CHAPTER 12
Urinary System
231
Figure 12-8B.
The nephron and collecting system of the kidney.
The kidney is composed of
nephrons
and a
collecting system
. Each
nephron
comprises a
renal corpuscle
, a
proximal convoluted tubule
, a
loop of Henle
, and a
distal convoluted tubule
. The U-shaped
loop of Henle
connects the
proximal convoluted tubules
to the
distal convo-
luted tubules
. The loop of Henle creates a high concentration of solutes in the interstitium of the medulla, which is essential in controlling
the concentration of urine. The
collecting system
(
yellow
) includes
cortical collecting tubules
,
collecting ducts
, and
papillary ducts
. The
dashed lines
indicate the junction between the cortex and the medulla and the medullary ray region in the cortex. When the blood pressure
in the glomerular capillaries is within certain limits, water and some solutes of the blood plasma are forced through the
± ltration barrier
into the
Bowman space
and then into the
proximal convoluted tubules
. Most glucose and amino acids and a large volume of water and
salt are reabsorbed by the
proximal convoluted
and
straight tubules
before the F ltrate enters the descending thin segment of the
loop
of Henle
. The thin descending segment is highly permeable to water and less permeable to salt, so water passes from the lumen to the
interstitium of the medulla and returns back to the blood circulation via the vasa recta. The thin ascending limb is impermeable to water
but permeable to salt, and the thick ascending segment, which is also impermeable to water, actively pumps salt into the interstitium. As
a result, the concentration of solutes increases to about four times the normal amount in the interstitium of the deep medulla. This hyper-
osmotic environment drives the movement of water from the lumens of the collecting ducts, therefore increasing the concentration of the
urine. The permeability to water in the cortical collecting tubules and collecting ducts (and, therefore, the F nal concentration of urine) is
controlled by the level of
ADH
released by pituitary glands. The reabsorption of various ions by the distal convoluted tubule
is controlled
by hormones, primarily
aldosterone
. The F nal urine is collected by papillary ducts and emptied at the
area cribrosa
into the minor calyx.
Collecting
Collecting
duct
duct
Collecting
duct
Collecting
Collecting
duct
duct
Collecting
duct
Collecting
Collecting
duct
duct
Collecting
duct
Glomerulus
Glomerulus
Glomerulus
Proximal
Proximal
straight tubule
straight tubule
Proximal
straight tubule
Distal straight
Distal straight
tubule
tubule
Distal straight
tubule
Proximal
Proximal
straight
straight
tubule
tubule
Proximal
straight
tubule
Distal
Distal
straight
straight
tubule
tubule
Distal
straight
tubule
Medullary ray
Medullary ray
Medullary ray
A
D. Cui &T. Yang
Area cribrosa
Papillary duct
(duct of Bellini)
Papillary duct
(duct of Bellini)
Collecting duct
Medullary
ray
Thick descending limb
(proximal straight tubule)
Loop of
Henle
Thick ascending limb
(distal straight tubule)
Thin descending limb
(descending thin segment)
Thin ascending limb
(ascending thin segment)
Distal
convoluted tubule
Macula densa (region)
Glomerulus
Proximal
convoluted tubule
Cortical
collecting tubule
Cortex
Medulla
Renal
medulla
Renal
cortex
H O (+ ADH)
2
HO
2
Cl
,Na
+
B
Cl
,Na
+
Figure 12-8A.
Medullary ray, renal cortex.
H&E,
3
142; insets
3
448
Each
medullary ray
is composed of
proximal straight
tubules
,
distal straight tubules
, and
collecting ducts
. These
tubules run parallel to each other within a medullary ray,
which separates the glomeruli into groups. Although med-
ullary rays are located in the renal cortex region, they are
an extension of the renal medulla. The
proximal straight
tubules
are lined by cuboidal cells with acidophilic cyto-
plasm and long microvilli. The
distal straight tubules
and
collecting ducts
are lined by cuboidal cells with clear cyto-
plasm. However, the collecting ducts have a larger lumen
and more distinct cell-to-cell borders than do distal straight
tubules. The proximal straight tubules convey F
ltrate from
the proximal convoluted tubules into the thin segment
tubules. The distal straight tubules convey the F ltrate into
the distal convoluted tubules, from which it drains into the
collecting tubules and ducts (see below).
previous page 246 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online next page 248 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online Home Toggle text on/off