Chapter 25
The Urinary System
965
25
Pressures That Affect Filtration
Te principles that govern filtration from the glomerulus are the
same as those that govern filtration from any capillary bed.
Focus
on Bulk Flow Across Capillary Walls
(Figure 19.17 on p. 718)
shows filtration in normal capillary beds.
Figure 25.11
applies
these principles to the glomerular capillaries of the nephron.
Outward Pressures
Outward pressures promote filtrate
formation.
Te
hydrostatic pressure
in glomerular capillaries (HP
gc
)
is essentially glomerular blood pressure. It is the chief force
pushing water and solutes out of the blood and across the
filtration membrane. Te blood pressure in the glomerulus is
extraordinarily high (approximately 55 mm Hg compared to
an average of 26 mm Hg or so in other capillary beds) and it
remains high across the entire capillary bed. Tis is because
the glomerular capillaries are drained by a high-resistance
efferent arteriole whose diameter is smaller than the afferent
arteriole that feeds them. As a result, filtration occurs along
the entire length of each glomerular capillary and reabsorp-
tion does not occur as it would in other capillary beds.
Teoretically, the
colloid osmotic pressure in the capsular
space
of the glomerular capsule would “pull” filtrate into the
tubule. However, this pressure is essentially zero because vir-
tually no proteins enter the capsule, so we will not consider it
further.
Urine Formation, Step 1:
Glomerular Filtration
Describe the forces (pressures) that promote or counteract
glomerular filtration.
Compare the intrinsic and extrinsic controls of the
glomerular filtration rate.
Glomerular
filtration
is a passive process in which hydrostatic
pressure forces fluids and solutes through a membrane. Te
glomeruli can be viewed as simple mechanical filters because
filtrate formation does not directly consume metabolic energy.
Let’s first look at the structure of the filtration membrane and
then see how it works.
The Filtration Membrane
Te
filtration membrane
lies between the blood and the in-
terior of the glomerular capsule. It is a porous membrane that
allows free passage of water and solutes smaller than plasma
proteins. As
Figure 25.10c
shows, its three layers are:
1.
Fenestrated endothelium of the glomerular capillaries.
Te fenestrations (capillary pores) allow all blood compo-
nents except blood cells to pass through.
2.
Basement membrane.
Te basement membrane lies be-
tween the other two layers and is composed of their fused
basal laminae. It forms a physical barrier that blocks all
but the smallest proteins while still permitting most other
solutes to pass. Te glycoproteins of the gel-like base-
ment membrane give it a negative charge. As a result, the
basement membrane electrically repels many negatively
charged macromolecular anions such as plasma proteins,
reinforcing the blockade based on molecular size.
3.
Foot processes of podocytes of the glomerular capsule.
Te visceral layer of the glomerular capsule is made of po-
docytes that have filtration slits between their foot pro-
cesses. If any macromolecules manage to make it through
the basement membrane,
slit diaphragms
—thin mem-
branes that extend across the filtration slits—prevent al-
most all of them from traveling farther.
Macromolecules that get “hung up” in the filtration mem-
brane are engulfed by specialized pericytes called
glomerular
mesangial cells
(Figure 25.8).
Molecules smaller than 3 nm in diameter—such as water,
glucose, amino acids, and nitrogenous wastes—pass freely from
the blood into the glomerular capsule. As a result, these sub-
stances usually have similar concentrations in the blood and the
glomerular filtrate. Larger molecules pass with greater difficulty,
and those larger than 5 nm are generally barred from entering
the tubule. Keeping the plasma proteins
in
the capillaries main-
tains the colloid osmotic (oncotic) pressure of the glomerular
blood, preventing the loss of all its water to the capsular space.
Te presence of proteins or blood cells in the urine usually indi-
cates a problem with the filtration membrane.
HP
gc
= 55 mm Hg
OP
gc
= 30 mm Hg
HP
cs
= 15 mm Hg
NFP = Net filtration pressure
= outward pressures – inward pressures
= (HP
gc
) – (HP
cs
+ OP
gc
)
= (55) – (15 + 30)
= 10 mm Hg
Afferent
arteriole
Glomerular
capsule
Efferent
arteriole
Figure 25.11
Forces determining net filtration pressure
(NFP).
The pressure values cited in the diagram are approximate.
HP
gc
5
hydrostatic pressure of glomerular capillaries, OP
gc
5
osmotic
pressure of glomerular capillaries, HP
cs
5
hydrostatic pressure of
capsular space.
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