798
UNIT FIVE
creatinine levels suggest that GFR is greatly reduced. Nearly
all of the creatinine the kidneys ± lter normally appears in
the urine, so a change in the rate of creatinine excretion may
refl
ect renal failure.
Another plasma clearance test uses
para-aminohippuric
acid
(PAH), which filters freely through the glomerular
membranes. However, unlike inulin, any PAH remaining in
the peritubular capillary plasma after ± ltration is secreted
into the proximal convoluted tubules. Therefore, essentially
all PAH passing through the kidneys appears in the urine.
For this reason, the rate of PAH clearance can be used to cal-
culate the rate of plasma fl
ow through the kidneys. Then, if
the hematocrit is known (see chapter 14, p. 523), the rate of
total blood fl
ow through the kidneys can also be calculated.
The kidneys of infants and young children are unable to concentrate
urine and conserve water as eF
ectively as those of adults, so they can
lose water rapidly, which may lead to dehydration. A 20-pound infant
can lose a pound in just a day of an acute viral illness, and this is a
su±
ciently signi²
cant proportion of body weight to warrant hospital-
ization. Intravenous ³
uids are given to restore water and electrolyte
balance (see chapter 21, p. 811).
PRACTICE
24
List the normal constituents of urine.
25
What is the normal hourly output of urine? The minimal hourly
output?
20.4
ELIMINATION OF URINE
After forming along the nephrons, urine passes from the
collecting ducts through openings in the renal papillae and
enters the minor and major calyces of the kidney. From there
it passes through the renal pelvis, into a ureter, and into the
urinary bladder. The urethra delivers urine to the outside.
Ureters
Each
ureter
is a tubular organ about 25 centimeters long, which
begins as the funnel-shaped renal pelvis. It extends downward
posterior to the parietal peritoneum and parallel to the verte-
bral column. In the pelvic cavity, each ureter courses forward
and medially to join the urinary bladder from underneath.
The wall of a ureter is composed of three layers. The
inner layer, or
mucous coat,
includes several thicknesses of
transitional epithelial cells and is continuous with the linings
of the renal tubules and the urinary bladder. The middle layer,
or
muscular coat,
largely consists of smooth muscle ± bers in
circular and longitudinal bundles. The outer layer, or
f
brous
coat,
is composed of connective tissue
(f g. 20.27)
.
Muscular peristaltic waves, originating in the renal pelvis,
help move the urine along the length of the ureter. The pres-
ence of urine in the renal pelvis initiates these waves, whose
Urine Composition
Urine composition refl ects the volumes of water and solutes
that the kidneys must eliminate from the body or retain in
the internal environment to maintain homeostasis. It var-
ies considerably from time to time because of differences
in dietary intake and physical activity. Urine is about 95%
water and usually also contains urea and uric acid from the
catabolism of amino acids and nucleic acids, and creatinine
from metabolism of creatine. Urine may also have a trace
of amino acids, as well as electrolytes whose concentrations
refl ect diet (see table 20.1). Appendix B (p. 943) lists the nor-
mal concentrations of urine components.
Not all abnormal constituents of urine indicate illness. Glucose may
enter urine after a sugary meal or toward the end of pregnancy; pro-
tein may appear in the urine following vigorous physical exercise;
ketones are in urine during a prolonged fast or when a person follows
a very low-calorie or low-carbohydrate diet.
The volume of urine produced usually varies between
0.6 and 2.5 liters per day. Such factors as fl uid intake; envi-
ronmental temperature; relative humidity of the surrounding
air; and a person’s emotional condition, respiratory rate, and
body temperature infl
uence the exact urine volume. An out-
put of 50–60 milliliters of urine per hour is considered nor-
mal, and an output of less than 30 milliliters per hour may
indicate kidney failure.
Renal Clearance
The rate at which a particular chemical is removed from the
plasma indicates kidney ef±
ciency. This rate is called
renal
clearance.
Tests of renal clearance detect glomerular damage or
monitor the progression of renal disease. One such test, the
inulin clearance test,
uses
inulin
(not to be confused with
insulin), a complex polysaccharide found in certain plant
roots. In the test, a known amount of inulin is infused into
the blood at a constant rate. The inulin passes freely through
the glomerular membranes, so its concentration in the glom-
erular ± ltrate equals that in the plasma. In the renal tubule,
inulin is not reabsorbed to any signi± cant degree, nor is it
secreted. Consequently, the rate at which it appears in the
urine can be used to calculate the rate of glomerular ± ltration.
Similarly, the kidneys remove creatinine from the blood.
Creatinine is produced at a constant rate during muscle
metabolism. Like inulin, creatinine is filtered but neither
reabsorbed nor secreted by the kidneys. The
creatinine clear-
ance test,
which compares a patient’s blood and urine crea-
tinine concentrations, can also be used to calculate the GFR.
A signi± cant advantage is that the bloodstream normally has
a constant level of creatinine. Therefore, a single measure-
ment of plasma creatinine levels provides a rough index of
kidney function. For example, signi± cantly elevated plasma
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