Glomerular Filtration Moves
a Large Amount of Fluid
into the Glomerular Capsule
Glomerular filtration occurs in the renal corpuscle and in-
volves a movement of fluid from the glomerulus into the
glomerular capsule. The endothelial cells of the glomer-
ular capillaries, the basal lamina, and the
podocytes
(the
cells of the inner layer of the glomerular capsule) make
up a three-layered filtration membrane. The filtration
membrane allows the passage of blood plasma and small
proteins but prevents the passage of blood cells and me-
dium- or large-sized proteins.
The pressure that causes filtration is the blood pres-
sure in the glomerular capillaries. Two other pressures op-
pose glomerular filtration: Blood colloid osmotic pressure
and glomerular capsule pressure. Normally, bood pressure
is greater than the two opposing forces, producing a net
filtration pressure (NFP) of about 10 mm Hg (Figure
15.5). NFP drives fluid flow through the nephron, about
150 liters daily in adult females and 180 liters daily in
adult males.
The afferent arteriole is larger in diameter than the
efferent arteriole, so blood is flowing from a larger di-
ameter to a smaller diameter. This helps to raise blood
G l o m e r u l a r f i l t r a t i o n
F i g u r e 1 5 . 5
Pressure of the blood in the glomerulus
(glomerular blood hydrostatic
pressure, GBHP, ~55 mm Hg) pushes
fluid through the filtration membrane into
the glomerular capsule.
Pressure of the blood in the glomerular
capsule
(capsular hydrostatic
pressure, CHP, ~15 mm Hg) pushes
fluid through the filtration membrane into
the glomerular capillaries.
Afferent arteriole
Efferent arteriole
The blood, which is now more concentrated,
exerts an osmotic pressure (blood colloidal
osmotic pressure, BCOP, ~30 mm Hg) that
pulls fluid back from the glomerular capsule into
the glomerular capillaries.
Proximal convoluted tubule
Glomerular
(Bowman's)
capsule
Capsu|ar
space
Net filtration pressure (NFP) involves all of these
pressures:
NFP =
GBHP - CHP - BCOP
= 55 mm Hg - 15 mm Hg - 30 mm Hg
=
10 Hg
NFP drives fluid flow from glomerular capillaries
into glomerular capsule and through the renal
tubule.
446 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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