236
UNIT 3
Organ Systems
Structure
Epithelial Lining
of the Tubules
Characteristics of the Tubules
Main
Locations
Main Functions
Nephron
Renal corpuscle
Simple squamous
epithelium
Composed of glomerulus (blood
vessels covered by podocytes) and
Bowman capsule
Renal cortex
Filters blood and forms
urine
Proximal convoluted
tubule
Simple cuboidal
epithelium with
long microvilli
(brush border)
Long and highly convoluted
tubule; relatively small lumen and
acidophilic cytoplasm; abundant
mitochondria; numerous
basolateral plasma membrane
enfoldings
Renal cortex
Drains fl uid from the renal
corpuscle to the loop of
Henle; reabsorbs 70%–80%
Na
+
and Cl
and water; also
reabsorbs glucose, amino
acids, and proteins and
produces calcitriol (active
form vitamin D)
The Loop of Henle
Thick descending
limb (
proximal
straight tubule
)
Simple cuboidal
epithelium with
long microvilli
(brush border)
Similar to proximal convoluted
tubule but shorter and straight;
small mitochondria; no
basolateral plasma membrane
enfoldings
Medullary
ray and outer
zone of the
renal medulla
Absorptive function
is
similar to proximal
convoluted tubule but
less signi± cant
Thin descending
limb (
descending
thin segment
)
Simple squamous
epithelium
Thin, small tubule; epithelial cells
may reveal basolateral
enfoldings
and small microvilli
Partial outer
zone and most
of the inner
zone of the
renal medulla
Highly permeable to water
(loss of water from lumen to
interstitium); less permeable
to salt (keeps or may gain
some Na
+
and Cl
in the
lumen)
Thin ascending
limb (
ascending
thin segment
)
Simple squamous
epithelium
Similar to descending thin
segment; may have basolateral
enfoldings and small microvilli
Inner zone
of the renal
medulla
Impermeable to water
(retains water); highly
permeable to salt (loss of
Na
+
and Cl
from the lumen
to the interstitium)
Thick ascend-
ing limb (
distal
straight tubule
)
Simple cuboidal
epithelium with
short microvilli
Straight tubule; numerous
mitochondria; less acidophilic
cytoplasm; many basolateral
plasma membrane enfoldings
Medullary
ray and outer
zone of the
renal medulla
Impermeable to water
(retains water); highly
permeable to salt (loss of
Na
+
and Cl
from the lumen
to the interstitium)
Distal convoluted
tubule
Simple cuboidal
epithelium with
short microvilli
Numerous mitochondria;
basolateral plasma membrane
enfoldings; less acidophilic
cytoplasm; highly convoluted
tubule
Renal cortex
Reabsorbs Na
+
and
secretes K
+
, if aldosterone
stimulation is present;
reabsorbs bicarbonate ions
and secretes ammonium to
adjust pH
Collecting System
Collecting tubule
Simple cuboidal
epithelium with
few microvilli
Straight tubule; much less
acidophilic cytoplasm; more than
one cell type
Renal cortex
Highly permeable to water;
loss of water from the
lumen to the interstitium
when ADH is present
Collecting duct
Simple columnar
Large straight tubule; clear
cytoplasm and distinct
boundaries between cells;
well- developed basal enfoldings
Medullary
ray and renal
medulla
Highly permeable to water;
loss of water from the
lumen to the interstitium
when ADH is present
Papillary duct
Simple columnar
epithelium
Short duct; links collecting duct
to the minor calyx
Bottom tip of
the pyramid
of the medulla
Conducts urine
TABLE 12-1
Kidneys
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