222
UNIT 3
Organ Systems
from collecting ducts into the interstitium. Some physiology
textbooks do not include the thick descending limb (proximal
straight tubule) as a part of the loop of Henle proper because
it does not
signiF cantly
contribute to its physiological function.
The proximal straight tubules are similar in structure to the
proximal convoluted tubules. The descending and ascending
thin segment tubules are lined by squamous cells and are struc-
turally similar to each other. The
descending limb
is perme-
able to water, Cl
-
, and Na
+
. The tubules of the descending limb
reabsorb water and salts and reduce the volume of the F ltrate
that has passed through the proximal convoluted tubules. The
ascending limb
is very active physiologically. It is impermeable
to water, and it actively pumps Cl
-
and Na
+
from the lumen into
the medullary interstitium.
Distal Convoluted Tubules
are lined by small, simple
cuboidal epithelial cells, which have no brush border. They
may show a few short, irregular microvilli on their apical sur-
faces and plasma membrane infoldings on their basal region
at the EM level (±ig. 12-10A,B). Their lumens appear clearer
and wider than those of proximal tubules. The distal convo-
luted tubules are located in the cortex of the kidney and are
closely associated with the renal corpuscles. At the junction
between the distal straight and the convoluted tubules, there is
an important specialized sensory structure, the
macula densa
,
which senses and monitors ionic content and water volume
of the F
ltrate. The macula densa is composed of cells that are
taller and more tightly packed than other cells of the distal
tubule (see ±ig. 12-5A,B). This portion of the distal tubule
is positioned between afferent and efferent arterioles at the
vascular pole of the renal corpuscle. The distal convoluted
tubules remove Na
+
and add K
+
to the F
ltrate if aldosterone
stimulation is present; they also reabsorb bicarbonate ions and
secrete ammonium to adjust the pH balance. The distal con-
voluted tubules connect distal straight tubules (thick ascend-
ing limb of the loop of Henle) to the collecting tubules. The
distal convoluted and straight tubules are structurally similar
to each other, differing mainly in their locations and courses.
THE COLLECTING SYSTEM
consists of
cortical collecting
tubules
,
collecting ducts
, and
papillary ducts
. The
collecting
tubules
are small and lined by cuboidal cells. They are located
in the renal cortex, so they are also called
cortical collecting
tubules
. They drain the F
ltrate from distal convoluted tubules
into the
collecting ducts
of the medullary rays, which, in turn,
drain into larger collecting ducts in the medulla.
Collecting
ducts
have larger lumens than collecting tubules, and they
are lined by taller cuboidal or columnar cells. Both collecting
tubules and ducts have clear cytoplasm and distinct cell-to-cell
boundaries. These tubules become highly permeable to water
under the infl
uence of antidiuretic hormone (ADH). Depending
on ADH levels, the tubules passively diffuse a variable volume
of water from their lumens into the medullary interstitium, thus
increasing the concentration of urine. The collecting ducts are
the last components of the kidney that process and determine
the F
nal urine composition.
Papillary ducts
, also called
ducts
of Bellini
, are continuations of the collecting ducts. They are
located in the papilla of the renal medulla. Several collecting
ducts merge into a single papillary duct, which empties urine
into the minor calyx at the tip of the renal papilla.
THE VASCULAR SUPPLY TO THE KIDNEY
comes from the
renal artery, which enters the kidney at the hilum; segmental
branches of the renal artery give rise to the
interlobar arteries
.
These pass through the renal columns between the renal pyra-
mids and give rise to
arcuate arteries
. The arcuate arteries run
along the junction between the cortex and the medulla of the
kidney and give rise to the
interlobular arteries
, which extend
into the medulla to supply the
afferent arterioles
of renal cor-
puscles. Each afferent arteriole supplies a glomerulus of capil-
laries from which blood is drained by an
efferent arteriole
at the
vascular pole
. The efferent arterioles of corpuscles in the outer
cortex feed into the
peritubular capillary network
, which sup-
plies the cortical tissue surrounding the cortical tubules. These
peritubular capillaries provide for gas and material exchange
and also receive renal interstitial fl
uid, which is reabsorbed out
of the tubules and goes back into the vascular bed. Venules
carry blood to the
interlobular veins
and to the
arcuate veins
in the renal corticomedullary junction. The efferent arterioles
of deeper (juxtamedullary) corpuscles extend into the medulla
where they give rise to capillaries called
vasa recta
, which
receive interstitial fl
uid (reabsorbed from
F ltrate) in the medulla
and send it back to the circulation. The vasa rectae take a hair-
pin course in the medulla following the loop of Henle. They
return to the corticomedullary junction to join the interlobular
veins and then drain into the arcuate veins. The arcuate veins
drain blood into the interlobar veins, which then merge to form
the branches of the segmental renal veins, which in turn F
nally
merge into the renal vein (see ±ig. 11-2).
Ureters
The two
ureters
lie in the extraperitoneal connective tissue,
laterally positioned on each side of the vertebral column. The
ureters are long, relatively small tubules lined by
transitional
epithelium
and surrounded by a thin layer of smooth muscle
and connective tissue. Superiorly, they drain the funnel-shaped
renal pelvis
, and inferiorly, they empty into the bladder by pen-
etrating its posterior wall. The ureters have a much thinner wall
than the bladder. Like most tubular organs, the wall of the ure-
ter is composed of several layers of tissues:
mucosa
,
muscularis
,
and
adventitia
(±ig. 12-13A,B).
Urinary Bladder
The
urinary bladder
, a distensible sac-shaped organ located in
the pelvic cavity, temporarily stores urine. The wall of the blad-
der has three openings, two of them for ureters to enter and
one for emptying urine into the urethra. Like the ureter, the uri-
nary bladder wall consists of
mucosal
,
muscularis
, and
adven-
titial
layers, but the bladder wall is much thicker, having three
substantial layers of smooth muscle in the muscularis. (1) The
mucosa
consists of a transitional epithelium lining and a layer
of connective tissue (lamina propria) containing blood vessels
and nerve F
bers. (2) The
muscularis
contains the three layers
of smooth muscle: inner longitudinal smooth muscle, middle
circular smooth muscle, and outer longitudinal smooth mus-
cle. The muscularis contracts in different directions to enable
the urinary bladder to empty urine. (3) The outer portion of
the bladder is protected by both a
serosa
and an
adventitia
depending on whether it projects into the peritoneal cavity. The
superior surface of the bladder is covered by
serosa
, which is a
layer of connective tissue covered by mesothelium; the inferior
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