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CHAPTER # 25(b)
THE URINARY SYSTEM
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Tubular Reabsorption
• A selective transepithelial process
• All organic nutrients are reabsorbed
• Water and ion reabsorption are hormonally
regulated
• Includes active and passive process
• Two routes
• Transcellular
• Paracellular
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Tubular Reabsorption
• Transcellular route
• Luminal membranes of tubule cells
• Cytosol of tubule cells
• Basolateral membranes of tubule cells
• Endothelium of peritubular capillaries
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Tubular Reabsorption
• Paracellular route
• Between cells
• Limited to water movement and reabsorption
of Ca2+, Mg2+, K+, and some Na+ in the PCT
where tight junctions are leaky
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Movement via the
transcellular route
involves:
1
Transport across the
luminal membrane.
2 Diffusion through the
cytosol.
3
Transport across the basolateral
membrane. (Often involves the lateral
intercellular spaces because
membrane transporters transport ions
into these spaces.)
4 Movement through the interstitial
fluid and into the capillary.
Lateral intercellular space
Tight junction
Filtrate
in tubule
lumen
The paracellular route
involves:
• Movement through
leaky tight junctions,
particularly in the PCT.
Interstitial
fluid
Capillary
endothelial
cell
Tubule cell
Peritubular
capillary
Paracellular
H2O
1
2
3
4
Transcellular
Luminal
membrane
1
Transcellular
3
4
2
Solutes
3
Paracellular
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4
Basolateral
membranes
Active
transport
Passive
transport
Figure 25.13
Sodium Reabsorption
• Na+ (most abundant cation in filtrate)
• Primary active transport out of the tubule cell
by
Na+-K+ ATPase in the basolateral membrane
• Na+ passes in through the luminal membrane
by secondary active transport or facilitated
diffusion mechanisms
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Sodium Reabsorption
• Low hydrostatic pressure and high osmotic
pressure in the peritubular capillaries
• Promotes bulk flow of water and solutes
(including Na+)
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Reabsorption of Nutrients, Water, and Ions
• Na+ reabsorption provides the energy and the
means for reabsorbing most other substances
• Organic nutrients are reabsorbed by
secondary active transport
• Transport maximum (Tm) reflects the number
of carriers in the renal tubules available
• When the carriers are saturated, excess of that
substance is excreted
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Reabsorption of Nutrients, Water, and Ions
• Water is reabsorbed by osmosis (obligatory
water reabsorption), aided by water-filled
pores called aquaporins
• Cations and fat-soluble substances follow by
diffusion
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1 At the basolateral membrane,
Nucleus
Filtrate
in tubule
lumen
Na+
Tubule cell
3Na+
1
2K+
2K+
3
K+
4
Lipid-soluble 5
substances
Cl–, Ca2+, K+
and other
ions, urea
6
Tight junction
Primary active transport
Secondary active transport
Passive transport (diffusion)
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Peritubular
capillary
3 Reabsorption of organic
2
3Na+
Glucose
Amino
acids
Some
ions
Vitamins
H2O
Interstitial
fluid
Na+ is pumped into the
interstitial space by the Na+-K+
ATPase. Active Na+ transport
creates concentration gradients
that drive:
2 “Downhill” Na+ entry at the
luminal membrane.
Cl–
Paracellular
route
Transport protein
Ion channel or aquaporin
nutrients and certain ions by
cotransport at the luminal
membrane.
4 Reabsorption of water by
osmosis. Water reabsorption
increases the concentration of
the solutes that are left
behind. These solutes can
then be reabsorbed as
they move down their
concentration gradients:
5 Lipid-soluble
substances diffuse by the
transcellular route.
6 Cl– (and other anions),
K+, and urea diffuse by the
paracellular route.
Figure 25.14
Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• PCT
• Site of most reabsorption
• 65% of Na+ and water
• All nutrients
• Ions
• Small proteins
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Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• Loop of Henle
• Descending limb: H2O
• Ascending limb: Na+, K+, Cl
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Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• DCT and collecting duct
• Reabsorption is hormonally regulated
• Ca2+ (PTH)
• Water (ADH)
• Na+ (aldosterone and ANP)
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Reabsorptive Capabilities of Renal Tubules
and Collecting Ducts
• Mechanism of aldosterone
• Targets collecting ducts (principal cells) and
distal DCT
• Promotes synthesis of luminal Na+ and K+
channels
• Promotes synthesis of basolateral Na+-K+
ATPases
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Tubular Secretion
• Reabsorption in reverse
• K+, H+, NH4+, creatinine, and organic acids
move from peritubular capillaries or tubule
cells into filtrate
• Disposes of substances that are bound to
plasma proteins
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Tubular Secretion
• Eliminates undesirable substances that have
been passively reabsorbed (e.g., urea and
uric acid)
• Rids the body of excess K+
• Controls blood pH by altering amounts of H+
or HCO3– in urine
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Regulation of Urine Concentration and
Volume
• Osmolality
• Number of solute particles in 1 kg of H2O
• Reflects ability to cause osmosis
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Regulation of Urine Concentration and
Volume
• Osmolality of body fluids
• Expressed in milliosmols (mOsm)
• The kidneys maintain osmolality of plasma at
~300 mOsm, using countercurrent
mechanisms
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Countercurrent Mechanism
• Occurs when fluid flows in opposite directions
in two adjacent segments of the same tube
• Filtrate flow in the loop of Henle
(countercurrent multiplier)
• Blood flow in the vasa recta (countercurrent
exchanger)
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Countercurrent Mechanism
• Role of countercurrent mechanisms
• Establish and maintain an osmotic gradient
(300 mOsm to 1200 mOsm) from renal cortex
through the medulla
• Allow the kidneys to vary urine concentration
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Cortex
Medulla
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Figure 25.15
Countercurrent Multiplier: Loop of Henle
• Descending limb
• Freely permeable to H2O, which passes out of
the filtrate into the hyperosmotic medullary
interstitial fluid
• Filtrate osmolality increases to ~1200 mOsm
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Countercurrent Multiplier: Loop of Henle
• Ascending limb
• Impermeable to H2O
• Selectively permeable to solutes
• Na+ and Cl– are passively reabsorbed in the
thin segment, actively reabsorbed in the
thick segment
• Filtrate osmolality decreases to 100 mOsm
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Osmolality
of interstitial
fluid
(mOsm)
Active transport
Passive transport
Water impermeable
Filtrate entering the
loop of Henle is
isosmotic to both
blood plasma and
cortical interstitial
fluid.
H2O
NaCI
H2O
NaCI
H2O
NaCI
H2O
The descending limb:
• Permeable to H2O
• Impermeable to NaCl
H2O
As filtrate flows, it
becomes increasingly
H2O
concentrated as H2O
leaves the tubule by
H2O
osmosis. The filtrate
osmolality increases from
300 to 1200 mOsm.
Loop of Henle
(a) Countercurrent multiplier.
The long loops of Henle of the
juxtamedullary nephrons
create the medullary
osmotic gradient.
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NaCI
Cortex
Outer
medulla
NaCI
Inner
medulla
The ascending limb:
• Impermeable to H2O
• Permeable to NaCl
Filtrate becomes increasingly dilute as NaCl leaves, eventually becoming
hypo-osmotic to blood at 100 mOsm in the cortex. NaCl leaving the
ascending limb increases the osmolality of the medullary interstitial fluid.
Figure 25.16a
Urea Recycling
• Urea moves between the collecting ducts and
the loop of Henle
• Secreted into filtrate by facilitated diffusion in
the ascending thin segment
• Reabsorbed by facilitated diffusion in the
collecting ducts deep in the medulla
• Contributes to the high osmolality in the
medulla
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Countercurrent Exchanger: Vasa Recta
• The vasa recta
• Maintain the osmotic gradient
• Deliver blood to the medullary tissues
• Protect the medullary osmotic gradient by
preventing rapid removal of salt, and by
removing reabsorbed H2O
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Osmolality
of interstitial
fluid
(mOsm)
Cortex
Blood from
efferent
arteriole
Passive transport
To vein
NaCI
H2O
NaCI
H2O
NaCI
H2O
NaCI
H2O
NaCI
H2O
NaCI
H2O
NaCI
H2O
NaCI
H2O
Outer
medulla
Inner
medulla
Vasa recta
(b) Countercurrent exchanger.
The vasa recta preserve the
medullary gradient while
removing reabsorbed water
and solutes.
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The vasa recta:
• Highly permeable to H2O and solute
• Nearly isosmotic to interstitial fluid due to sluggish blood flow
Blood becomes more concentrated as it descends deeper into
the medulla and less concentrated as it approaches the cortex.
Figure 25.16b
Formation of Dilute Urine
• Filtrate is diluted in the ascending loop of
Henle
• In the absence of ADH, dilute filtrate
continues into the renal pelvis as dilute urine
• Na+ and other ions may be selectively
removed in the DCT and collecting duct,
decreasing osmolality to as low as 50 mOsm
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Active transport
Passive transport
Collecting duct
Descending limb
of loop of Henle
DCT
Cortex
NaCI
H2O
NaCI
Outer
medulla
NaCI
H2O
Urea
Inner
medulla
(a) Absence of ADH
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Large volume
of dilute urine
Figure 25.17a
Formation of Concentrated Urine
• Depends on the medullary osmotic gradient
and ADH
• ADH triggers reabsorption of H2O in the
collecting ducts
• Facultative water reabsorption occurs in the
presence of ADH so that 99% of H2O in filtrate
is reabsorbed
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Active transport
Passive transport
Collecting duct
H2O
Descending limb
of loop of Henle
DCT
H2O
Cortex
NaCI
H2O
H2O
H2O
NaCI
Outer
medulla
NaCI
H2O
Inner
medulla
(b) Maximal ADH
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H2O
Urea
H2O
Urea
H2O
Small volume of
concentrated urine
Figure 25.17b
Diuretics
• Chemicals that enhance the urinary output
• Osmotic diuretics: substances not reabsorbed,
(e.g., high glucose in a diabetic patient)
• ADH inhibitors such as alcohol
• Substances that inhibit Na+ reabsorption and
obligatory H2O reabsorption such as caffeine
and many drugs
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Na+ (65%)
Glucose
Amino acids
H2O (65%) and
many ions (e.g.
Cl– and K+)
Milliosmols
Cortex
(d)
(a)
300
(e)
Outer
medulla
(b)
(c)
600
Some
drugs
–
H+, HCO3
NH4+
Inner
medulla
Blood pH regulation
1200
(a) Proximal convoluted tubule:
• 65% of filtrate volume reabsorbed
• Na+, glucose, amino acids, and other nutrients actively
transported; H2O and many ions follow passively
• H+ and NH4+ secretion and HCO3– reabsorption to
maintain blood pH (see Chapter 26)
• Some drugs are secreted
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Active transport
(primary or
secondary)
Passive transport
Figure 25.18a
Milliosmols
Cortex
(d)
(a)
300
(e)
Outer
medulla
H2O
(b)
(c)
600
(b) Descending limb of loop
of Henle
• Freely permeable to H2O
• Not permeable to NaCl
• Filtrate becomes increasingly
concentrated as H2O leaves by
osmosis
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Inner
medulla
1200
Active transport
(primary or
secondary)
Passive transport
Figure 25.18b
Milliosmols
Na+
Cl–
K+
Cortex
(d)
(a)
300
(e)
Outer
medulla
Urea
Na+
Cl–
(c) Ascending limb of loop of Henle
• Impermeable to H2O
• Permeable to NaCl
• Filtrate becomes
increasingly dilute as salt is
reabsorbed
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(b)
(c)
600
Inner
medulla
1200
Active transport
(primary or
secondary)
Passive transport
Figure 25.18c
Na+; aldosterone-regulated
Ca2+; PTH-regulated
Cl–; follows Na+
Milliosmols
Cortex
(d)
(a)
300
(e)
Outer
medulla
(b)
(c)
600
Inner
medulla
(d) Distal convoluted tubule
• Na+ reabsorption regulated by
aldosterone
• Ca2+ reabsortion regulated by
parathyroid hormone (PTH)
• Cl– cotransported with Na+
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1200
Active transport
(primary or
secondary)
Passive transport
Figure 25.18d
Milliosmols
Cortex
H2O regulated
by ADH
(d)
(a)
300
Regulated by
aldosterone:
Na+
K+
Blood pH
regulation
H+
Urea;
increased
by ADH
(e)
Outer
medulla
(c)
600
HCO3–
NH4+
(b)
Inner
medulla
1200
(e) Collecting duct
• H2O reabsorption through aquaporins regulated by ADH
• Na+ reabsorption and K+ secretion regulated by aldosterone
• H+ and HCO3– reabsorption or secretion
Active transport
to maintain blood pH (see Chapter 26)
(primary or secondary)
Passive transport
• Urea reabsorption increased by ADH
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Figure 25.18e
Renal Clearance
• Volume of plasma cleared of a particular
substance in a given time
• Renal clearance tests are used to
• Determine GFR
• Detect glomerular damage
• Follow the progress of renal disease
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Renal Clearance
RC = UV/P
RC = renal clearance rate (ml/min)
U = concentration (mg/ml) of the substance in
urine
V = flow rate of urine formation (ml/min)
P = concentration of the same substance in
plasma
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Renal Clearance
• For any substance freely filtered and neither
reabsorbed nor secreted by the kidneys (e.g.,
insulin),
RC = GFR = 125 ml/min
• If RC < 125 ml/min, the substance is reabsorbed
• If RC = 0, the substance is completely reabsorbed
• If RC > 125 ml/min, the substance is secreted (most
drug metabolites)
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Physical Characteristics of Urine
• Color and transparency
• Clear, pale to deep yellow (due to urochrome)
• Drugs, vitamin supplements, and diet can alter
the color
• Cloudy urine may indicate a urinary tract
infection
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Physical Characteristics of Urine
• Odor
• Slightly aromatic when fresh
• Develops ammonia odor upon standing
• May be altered by some drugs and vegetables
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Physical Characteristics of Urine
• pH
• Slightly acidic (~pH 6, with a range of 4.5 to
8.0)
• Diet, prolonged vomiting, or urinary tract
infections may alter pH
• Specific gravity
• 1.001 to 1.035, dependent on solute
concentration
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Chemical Composition of Urine
• 95% water and 5% solutes
• Nitrogenous wastes: urea, uric acid, and
creatinine
• Other normal solutes
• Na+, K+, PO43–, and SO42–,
• Ca2+, Mg2+ and HCO3–
• Abnormally high concentrations of any
constituent may indicate pathology
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Ureters
• Convey urine from kidneys to bladder
• Retroperitoneal
• Enter the base of the bladder through the
posterior wall
• As bladder pressure increases, distal ends of
the ureters close, preventing backflow of urine
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Ureters
• Three layers of wall of ureter
1. Lining of transitional epithelium
2. Smooth muscle muscularis
• Contracts in response to stretch
3. Outer adventitia of fibrous connective tissue
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Lumen
Adventitia
Circular
layer
Longitudinal
layer
Transitional
epithelium
Lamina
propria
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Figure 25.20
Renal Calculi
• Kidney stones form in renal pelvis
• Crystallized calcium, magnesium, or uric acid
salts
• Larger stones block ureter, cause pressure
and pain in kidneys
• May be due to chronic bacterial infection,
urine retention, Ca2+ in blood, pH of urine
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Urinary Bladder
• Muscular sac for temporary storage of urine
• Retroperitoneal, on pelvic floor posterior to
pubic symphysis
• Males—prostate gland surrounds the neck
inferiorly
• Females—anterior to the vagina and uterus
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Urinary Bladder
• Trigone
• Smooth triangular area outlined by the
openings for the ureters and the urethra
• Infections tend to persist in this region
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Urinary Bladder
• Layers of the bladder wall
1. Transitional epithelial mucosa
2. Thick detrusor muscle (three layers of smooth
muscle)
3. Fibrous adventitia (peritoneum on superior
surface only)
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Urinary Bladder
• Collapses when empty; rugae appear
• Expands and rises superiorly during filling
without significant rise in internal pressure
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Peritoneum
Ureter
Rugae
Detrusor
muscle
Ureteric orifices
Bladder neck
Internal urethral
sphincter
External urethral
sphincter
Urogenital diaphragm
(b) Female.
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Trigone
Urethra
External urethral
orifice
Figure 25.21b
Urethra
• Muscular tube
• Lining epithelium
• Mostly pseudostratified columnar epithelium,
except
• Transitional epithelium near bladder
• Stratified squamous epithelium near
external urethral orifice
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Urethra
• Sphincters
• Internal urethral sphincter
• Involuntary (smooth muscle) at bladderurethra junction
• Contracts to open
• External urethral sphincter
• Voluntary (skeletal) muscle surrounding the
urethra as it passes through the pelvic floor
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Urethra
• Female urethra (3–4 cm):
• Tightly bound to the anterior vaginal wall
• External urethral orifice is anterior to the
vaginal opening, posterior to the clitoris
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Peritoneum
Ureter
Rugae
Detrusor
muscle
Ureteric orifices
Bladder neck
Internal urethral
sphincter
External urethral
sphincter
Urogenital diaphragm
(b) Female.
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Trigone
Urethra
External urethral
orifice
Figure 25.21b
Urethra
• Male urethra
• Carries semen and urine
• Three named regions
1. Prostatic urethra (2.5 cm)—within prostate
gland
2. Membranous urethra (2 cm)—passes
through the urogenital diaphragm
3. Spongy urethra (15 cm)—passes through
the penis and opens via the external
urethral orifice
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Peritoneum
Ureter
Rugae
Detrusor muscle
Adventitia
Ureteric orifices
Trigone of bladder
Bladder neck
Internal urethral sphincter
Prostate
Prostatic urethra
Urogenital diaphragm
External urethral sphincter
Membranous urethra
Spongy urethra
Erectile tissue of penis
External urethral orifice
(a) Male. The long male urethra has three
regions: prostatic, membranous and spongy.
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Figure 25.21a
Micturition
• Urination or voiding
• Three simultaneous events
1. Contraction of detrusor muscle by ANS
2. Opening of internal urethral sphincter by ANS
3. Opening of external urethral sphincter by
somatic nervous system
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Micturition
• Reflexive urination (urination in infants)
• Distension of bladder activates stretch
receptors
• Excitation of parasympathetic neurons in reflex
center in sacral region of spinal cord
• Contraction of the detrusor muscle
• Contraction (opening) of internal sphincter
• Inhibition of somatic pathways to external
sphincter, allowing its relaxation (opening)
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Micturition
• Pontine control centers mature between ages 2
and 3
1. Pontine storage center inhibits micturition:
• Inhibits parasympathetic pathways
• Excites sympathetic and somatic efferent pathways
2. Pontine micturition center promotes micturition:
• Excites parasympathetic pathways
• Inhibits sympathetic and somatic efferent pathways
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Brain
Higher brain
centers
Urinary bladder
filling stretches
bladder wall
Allow or inhibit micturition
as appropriate
Pontine micturition
center
Afferent impulses
from stretch
receptors
Simple
spinal
reflex
Promotes micturition
by acting on all three
spinal efferents
Pontine storage
center
Inhibits micturition
by acting on all three
spinal efferents
Spinal
cord
Spinal
cord
Parasympathetic
activity
Sympathetic
activity
Detrusor muscle
contracts; internal
urethral sphincter
opens
Parasympathetic activity
Sympathetic activity
Somatic motor nerve activity
External urethral
sphincter opens
Micturition
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Somatic motor
nerve activity
Inhibits
Figure 25.22
Developmental Aspects
• Three sets of embryonic kidneys forming
succession
1. Pronephros degenerates but pronephric duct
persists
2. Mesonephros claims this duct and it becomes
the mesonephric duct
3. Metanephros develops by the fifth week,
develops into adult kidneys and ascends
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Degenerating
pronephros
Urogenital
ridge
Developing
digestive
tract
Duct to
yolk sac
Mesonephros
Allantois
Mesonephric duct
(initially, pronephric
duct)
Hindgut
Cloaca
Ureteric
bud
(a) Week 5
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Figure 25.23a
Degenerating
pronephros
Duct to yolk
sac
Allantois
Body stalk
Mesonephros
Mesonephric
duct
(b) Week 6
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Urogenital
sinus
Rectum
Ureteric bud
Metanephros
Figure 25.23b
Developmental Aspects
• Metanephros develops as ureteric buds that induce
mesoderm of urogenital ridge to form nephrons
• Distal ends of ureteric buds form renal pelves, calyces,
and collecting ducts
• Proximal ends become ureters
• Kidneys excrete urine into amniotic fluid by the third
month
• Cloaca subdivides into rectum, anal canal, and
urogenital sinus
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Gonad
Urogenital
sinus
(developing
urinary
bladder)
Rectum
Metanephros
(kidney)
(c) Week 7
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Figure 25.23c
Urinary bladder
Gonad
Urethra
Kidney
Anus
Ureter
Rectum
(d) Week 8
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Figure 25.23d
Developmental Aspects
• Frequent micturition in infants due to small bladders
and less-concentrated urine
• Incontinence is normal in infants: control of the
voluntary urethral sphincter develops with the
nervous system
• E. coli bacteria account for 80% of all urinary tract
infections
• Streptococcal infections may cause long-term renal
damage
• Sexually transmitted diseases can also inflame the
urinary tract
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