784
UNIT FIVE
cells of the macula densa, they constitute the
juxtaglom-
erular apparatus
(juks
tah-glo-mer
u-lar ap
ah-ra
tus). This
structure is important in regulating the secretion of renin
(see chapter 13, p. 506)
(f
g. 20.12)
.
Cortical and Juxtamedullary Nephrons
The corpuscles of most nephrons are in the renal cortex near
the surface of the kidney. These
cortical nephrons
have short
nephron loops that usually do not extend as far as the renal
medulla.
Another group of kidney subunits, called
juxtamedullary
nephrons,
have corpuscles close to the renal medulla, and
their nephron loops extend deep into the medulla. Although
these nephrons represent only about 20% of the total, they
are important in regulating water balance
(f
g. 20.13)
.
Glomerular
capsule
Glomerulus
Afferent
arteriole
Efferent
arteriole
Juxtaglomerular
apparatus
Distal
convoluted
tubule
Proximal
convoluted
tubule
Nephron loop
Glomerulus
Afferent arteriole
Glomerular capsule
Efferent arteriole
Ascending limb
of nephron loop
Juxtaglomerular
cells
Podocyte
Macula densa
(a)
(b)
Juxtaglomerular
apparatus
FIGURE 20.12
±Juxtaglomerular±
apparatus. (
a
) Location of and (
b
)
enlargement of a section of the
juxtaglomerular apparatus, which
consists of the macula densa and the
juxtaglomerular cells.
Blood Supply of a Nephron
The cluster of capillaries that forms a glomerulus arises
from an afferent arteriole whose diameter is greater than
that of other arterioles. Blood passes through the capillaries
of the glomerulus, then (minus any F
ltered fl uid) enters an
efferent arteriole (rather than a venule), whose diameter is
smaller than that of the afferent arteriole. The greater resis-
tance to blood fl
ow of the efferent arteriole causes blood to
back up into the glomerulus. This results in a higher pres-
sure in the glomerular capillaries compared to capillaries
elsewhere.
The efferent arteriole branches into a complex network
of capillaries that surrounds the renal tubule called the
peritubular capillary
(per
ı˘-tu
bu-lar kap
ı˘-ler
e)
system.
Blood in the system has passed through two arterioles and
previous page 814 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 816 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off