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CHAPTER TWENTY
Urinary System
Net Outward Pressure
Outward force, glomerular hydrostatic pressure
Inward force of plasma colloid osmotic pressure
Inward force of capsular hydrostatic pressure
Net filtration pressure
=
+60 mm
=
–32 mm
=
–18 mm
=
+10 mm
Plasma colloid
osmotic pressure
Hydrostatic
pressure of
blood
Glomerular hydrostatic
pressure
Capsular
hydrostatic
pressure
The hydrostatic pressure in the glomerular capsule is
another factor that may affect net F ltration pressure and rate.
This capsular pressure can change as a result of an obstruc-
tion, such as a stone in a ureter or an enlarged prostate gland
pressing on the urethra. If this occurs, fl uids back up into the
renal tubules and raise the hydrostatic pressure in the glomer-
ular capsules. Any increase in capsular pressure opposes glom-
erular F ltration, so F ltration rate may signiF cantly decrease.
At rest, the kidneys receive approximately 25% of the
cardiac output, and about 20% of the blood plasma is F
l-
tered as it flows through the glomerular capillaries. This
means that in an average adult, the glomerular filtration
rate for the nephrons of both kidneys is about 125 milliliters
per minute, or 180,000 milliliters (180 liters) in twenty-four
hours. Assuming that the blood plasma volume is about 3
liters, the production of 180 liters of F
ltrate in twenty-four
hours means that all of the plasma must be F ltered through
the glomeruli about sixty times each day
(f g. 20.19)
. This
twenty-four-hour volume is nearly 45 gallons, so it is obvi-
ous that not all of it is excreted as urine. Instead, most of the
uid that passes through the renal tubules is reabsorbed and
reenters the plasma.
The colloid osmotic pressure of the glomerular plasma
also infl uences net F
ltration pressure and the rate of F
ltration.
In other systemic capillaries, F ltration occurs at the beginning
of the capillary, but the osmotic effect of the plasma proteins
predominates at the end of the capillary, and most F ltered
fl uid is thus reabsorbed. The small excess remaining eventu-
ally becomes lymph.
Much more fl uid is F ltered by glomerular capillaries than
by capillaries elsewhere because of the relatively high hydro-
static pressure in the glomerular capillaries. In fact, as F l-
tration occurs through the capillary wall, proteins remaining
in the plasma raise the colloid osmotic pressure within the
glomerular capillaries. Despite this, the glomerular capillary
hydrostatic pressure is sufF
ciently great that the net F
ltration
pressure is normally positive. That is, the forces favoring F
l-
tration in the glomerular capillaries always predominate. Of
course, conditions that lower plasma colloid osmotic pres-
sure, such as a decrease in plasma protein concentration,
would increase F
ltration rate.
If arterial blood pressure drops drastically, such as during
shock,
the
glomerular hydrostatic pressure may fall below the level required for
F
ltration, leading to acute renal failure. At the same time, the epithe-
lial cells of the renal tubules may not receive su±
cient nutrients to
maintain their high rates of metabolism. Cells may die (tubular necro-
sis), and renal functions may be lost permanently, resulting in chronic
renal failure.
FIGURE 20.18
Normally the glomerular net F
ltration pressure
is positive, causing F
ltration. The responsible forces include the
hydrostatic and osmotic pressure of the plasma and the hydrostatic
pressure of the ²
uid in the glomerular capsule.
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Liters
140
150
160
170
180
180 Liters
Glomerular filtrate
(a)
(b)
Urine
0.6 – 2.5 Liters
FIGURE 20.19
Relative volumes of (
a
) glomerular F
ltrate and (
b
)
urine formed in twenty-four hours.
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