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Lecture 10
The Urinary System
Kidney Function
• Regulating total water volume and total solute
concentration in water
• Regulating ECF ion concentrations
• Ensuring long-term acid-base balance
• Removal of metabolic wastes, toxins, drugs
Kidney Function
• Endocrine functions
– Renin - regulation of blood pressure
– Erythropoietin - regulation of RBC production
• Activation of vitamin D
Kidney System Organs
• Kidneys
– major excretory organs
• Ureters
– transport urine from kidneys to urinary bladder
• Urinary bladder
– temporary storage reservoir for urine
• Urethra
– transports urine out of body
Figure 25.1 The urinary system.
Hepatic veins (cut)
Esophagus (cut)
Inferior vena cava
Adrenal gland
Renal artery
Renal hilum
Aorta
Renal vein
Kidney
Iliac crest
Ureter
Rectum (cut)
Uterus (part of female
reproductive system)
Urinary
bladder
Urethra
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Kidney Anatomy
• Layers of surrounding supportive tissue:
• Renal fascia
– Anchoring outer layer of dense fibrous connective
tissue
• Perirenal fat capsule
– Fatty cushion for protection
• Fibrous capsule
– Prevents spread of infection to kidney
Internal Anatomy
• Three distinct regions
• Renal cortex
– Granular-appearing superficial region
• Renal medulla
– Composed of cone-shaped medullary (renal)
pyramids (lobe and papilla)
• Pelvis
– funnel shaped tube leaving the hilum
Figure 25.3 Internal anatomy of the kidney.
Renal
hilum
Renal cortex
Renal medulla
Major calyx
Papilla of
pyramid
Renal pelvis
Minor calyx
Ureter
Renal pyramid in
renal medulla
Renal column
Fibrous capsule
Photograph of right kidney, frontal section
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Diagrammatic view
Kidney Function
• Kidneys cleanse blood
– Renal arteries deliver ~ ¼ (1200 ml) of cardiac output
to kidneys each minute
• Arterial flow into and venous flow out of kidneys
follow similar paths
• Nerve supply via sympathetic fibers from renal
plexus
Figure 25.4a Blood vessels of the kidney.
Cortical radiate
vein
Cortical radiate
artery
Arcuate vein
Arcuate artery
Interlobar vein
Interlobar artery
Segmental arteries
Renal vein
Renal artery
Renal pelvis
Ureter
Renal medulla
Renal cortex
Frontal section illustrating major blood vessels
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Nephrons
• Structural and functional units that form urine
• > 1 million per kidney
• Two main parts
– Renal corpuscle
– Renal tubule
Renal Corpuscle
• Two parts of renal corpuscle
• Glomerulus
– Tuft of capillaries; fenestrated
– highly porous, allows filtrate formation
• Glomerular capsule (Bowman's capsule)
– Cup-shaped, hollow structure surrounding
glomerulus
Figure 25.5 Location and structure of nephrons.
Renal cortex
Renal medulla
Glomerular capsule: parietal layer
Renal pelvis
Ureter
Kidney
Renal corpuscle
• Glomerular capsule
• Glomerulus
Distal
convoluted
tubule
Basement
membrane
Podocyte
Fenestrated endothelium
of the glomerulus
Glomerular capsule: visceral layer
Apical
microvilli
Mitochondria
Highly infolded basolateral
membrane
Proximal convoluted tubule cells
Proximal
convoluted
tubule
Cortex
Apical side
Medulla
Thin segment
Nephron loop
• Descending limb
• Ascending limb
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Thick
segment
Basolateral side
Distal convoluted tubule cells
Nephron loop (thin-segment) cells
Collecting
duct
Principal
cell
Intercalated cell
Collecting duct cells
Renal Tube
• Three parts:
– Proximal convoluted tubule
– Nephron loop
– Distal convoluted tubule
• Collecting Duct
– receives filtrate from many nephrons
– run through medullary pyramids
– fuse together at renal pelvis to deliver urine
Two Types of Nephrons
• Cortical nephrons
– 85% of nephrons; almost entirely in cortex
• Juxtamedullary nephrons
– Long nephron loops deeply invade medulla
– Important in production of concentrated urine
Cortical nephron
• Short nephron loop
• Glomerulus further from the cortex-medulla junction
• Efferent arteriole supplies peritubular capillaries
Renal
Glomerulus
corpuscle
Juxtamedullary nephron
• Long nephron loop
• Glomerulus closer to the cortex-medulla junction
• Efferent arteriole supplies vasa recta
Efferent
arteriole
Afferent arteriole
Proximal
convoluted
tubule
Afferent
arteriole
Efferent
arteriole
Peritubular
capillaries
Ascending
limb of
nephron loop
Cortex-medulla
junction
Vasa recta
Kidney
Nephron loop
Descending
limb of
nephron loop
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Nephron Capillary Beds
• Renal tubules associated with two capillary
beds
– Glomerulus
– Peritubular capillaries
• Juxtamedullary nephrons also associated with
– Vasa recta
Glomerulus Capillaries
• Specialized for filtration
• Fed and drained by arteriole
• Afferent arteriole glomerulus efferent
arteriole capillaries
• Blood pressure in glomerulus high for filtration
– Afferent arterioles larger in diameter than efferent
arterioles
Peritubular Capillaries
• Low-pressure, porous capillaries adapted for
absorption of water and solutes
• Arise from efferent arterioles
• Cling to adjacent renal tubules in cortex
• Empty into venules
Vasa Recta
• Long, thin-walled vessels parallel to long
nephron loops of juxtamedullary nephrons
• Arise from efferent arterioles serving
juxtamedullary nephrons
• Function in formation of concentrated urine
Overall Nephron/Capillary Function
• Two functionally different capillary beds
• Glomerus
– produces the filtrate from the blood
• Peritubular and vasa recta
– reclaims most of the filtrate
Juxtaglomerular Complex
• Important in regulation of rate of filtrate
formation and blood pressure
• Three cell populations
– Macula densa
– Granular cells
– Extraglomerular mesangial cells
Figure 25.8 Juxtaglomerular complex (JGC) of a nephron.
Glomerular
capsule
Efferent
arteriole
Glomerulus
Capsular
space
Afferent
arteriole
Red blood cell
Proximal
tubule cell
Efferent
arteriole
Juxtaglomerular
complex
• Macula densa
cells
of the ascending
limb of nephron loop
• Extraglomerular
mesangial cells
• Granular
cells
Afferent
arteriole
Juxtaglomerular complex
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Lumens of
glomerular
capillaries
Renal corpuscle
Cells of JGC
• Macula densa
– Tall, closely packed cells of ascending limb
– Chemoreceptors; sense NaCl content of filtrate
• Granular cells (juxtaglomerular, or JG cells)
– Enlarged, smooth muscle cells of arteriole
– Secretory granules contain renin
– Mechanoreceptors; sense blood pressure in
afferent arteriole
Cells of JGC
• Extraglomerular mesangial cells
– Between arteriole and tubule cells
– Interconnected with gap junctions
– May pass signals between macula densa and
granular cells
– help recycle large macromolecules to maintain
osmotic pressure
Physiology of Kidney
• 180 L fluid processed daily
– only 1.5 L urine
• Three processes in urine formation and
adjustment of blood composition
– Glomerular filtration
– Tubular reabsorption
– Tubular secretion
Mechanisms of Urine Formation
• Glomerular filtration
– produces cell- and protein-free filtrate
• Tubular reabsorption
– Selectively returns 99% of substances from filtrate
to blood in renal tubules and collecting ducts
• Tubular secretion
– Selectively moves substances from blood to
filtrate in renal tubules and collecting ducts
Kidney Filtering Capacity
• Kidneys filter body's entire plasma volume 60
times each day
– 1% body weight consume 20-25% oxygen used
• Filtrate (produced by glomerular filtration)
– Blood plasma minus proteins
• Urine
– <1% of original filtrate
– Contains metabolic wastes and unneeded substances
Glomerular Filtration
• Passive process
– No metabolic energy required
• Hydrostatic pressure forces fluids and solutes
through filtration membrane
– similar to gas exchange
• No reabsorption into capillaries of glomerulus
Filtration Membrane
• Porous membrane between capillary and
interior of glomerular capsule
• Water, solutes smaller than plasma proteins
pass; normally no cells pass
• Layers include fenestrated endothelium of
glomerular capillaries and basement
membrane of glomerular capsule
Figure 25.10a The filtration membrane.
Efferent
arteriole
Glomerular
capsular space
Cytoplasmic extensions
of podocytes
Filtration slits
Podocyte
cell body
Afferent
arteriole
Glomerular
capillary covered by
podocytes that form
the visceral layer of
glomerular capsule
Proximal
convoluted
tubule
Parietal layer
Fenestrations
of glomerular
(pores)
capsule
Glomerular capillaries and the
visceral layer of the glomerular
capsule
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Glomerular
capillary endothelium
(podocyte covering
and basement
membrane removed)
Foot
processes
of podocyte
Filtration Membrane
• Macromolecules "stuck" in filtration membrane
engulfed by glomerular mesangial cells
• Allows molecules smaller than 3 nm to pass
– Water, glucose, amino acids, nitrogenous wastes
• Plasma proteins remain in blood that maintains
colloid osmotic pressure
– prevents loss of all water to capsular space
Pressures for Filtration: Part I
• Outward pressures promote filtrate formation
• Hydrostatic pressure in glomerular capillaries =
Glomerular blood pressure
• Chief force pushing water, solutes out of blood
– Quite high (55 mm Hg) (most capillary beds ~ 26 mm
Hg)
– Efferent arteriole is high resistance vessel with
diameter smaller than afferent arteriole
Pressures for Filtration: Part II
• Inward forces inhibiting filtrate formation
• Hydrostatic pressure in capsular space (HPcs)
• Pressure of filtrate in capsule: 15 mm Hg
• Colloid osmotic pressure in capillaries (OPgc)
– "Pull" of proteins in blood – 30 mm Hg
Net Filtration Pressure
• NFP =
55 mm Hg forcing out
45 mm Hg forcing in
Net outward force of 10 mm Hg
Figure 25.11 Forces determining net filtration pressure (NFP).
Glomerular
capsule
Efferent
arteriole
HPgc = 55 mm Hg
OPgc = 30 mm Hg
Afferent
arteriole
HPcs = 15 mm Hg
NFP = Net filtration pressure
= outward pressures – inward pressures
= (HPgc) – (HPcs + OPgc)
= (55) – (15 + 30)
= 10 mm Hg
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Glomerular Filtration Rate
• Volume of filtrate formed per minute by both
kidneys (normal = 120–125 ml/min)
• GFR directly proportional to
– NFP – primary pressure is hydrostatic pressure in
glomerulus
– Total surface area available for filtration
• glomerular mesangial cells control by contracting
– Filtration membrane permeability
• much more permeable than other capillaries
Regulating GFR
• Constant GFR allows kidneys to make filtrate
and maintain extracellular homeostasis
• GFR affects systemic blood pressure:
•
GFR = urine output = blood pressure, and
vice versa
Renal Autoregulation
• Intrinsic Control
• Maintains nearly constant GFR when MAP: 80–
180 mm Hg
– Autoregulation ceases if out of that range
• Two types of renal autoregulation
– Myogenic mechanism
– Tubuloglomerular feedback mechanism
Myogenic Mechanism
• Smooth muscle contracts when stretched:
•  BP  muscle stretch  constriction of afferent
arterioles  restricts blood flow into glomerulus
– Protects glomeruli from damaging high BP
•  BP  dilation of afferent arterioles
• Both help maintain normal GFR despite normal
fluctuations in blood pressure
Tubuloglomerular Feedback
• Flow-dependent mechanism directed by
macula densa cells
– respond to filtrate NaCl concentration
• If GFR  filtrate flow rate  
reabsorption time  high filtrate NaCl levels
 constriction of afferent arteriole   NFP
& GFR  more time for NaCl reabsorption
• Opposite for  GFR
Figure 25.8 Juxtaglomerular complex (JGC) of a nephron.
Glomerular
capsule
Efferent
arteriole
Glomerulus
Capsular
space
Afferent
arteriole
Red blood cell
Proximal
tubule cell
Efferent
arteriole
Juxtaglomerular
complex
• Macula densa
cells
of the ascending
limb of nephron loop
• Extraglomerular
mesangial cells
• Granular
cells
Afferent
arteriole
Juxtaglomerular complex
© 2013 Pearson Education, Inc.
Lumens of
glomerular
capillaries
Renal corpuscle
Extrinsic Controls
• Hormonal
– Renin-Angiotensin-Aldosterone
• Neural
–  BP inhibits baroreceptors sympathetic nervous
system:
• Release of renin from kidneys
• Epinephrine from adrenal medulla
• Vasocontriction of arterioles
Tubular Reabsorption
• Most of tubular contents reabsorbed to blood
• Selective transepithelial process
– ~ All organic nutrients reabsorbed
– Water and ion reabsorption hormonally regulated
and adjusted
• Combination of passive and active transport
Slide 1
The paracellular route
involves:
The transcellular route
involves:
1 Transport across the
apical membrane.
• Movement through leaky
tight junctions
• Movement through the
interstitial fluid and into
the capillary.
2 Diffusion through the
cytosol.
Filtrate
in tubule
lumen
Tubule cell
Interstitial fluid
Lateral
intercellular
space
Tight junction
3
H2O and
solutes
1
Apical
membrane
H2O and
solutes
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2
4
3
4
Transcellular Capillary
endothelial
route
cell
Paracellular route
Basolateral
membranes
Peritubular
capillary
Slide 1
1 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
2 “Downhill” Na+ entry at the
apical membrane.
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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Paracellular
route
Transport protein
Ion channel
Aquaporin
6 Various ions (e.g., Cl−,
Ca2+, K+) and urea diffuse
by the paracellular route.
Proximal Convoluted Tubules (PCT)
• The entire renal tubule is involved in
reabsorption but PCT are the most active
• 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)
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;
Distal Convoluted Tubules and
Collecting Ducts
• Reabsorption hormonally regulated
– Antidiuretic hormone (ADH) – Water
– Aldosterone – Na+ (therefore water)
– Atrial natriuretic peptide (ANP) – Na+
– PTH – Ca2+
Hormone Regulation
• Antidiuretic hormone (ADH)
– Released by posterior pituitary gland
– Causes collecting ducts to insert aquaporins in
apical membranes for water reabsorption
• As ADH levels increase so does increased
water reabsorption
Hormone Regulation
• Aldosterone
– Targets collecting ducts and DCT
• Promotes synthesis of apical Na+ and K+ channels,
and basolateral Na+-K+ ATPases for Na+
reabsorption; water follows
• Functions – increase blood pressure; decrease K+
levels
Hormone Regulation
• Atrial natriuretic peptide
– Reduces blood Na+
– decreased blood volume and blood pressure
– Released by cardiac atrial cells if blood volume or
pressure elevated
• Parathyroid hormone acts on DCT to increase
Ca2+ reabsorption
Tubular Secretion
• Reabsorption in reverse; almost all in PCT
• Selected substances
– K+, H+, NH4+, creatinine, organic acids and bases
– move from peritubular capillaries through tubule
cells into filtrate
• Substances synthesized in tubule cells also
secreted – HCO3-
Regulation of Urine Volume
• Defined by Osmolality (milliosmols = mOsm)
– Number of solute particles in 1 kg of H2O
• Reflects ability to cause osmosis
• Kidneys maintain osmolality of plasma at
~300 mOsm by regulating urine concentration
and volume
– regulate with countercurrent mechanism
Countercurrent mechanism
• Occurs when fluid flows in opposite directions in
two adjacent segments of same tube with hair
pin turn
• Countercurrent multiplier
– interaction of filtrate flow in ascending/descending
limbs of nephron loops of juxtamedullary nephrons
• Countercurrent exchanger
– Blood flow in ascending/descending limbs of vasa
recta
What does this do?
• Establish and maintain osmotic gradient
(300 mOsm to 1200 mOsm) from renal cortex
through medulla
• Allow kidneys to vary urine concentration
while keeping you properly hydrated
throughout your body
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.
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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.
Countercurrent Multiplier
• Descending limb
– Freely permeable to H2O
– H2O passes out of filtrate into hyperosmotic
medullary interstitial fluid
– Filtrate osmolality increases to ~1200 mOsm
Countercurrent Multiplier
• Ascending limb
– Impermeable to H2O
– Selectively permeable to solutes
– Na+ and Cl– actively reabsorbed in thick segment;
some passively reabsorbed in thin segment
– Filtrate osmolality decreases to 100 mOsm
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.
© 2013 Pearson Education, Inc.
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
Inner
medulla
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3 Filtrate reaches its highest
concentration at the bend of the
loop.
Nephron loop
1200
Countercurrent Multiplier
• 300mOsm going in – 100mOsm going out
– Constant 200 mOsm difference between two
limbs of nephron loop and between ascending
limb and interstitial fluid
– difference not enough for filtration/secretion
• Countercurrent flow “multiplies” the small
changes in solute concentrations to allow
Δ900mOsm
Countercurrent Exchanger
• Vasa recta preserves medullary gradient
– Prevent rapid removal of salt from interstitial space
– Remove reabsorbed water
• Blood within the vasa recta remains nearly
isosmotic to the surrounding fluid.
– The vasa recta does not undo the osmotic gradient as
they remove reabsorbed water and solutes
Cortical nephron
• Short nephron loop
• Glomerulus further from the cortex-medulla junction
• Efferent arteriole supplies peritubular capillaries
Renal
Glomerulus
corpuscle
Juxtamedullary nephron
• Long nephron loop
• Glomerulus closer to the cortex-medulla junction
• Efferent arteriole supplies vasa recta
Efferent
arteriole
Afferent arteriole
Proximal
convoluted
tubule
Afferent
arteriole
Efferent
arteriole
Peritubular
capillaries
Ascending
limb of
nephron loop
Cortex-medulla
junction
Vasa recta
Kidney
Nephron loop
Descending
limb of
nephron loop
© 2013 Pearson Education, Inc.
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
Inner
medulla
© 2013 Pearson Education, Inc.
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
Countercurrent Exchanger
• Water picked by the ascending vasa recta
includes:
– water lost from the descending vasa recta
– water reabsorbed from nephron loop and
collecting duct
• Volume of blood at end of vasa recta is greater
than in the beginning
Regulating Urine
Volume/Concentration
• Osmotic gradient used to raise urine
concentration > 300 mOsm to conserve water
• Overhydration
– large volume, dilute urine
• Dehydration
– small volume, concentrated urine
• Severe dehydration
– 99% water reabsorbed, little to no volume
Regulating Hydration
• 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)
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
1200
1200
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
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
The role of urea
• Urea helps form medullary gradient
– Enters filtrate in ascending thin limb of nephron
loop by facilitated diffusion
– Collecting duct reabsorbs water; leaves urea at a
high concentration
• Highly concentrated urea in collecting duct
– transports into interstitial fluid and back to
ascending limb = high osmolality in medulla
ADH and Urea
• ADH enhances urea transport out of the
medullary collecting ducts
– strengthens the medullary osmotic gradient
allowing more concentrated urine to be formed
• Urea contributes to the osmotic gradient, ADH
uses this mechanism to rid the body of high
solute levels
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
© 2013 Pearson Education, Inc.
300
300
100
300
300
400
600
400
600
600
900
900
1200
1200
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
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
Lab Exercises for Today
• Labs 40 and 41
• Wet lab today

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