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prepared by
Barbara Heard,
Atlantic Cape Community
College
CHAPTER
25
The Urinary
System:
Part B
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
Tubular Reabsorption
• Most of tubular contents reabsorbed to
blood
• Selective process
– ~ All organic nutrients reabsorbed
– Water and ion reabsorption hormonally
regulated and adjusted
• Includes active and passive tubular
reabsorption
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Figure 25.13 Transcellular and paracellular routes of tubular reabsorption.
Slide 1
The paracellular route
The transcellular route 3 Transport across the
involves:
basolateral membrane. (Often
involves:
•
Movement through leaky
involves the lateral intercellular
1 Transport across the spaces because membrane
tight junctions, particularly in
apical membrane.
the PCT.
transporters transport ions into
• Movement through the inter2 Diffusion through the these spaces.)
stitial fluid and into the
4 Movement through the intercytosol.
capillary.
stitial fluid and into the capillary.
Filtrate
Tubule cell
Interstitial fluid
in tubule
PeriLateral
Tight junction
lumen
tubular
intercellular capillary
space
3
H2O and
solutes
Apical
membrane
H2O and
solutes
1
2
4
3
4
Transcellular Capillary
endothelial
route
cell
Paracellular route
Basolateral
membranes
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Reabsorption of Nutrients, Water, and Ions
• Na+ reabsorption by primary active
transport provides energy and means for
reabsorbing most other substances
• Creates electrical gradient  passive
reabsorption of anions
• Organic nutrients reabsorbed by
secondary active transport
– Glucose, amino acids, some ions, vitamins
© 2013 Pearson Education, Inc.
Figure 25.14 Reabsorption by PCT cells.
Slide 1
1 At the basolateral membrane, Na+ is
pumped into the interstitial space by the
Na+-K+ ATPase. Active Na+ transport
creates concentration gradients that drive:
Nucleus
Filtrate
in tubule
lumen
Tubule cell
Interstitial
fluid
Peritubular
capillary
2
Glucose
Amino
acids
Some
ions
Vitamins
1
3
4
Lipid5
soluble
substances
6
Various
Ions
and urea
3 Reabsorption of organic
nutrients and certain ions by
cotransport at the apical
membrane.
4 Reabsorption of water by
osmosis through
aquaporins. Water
reabsorption increases the
concentration of the
solutes that are left behind.
These solutes can then be
reabsorbed as they move
down their gradients:
5 Lipid-soluble substances
diffuse by the transcellular
route.
Tight junction
Primary active transport
Secondary active transport
Passive transport (diffusion)
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2 “Downhill” Na+ entry at the
apical membrane.
Paracellular
route
Transport protein
Ion channel
Aquaporin
6 Various ions (e.g., Cl−,
Ca2+, K+) and urea diffuse
by the paracellular route.
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• PCT
– Site of most reabsorption
•
•
•
•
All nutrients, e.g., glucose and amino acids
65% of Na+ and water
Many ions
~ All uric acid; ½ urea (later secreted back into
filtrate)
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Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• Nephron loop
– Descending limb - H2O can leave; solutes
cannot
– Ascending limb – H2O cannot leave; solutes
can
• Thin segment – passive Na+ movement
• Thick segment – Na+-K+-2Cl- symporter and Na+H+ antiporter; some passes by paracellular route
© 2013 Pearson Education, Inc.
Figure 25.15 Summary of tubular reabsorption and secretion.
Cortex
65% of filtrate volume
reabsorbed
• H2O
• Na+, HCO3−, and
many other ions
• Glucose, amino acids,
and other nutrients
• H+ and NH4+
• Some drugs
Outer
medulla
Regulated reabsorption
• Na+ (by aldosterone;
Cl− follows)
• Ca2+ (by parathyroid
hormone)
Regulated
secretion
• K+ (by
aldosterone)
Regulated
reabsorption
• H2O (by ADH)
• Na+ (by
aldosterone; Cl−
follows)
• Urea (increased
by ADH)
• Urea
Inner
medulla
Regulated
secretion
• K+ (by
aldosterone)
• Reabsorption or secretion
to maintain blood pH
described in Chapter 26;
involves H+, HCO3−,
and NH4+
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Reabsorption
Secretion
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (1 of 4)
The three key players and their
orientation in the osmotic gradient:
(c) The collecting ducts of
all nephrons use the gradient
to adjust urine osmolality.
300
300
(a) The long nephron loops of
juxtamedullary nephrons create
the gradient. They act as
countercurrent multipliers.
400
600
900
(b) The vasa recta preserve the
gradient. They act as
countercurrent exchangers.
© 2013 Pearson Education, Inc.
1200
The osmolality of the medullary
interstitial fluid progressively
increases from the 300 mOsm of
normal body fluid to 1200 mOsm
at the deepest part of the medulla.
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (2 of 4)
Long nephron loops of juxtamedullary nephrons create the gradient.
The countercurrent multiplier depends on three properties
of the nephron loop to establish the osmotic gradient.
Fluid flows in the
opposite direction
(countercurrent)
through two
adjacent parallel
sections of a
nephron loop.
The descending
limb is permeable
to water, but not
to salt.
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The ascending limb
is impermeable to
water, and pumps
out salt.
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (3 of 4)
Long nephron loops of juxtamedullary nephrons create the gradient.
These properties establish a positive feedback cycle that
uses the flow of fluid to multiply the power of the salt pumps.
Interstitial fluid
osmolality
Start
here
Water leaves the
descending limb
Osmolality of filtrate
in descending limb
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Salt is pumped out
of the ascending limb
Osmolality of filtrate
entering the ascending
limb
Figure 25.16a Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration. (4 of 4)
(continued) As water and solutes are reabsorbed, the loop first concentrates the filtrate, then dilutes it.
Active transport
Passive transport
Water impermeable
300
300
Osmolality of interstitial fluid (mOsm)
300
100
Cortex
1 Filtrate entering the
nephron loop is isosmotic to
both blood plasma and
cortical interstitial fluid.
400
600
300
100
5 Filtrate is at its most dilute as it
leaves the nephron loop. At
100 mOsm, it is hypo-osmotic
to the interstitial fluid.
400
200
4 Na+ and Cl- are pumped out
of the filtrate. This increases the
interstitial fluid osmolality.
Outer
medulla
600
400
900
700
2 Water moves out of the
filtrate in the descending limb
down its osmotic gradient.
This concentrates the filtrate.
900
1200
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Inner
medulla
3 Filtrate reaches its highest
concentration at the bend of the
loop.
Nephron loop
1200
Figure 25.16b Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration.
Vasa recta preserve the gradient.
The entire length of the vasa recta is highly permeable to water
and solutes. Due to countercurrent exchanges between each
section of the vasa recta and its surrounding interstitial fluid, the
blood within the vasa recta remains nearly isosmotic to the
surrounding fluid. As a result, the vasa recta do not undo the
osmotic gradient as they remove reabsorbed water and solutes.
Blood from
efferent
arteriole
To vein
325
300
300
400
The countercurrent
flow of fluid moves
through two adjacent
parallel sections of
the vasa recta.
400
600
600
900
900
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Vasa recta
1200
Figure 25.16c Juxtamedullary nephrons create an osmotic gradient within the renal medulla that allows the kidney to
produce urine of varying concentration.
Collecting ducts use the gradient.
Under the control of antidiuretic hormone, the collecting
ducts determine the final concentration and volume of
urine. This process is fully described in Figure 25.17.
Collecting duct
400
600
900
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Urine
1200
Osmolality of interstitial fluid (mOsm)
300
Formation of Dilute or Concentrated Urine
• Osmotic gradient used to raise urine
concentration > 300 mOsm to conserve
water
– Overhydration  large volume dilute urine
• ADH production ; urine ~ 100 mOsm
• If aldosterone present, additional ions removed 
~ 50 mOsm
– Dehydration  small volume concentrated
urine
• Maximal ADH released; urine ~ 1200 mOsm
• Severe dehydration – 99% water reabsorbed
© 2013 Pearson Education, Inc.
Figure 25.17 Mechanism for forming dilute or concentrated urine.
If we were so overhydrated we had no ADH...
If we were so dehydrated we had maximal ADH...
Osmolality of extracellular fluids
Osmolality of extracellular fluids
ADH release from posterior pituitary
ADH release from posterior pituitary
Number of aquaporins (H2O channels) in collecting duct
Number of aquaporins (H2O channels) in collecting duct
H2O reabsorption from collecting duct
H2O reabsorption from collecting duct
Large volume of dilute urine
Small volume of concentrated urine
Collecting
duct
Cortex
100
600
300
400
600
100
Outer
medulla
900
700
900
1200
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300
300
100
300
300
400
600
400
600
600
900
900
Outer
medulla
Urea
700
900
Urea
100
Inner
medulla
1200
Large volume
of dilute urine
Active transport
Passive transport
150
Cortex
Urea
Inner
medulla
300
100
DCT
100
Osmolality of interstitial fluid (mOsm)
DCT
300
Descending limb
of nephron loop
300
100
1200
1200
1200
Small volume of
Urea contributes to concentrated urine
the osmotic gradient.
ADH increases its
recycling.
Osmolality of interstitial fluid (mOsm)
Descending limb
of nephron loop
Collecting duct
Physical Characteristics of Urine
• Color and transparency
– Clear
• Cloudy may indicate urinary tract infection
– Pale to deep yellow from urochrome
• Pigment from hemoglobin breakdown; more
concentrated urine  deeper color
– Abnormal color (pink, brown, smoky)
• Food ingestion, bile pigments, blood, drugs
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Physical Characteristics of Urine
• Odor
– Slightly aromatic when fresh
– Develops ammonia odor upon standing
• As bacteria metabolize solutes
– May be altered by some drugs and
vegetables
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Chemical Composition of Urine
• 95% water and 5% solutes
• Nitrogenous wastes
– Urea (from amino acid breakdown) – largest
solute component
– Uric acid (from nucleic acid metabolism)
– Creatinine (metabolite of creatine phosphate)
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Urinary Bladder
• Collapses when empty; rugae appear
• Expands and rises superiorly during filling
without significant rise in internal pressure
• ~ Full bladder 12 cm long; holds ~ 500 ml
– Can hold ~ twice that if necessary
– Can burst if overdistended
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Figure 25.18 Pyelogram.
Kidney
Renal
pelvis
Ureter
Urinary
bladder
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Figure 25.20a Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Adventitia
Ureteric orifices
Trigone of bladder
Bladder neck
Internal urethral sphincter
Prostate
Prostatic urethra
Intermediate part of the urethra
External urethral sphincter
Urogenital diaphragm
Spongy urethra
Erectile tissue of penis
External urethral orifice
Male. The long male urethra has three regions:
prostatic, intermediate, and spongy.
© 2013 Pearson Education, Inc.
Figure 25.20b Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Ureteric orifices
Bladder neck
Internal urethral
sphincter
Trigone
External urethral
sphincter
Urogenital diaphragm
Urethra
External urethral
orifice
Female.
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Urethra
• Muscular tube draining urinary bladder
– Lining epithelium
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Urethra
• Sphincters
– Internal urethral sphincter
• Involuntary (smooth muscle) at bladder-urethra
junction
• Contracts to open
– External urethral sphincter
• Voluntary (skeletal) muscle surrounding urethra as
it passes through pelvic floor
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Urethra
• Female urethra (3–4 cm)
– Tightly bound to anterior vaginal wall
– External urethral orifice
• Anterior to vaginal opening; posterior to clitoris
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Figure 25.20b Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Ureteric orifices
Bladder neck
Internal urethral
sphincter
Trigone
External urethral
sphincter
Urogenital diaphragm
Urethra
External urethral
orifice
Female.
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Urethra
• Male urethra carries semen and urine
– Three named regions
• Prostatic urethra (2.5 cm)—within prostate
• Intermediate part of the urethra (membranous
urethra) (2 cm)—passes through urogenital
diaphragm from prostate to beginning of penis
• Spongy urethra (15 cm)—passes through penis;
opens via external urethral orifice
© 2013 Pearson Education, Inc.
Figure 25.20a Structure of the urinary bladder and urethra.
Peritoneum
Ureter
Rugae
Detrusor
Adventitia
Ureteric orifices
Trigone of bladder
Bladder neck
Internal urethral sphincter
Prostate
Prostatic urethra
Intermediate part of the urethra
External urethral sphincter
Urogenital diaphragm
Spongy urethra
Erectile tissue of penis
External urethral orifice
Male. The long male urethra has three regions:
prostatic, intermediate, and spongy.
© 2013 Pearson Education, Inc.
Micturition
• Urination or voiding
• Three simultaneous events must occur
– Contraction of detrusor by ANS
– Opening of internal urethral sphincter by ANS
– Opening of external urethral sphincter by
somatic nervous system
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Figure 25.21 Control of micturition.
Brain
Higher brain
centers
Urinary bladder
fills, stretching
bladder wall
Allow or inhibit micturition
as appropriate
Pontine micturition
center
Afferent impulses
from stretch
receptors
Inhibits micturition by
acting on all three
Spinal efferents
Promotes micturition
by acting on all three
spinal efferents
Simple
spinal
reflex
Pontine storage
center
Spinal
cord
Spinal
cord
Parasympathetic
activity
Sympathetic
activity
Detrusor contracts;
internal urethral
sphincter opens
External urethral
sphincter opens
Micturition
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Somatic motor
nerve activity
Inhibits
Parasympathetic activity
Sympathetic activity
Somatic motor nerve activity