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U N I T 7
Kidney and Urinary Tract Function
constancy of the internal environment are reabsorbed
into the bloodstream while other, unneeded materials are
secreted into the tubular filtrate for elimination.
Nephrons can be roughly grouped into two catego-
ries. Approximately 85% of the nephrons originate in
the superficial part of the cortex and are called
corti-
cal nephrons
(Fig. 24-4B). They have short, thick loops
of Henle that penetrate only a short distance into the
medulla. The remaining 15% are called
juxtamedullary
nephrons.
They originate deeper in the cortex and have
longer and thinner loops of Henle that penetrate the
entire length of the medulla. The juxtamedullary neph-
rons are largely concerned with urine concentration.
The nephrons are supplied by two capillary systems,
the glomerulus and peritubular capillary network (see
Fig. 24-4A). The
glomerulus
is a unique, high-pressure
capillary filtration system located between two arteri-
oles—the afferent and the efferent arterioles. The
peri-
tubular capillaries
originate from the efferent arteriole.
They are low-pressure vessels that are adapted for reab-
sorption rather than filtration. These capillaries sur-
round all portions of the tubules, an arrangement that
permits rapid movement of solutes and water between
the fluid in the tubular lumen and the blood in the capil-
laries. In the deepest part of the renal cortex, the efferent
arterioles continue as long, thin-walled looping vessels
called the
vasa recta.
The vasa recta accompany the long
loops of Henle in the medullary portion of the kidney
to assist into the exchange of substances flowing in and
out of that portion of the kidney. The peritubular capil-
laries rejoin to form the venous channels through which
blood leaves the kidneys and empties into the inferior
vena cava.
The Renal Corpuscle
The renal corpuscle, commonly called the
glomerulus
,
consists of a compact tuft of capillaries, with a cen-
tral region of mesangial cells and surrounding matrix,
encased in a thin double-layered capsule called
Bowman
capsule
. The inner or
visceral layer
of the capsule envel-
ops the capillaries of the glomerulus and the external or
parietal layer forms the outer wall of the capsule. Blood
flows into the glomerular capillaries through the affer-
ent arterioles and flows out through the efferent arte-
rioles, which leads to a second capillary network, the
peritubular capillaries, that surrounds the renal tubules.
Fluid and particles from the blood are filtered through
the capillary wall into a fluid-filled space between the
visceral and parietal layers of Bowman capsule, called
Bowman space.
The portion of the blood that is filtered
into the capsule space is called the
glomerular filtrate
(Fig. 24-5A).
The glomerular capillary wall consists of a thin layer
of endothelial cells, a glomerular basement membrane,
and a surrounding layer of visceral epithelial cells of
Bowman capsule (Fig. 24-5B). The endothelium of the
glomerular capillary, which interfaces with blood as it
moves through the capillary, contains many small perfo-
rations, called
fenestrations.
These fenestrations allow
for the free passage of water, and of small particles such
as sodium, potassium, and glucose, but prevent the pas-
sage of red blood cells, white blood cells, or platelets. In
addition to their role as a filtration barrier, the endothe-
lial cells synthesize a number of vasoactive substances
such as nitric oxide (a vasodilator) and endothelin-1
(a vasoconstrictor) that control renal blood flow.
FIGURE 24-5.
Renal corpuscle.
(A)
Structures of the
glomerulus.
(B)
Cross-section of the glomerular membrane
showing the position of endothelium, basement membrane,
and mesangial cells.
(C)
Structures of the glomerulus in relation
to the filtration process.
Bowman
space
Efferent
arteriole
Afferent
arteriole
A
Proximal
tubule
Bowman
space
Glomerular
capsule
Mesangial
matrix
Endothelial
cell
Basement
membrane
Epithelial
podocytes
Mesangial cell
B
Filtration slit Glomerular basement
membrane
Glomerular capillary
Podocyte
Bowman
capsule
Ultrafiltrate
Foot process
of podocyte
Endothelial
cells with
fenestrations
Blood
C
