrosis depends on the duration,
degree, and level of obstruction. In far-advanced cases,
the kidney may be transformed into a thin-walled cystic
structure with parenchymal atrophy, total obliteration
of the pyramids, and thinning of the cortex (Fig. 25-14).
The condition is usually unilateral; bilateral hydrone-
phrosis occurs only when the obstruction is below the
level of the ureterovesical junction. When the obstruc-
tion affects the outflow of urine from the distal ureter,
the increased pressure dilates the ureter, a condition
called
hydroureter
. Bilateral hydroureter may develop
as a complication of bladder outflow obstruction due to
prostatic hyperplasia (see Chapter 39).
Clinical Features
The manifestations of urinary obstruction depend on
the site of obstruction, the cause, and the rapidity with
which the condition developed. Most of the early symp-
toms are produced by the underlying pathologic pro-
cess. Urinary tract obstruction encourages the growth
of microorganisms and should be suspected in persons
with recurrent urinary tract infections.
Complete or partial unilateral hydronephrosis may
remain silent for long periods because the unaffected
kidney can maintain adequate function. Obstruction
may provoke pain due to distention of the collecting
system and renal capsule. Whereas, acute supravesical
obstruction, such as that due to a kidney stone lodged
in the ureter, is associated with excruciating pain. More
insidious causes of obstruction, such as narrowing of
the ureteropelvic junction, may produce little pain but
cause total destruction of the kidney.
Complete bilateral obstruction results in oliguria and
anuria and renal failure. Acute bilateral obstruction may
mimic prerenal failure. With partial bilateral obstruction,
the earliest manifestation is an inability to concentrate
urine, reflected by polyuria and nocturia. Hypertension
is an occasional complication of urinary tract obstruc-
tion. It is more common in cases of unilateral obstruction
in which renin secretion is enhanced, probably second-
ary to impaired renal blood flow. In these circumstances,
removal of the obstruction often leads to a reduction in
blood pressure. When hypertension accompanies bilat-
eral obstruction, it is volume related. The relief of bilat-
eral obstruction leads to a loss of volume and a decrease
in blood pressure. In some cases, relieving the obstruc-
tion does not correct the hypertension.
Early diagnosis of urinary tract obstruction is impor-
tant because the condition usually is treatable and a
delay in therapy may result in permanent damage to the
kidneys. Diagnostic methods vary with the symptoms.
Ultrasonography has proved to be the single most use-
ful noninvasive diagnostic modality for urinary obstruc-
tion. Radiologic methods, CT scans, and intravenous
urography may also be used. Other diagnostic methods,
such as urinalysis, are used to determine the extent of
renal involvement and the presence of infection.
Treatment of urinary tract obstruction depends on
the cause. Urinary stone removal may be necessary, or
FIGURE 25-14.
Hydronephrosis. Bilateral urinary tract
obstruction has led to conspicuous dilation of the ureters,
pelves, and calyces. The kidney on the right shows severe
cortical atrophy. (From Jennette JC. The kidney. In: Rubin
R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic
Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters
Kluwer Health | Lippincott Williams &Wilkins; 2012:801.)
