962
UNIT 4
Maintenance of the Body
25
arterioles, they only experience low pressure. As a result, these
low-pressure, porous capillaries readily absorb solutes and
water from the tubule cells as these substances are reclaimed
from the filtrate. Renal tubules are closely packed together, so
the peritubular capillaries of each nephron absorb substances
from several adjacent nephrons.
Vasa Recta
Notice in Figure 25.7a that the efferent arterioles
serving the juxtamedullary nephrons tend
not
to break up into
meandering peritubular capillaries. Instead they form bun-
dles of long straight vessels called
vasa recta
(va
9
sah rek
9
tah;
“straight vessels”) that extend deep into the medulla paralleling
the longest nephron loops. Te thin-walled vasa recta play an
important role in forming concentrated urine, as we will de-
scribe shortly.
In summary, the nephrons have two functionally different
capillary beds separated by efferent arterioles. Te first capil-
lary bed (the glomerulus) produces the filtrate. Te second (a
combination of peritubular capillaries and vasa recta) reclaims
most of that filtrate.
Juxtaglomerular Complex (JGC)
Each nephron has a
juxtaglomerular complex (JGC)
(juks
0
tah-glo-mer
9
u-lar), a region where the most distal portion of
the ascending limb of the nephron loop lies against the afferent
arteriole feeding the glomerulus (and sometimes the efferent
arteriole)
(Figure 25.8)
. Both the ascending limb and the affer-
ent arteriole are modified at the point of contact.
Juxtamedullary nephrons
(juks
0
tah-mĕ
9
dul-ah-re) origi-
nate close to (
juxta
5
near to) the cortex-medulla junction,
and they play an important role in the kidneys’ ability to pro-
duce concentrated urine. Tey have long nephron loops that
deeply invade the medulla, and their ascending limbs have
both thin and thick segments.
Nephron Capillary Beds
Te renal tubule of every nephron is closely associated with two
capillary beds: the
glomerulus
and the
peritubular capillaries
(Figure 25.7). In addition, juxtamedullary nephrons are associ-
ated with special capillaries called the
vasa recta
.
Glomerulus
Te glomerulus, in which the capillaries run in
parallel, is specialized for filtration. It differs from all other
capillary beds in the body in that it is both fed and drained
by arterioles—the
afferent
arteriole
and
efferent
arteriole
, re-
spectively. Tis arrangement maintains the high pressure in the
glomerulus that is needed for filtration, a process we discuss on
p. 965. Filtration produces a large amount of fluid, most (99%)
of which is reabsorbed by the renal tubule cells and returned to
the blood in the peritubular capillary beds.
Te afferent arterioles arise from the
cortical radiate arteries
that run through the renal cortex. Te efferent arterioles feed
into either the peritubular capillaries or the vasa recta.
Peritubular Capillaries
Te
peritubular capillaries
cling
closely to adjacent renal tubules and empty into nearby
venules. Because they arise from the high-resistance efferent
Glomerulus
Glomerular capsule
Afferent
arteriole
Efferent
arteriole
Red blood cell
Podocyte cell body
(visceral layer)
Foot processes
of podocytes
Parietal layer
of glomerular
capsule
Proximal
tubule cell
Lumens of
glomerular
capillaries
Endothelial cell
of glomerular
capillary
Efferent arteriole
• Macula densa cells
of the ascending limb
of nephron loop
• Granular cells
• Extraglomerular
mesangial cells
Afferent arteriole
Capsular
space
Renal corpuscle
J
uxtaglomerular complex
Glomerular mesangial
cells
J
uxtaglomerular
complex
Figure 25.8
Juxtaglomerular complex (JGC) of a nephron.
Mesangial cells that surround
the glomerular capillaries (glomerular mesangial cells) are not part of the JGC.
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