The Urinary System
Pressures That Affect Filtration
Te principles that govern ﬁltration from the glomerulus are the
same as those that govern ﬁltration from any capillary bed.
on Bulk Flow Across Capillary Walls
(Figure 19.17 on p. 718)
shows ﬁltration in normal capillary beds.
these principles to the glomerular capillaries of the nephron.
Outward pressures promote ﬁltrate
in glomerular capillaries (HP
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.
colloid osmotic pressure in the capsular
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
Urine Formation, Step 1:
Describe the forces (pressures) that promote or counteract
Compare the intrinsic and extrinsic controls of the
glomerular ﬁltration rate.
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
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
shows, its three layers are:
Fenestrated endothelium of the glomerular capillaries.
Te fenestrations (capillary pores) allow all blood compo-
nents except blood cells to pass through.
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.
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,
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
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
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.
= 55 mm Hg
= 30 mm Hg
= 15 mm Hg
NFP = Net filtration pressure
= outward pressures – inward pressures
) – (HP
= (55) – (15 + 30)
= 10 mm Hg
Forces determining net ﬁltration pressure
The pressure values cited in the diagram are approximate.
hydrostatic pressure of glomerular capillaries, OP
pressure of glomerular capillaries, HP
hydrostatic pressure of