960
UNIT 4
Maintenance of the Body
25
Te
visceral layer
, which clings to the glomerular capillaries,
consists of highly modified, branching epithelial cells called
podocytes
(pod
9
o-sīts; “foot cells”) (see Figure 25.10a and
b). Te octopus-like podocytes terminate in
foot processes
,
which interdigitate as they cling to the basement membrane
of the glomerulus. Te cle±s or openings between the foot
processes are called
filtration slits
. Trough these slits, fil-
trate enters the
capsular space
inside the glomerular capsule.
We describe the
filtration membrane
, the filter that lies between
the blood in the glomerulus and the filtrate in the capsular
space, on p. 965.
Renal Tubule and Collecting Duct
Te
renal tubule
is about 3 cm (1.2 inches) long and has three ma-
jor parts. It leaves the glomerular capsule as the elaborately coiled
proximal convoluted tubule
, drops into a hairpin loop called the
nephron loop
, and then winds and twists again as the
distal con-
voluted tubule
before emptying into a collecting duct. Te terms
proximal
and
distal
indicate the relationship of the convoluted
tubules to the renal corpuscle—filtrate from the renal corpuscle
passes through the proximal convoluted tubule first and then the
distal convoluted tubule, which is thus “further away” from the
renal corpuscle. Te meandering nature of the renal tubule in-
creases its length and enhances its filtrate processing capabilities.
Troughout their length, the renal tubule and collecting duct
consist of a single layer of polar epithelial cells on a basement
membrane. However, each region has a unique histology that
reflects its role in processing filtrate.
Proximal Convoluted Tubule (PCT)
Te walls of the
proximal
convoluted tubule
are formed by cuboidal epithelial cells with
large mitochondria, and their apical (luminal) surfaces bear
dense microvilli (Figure 25.5 and
Figure 25.6
). Just as in the
intestine, this
brush border
dramatically increases the surface
area and capacity for reabsorbing water and solutes from the
filtrate and secreting substances into it.
Nephron Loop
Te U-shaped
nephron loop
(formerly called
the
loop of Henle
) has
descending
and
ascending limbs
. Te
proximal part of the descending limb is continuous with the
proximal tubule and its cells are similar. Te rest of the descend-
ing limb, called the
descending thin limb
, consists of a simple
squamous epithelium. Te epithelium becomes cuboidal or
even low columnar in the ascending part of the nephron loop,
which is therefore called the
thick ascending limb
. In most neph-
rons, the entire ascending limb is thick but in some nephrons,
the thin segment extends around the bend as the
ascending thin
limb
. Te thick and thin parts of the nephron loop are also re-
ferred to as thick and thin segments.
Distal Convoluted Tubule (DCT)
Te epithelial cells of the
dis-
tal
convoluted tubule
, like those of the PC², are cuboidal and
confined to the cortex, but they are thinner and almost entirely
lack microvilli (Figure 25.5).
Collecting Duct
Each
collecting duct
contains two cell types.
Te more numerous
principal cells
have sparse, short microvilli
and are responsible for maintaining the body’s water and Na
1
balance. Te
intercalated cells
are cuboidal cells with abundant
microvilli. Tere are two varieties of intercalated cells (types A
and B), and each plays a role in maintaining the acid-base bal-
ance of the blood.
Each collecting duct receives filtrate from many nephrons.
Te collecting ducts run through the medullary pyramids, giv-
ing them their striped appearance. As the collecting ducts ap-
proach the renal pelvis, they fuse together and deliver urine into
the minor calyces via papillae of the pyramids.
Classes of Nephrons
Nephrons are generally divided into two major groups, cortical
and juxtamedullary
(Figure 25.7a)
.
Cortical nephrons
account for 85% of the nephrons in the kid-
neys. Except for small parts of their nephron loops that dip into
the outer medulla, they are located entirely in the cortex.
(a)
Photomicrograph of renal cortical tissue (180
m
)
(b)
Scanning electron micrograph
of renal tubules (250
m
)
Renal corpuscle
Distal convoluted
tubules
(clear lumens)
Proximal convoluted
tubules
(fuzzy lumens
due to long microvilli)
Glomerulus
Glomerular capsular
space
Squamous epithelium
of parietal layer of
glomerular capsule
Figure 25.6
Renal cortical tissue.
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