788
UNIT FIVE
The net effect of these forces is called
net f
ltration pres-
sure,
and it is normally positive, favoring F ltration at the
glomerulus. Net F
ltration pressure is calculated as follows:
Net f
ltration pressure =
Force Favoring f
ltration – Forces opposing f
ltration
(glomerular capillary
(capsular hydrostatic
hydrostatic pressure)
pressure and glomerular
capillary osmotic pressure)
±iltration Rate
The glomerular F ltration rate (G±R) is directly proportional
to the net F ltration pressure. Consequently, the factors that
affect the glomerular hydrostatic pressure, glomerular plasma
osmotic pressure, or hydrostatic pressure in the glomerular
capsule also affect the rate of F
ltration (F
g. 20.18).
Normally, glomerular hydrostatic pressure is the most
important factor determining net filtration pressure and
G±R. Each glomerular capillary lies between two arterioles—
the afferent and efferent arterioles—so any change in the
diameters of these vessels is likely to change glomerular
hydrostatic pressure, affecting glomerular F
ltration rate.
The afferent arteriole, through which the blood enters the
glomerulus, may vasoconstrict in response to sympathetic
nerve stimulation. If this occurs, net F
ltration pressure in
that glomerulus decreases, and F
ltration rate drops. If, on
the other hand, the efferent arteriole (through which the
blood leaves the glomerulus) vasoconstricts, blood backs
up into the glomerulus, net F
ltration pressure increases,
and F
ltration rate rises. Vasodilation of these vessels pro-
duces opposite effects.
the F ltrate that becomes tissue fl uid elsewhere in the body.
Glomerular F ltrate is mostly water and the same solutes as in
blood plasma, except for the larger protein molecules. More
speciF cally, glomerular F
ltrate includes water, glucose, amino
acids, urea, uric acid, creatine, creatinine, sodium, chloride,
potassium, calcium, bicarbonate, phosphate, and sulfate ions.
Table 20.1
compares the concentrations of some of the sub-
stances in the blood plasma, glomerular F ltrate, and urine.
The concentrations of certain components of the blood plasma can
be used to evaluate kidney functions. For example, if the kidneys
are functioning inadequately, the plasma concentrations of urea (a
nitrogenous waste) indicated by a blood urea nitrogen test and crea-
tinine may increase as much as tenfold above normal.
±iltration Pressure
The main force that moves substances by F ltration through
the glomerular capillary wall is the hydrostatic pressure of the
blood inside, as in other capillaries. (Recall that glomerular
capillary pressure is high compared to other capillaries.) The
osmotic pressure of the blood plasma in the glomerulus and
the hydrostatic pressure inside the glomerular capsule also
infl uence glomerular F
ltration.
The colloid osmotic pressure of the plasma caused by
plasma proteins is always higher than that of the glomerular
F ltrate (except in some types of kidney disease). This draws
water back into the glomerular capillaries, opposing F ltra-
tion. Any increase in glomerular capsule hydrostatic pres-
sure also opposes F
ltration
(f
g. 20.18)
.
TABLE
20.1
|
Relative Concentrations of Plasma, Glomerular Filtrate, and Urine Components
Substance
Plasma
Glomerular Filtrate Concentrations (mEq/L)
Urine
Sodium (Na
+
)
142
142
128
Potassium (K
+
)5
5
6
0
Calcium (Ca
+2
)4
4
5
Magnesium (Mg
+2
)3
3
1
5
Chloride (Cl
)
103
103
134
Bicarbonate (HCO
3
)2
7
2
7
1
4
Sulfate (SO
4
–2
)1
1
3
3
Phosphate (PO
4
–3
)2
2
4
0
(mEq/L [milliequivalents per liter] is a commonly used measure of concentration based on how many charges an ion carries. For a substance with a charge of 1,
such as Cl
, a mEq is equal to a millimole.)
Substance
Plasma
Glomerular Filtrate Concentrations (mEq/L)
Urine
Glucose
100
100
0
Urea
26
26
1,820
Uric acid
4
4
53
Creatinine
1
1
196
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