Transcript Chapter 25 - The Urinary System

Chapter 25
The Urinary System
G.R. Pitts, J.R. Schiller, and
James F. Thompson, Ph.D.
General
• During metabolism cells produce wastes
– waste - any substance acquired or produced in
excess with no function in body
– e.g. - CO2, H2O, heat
• All wastes must be eliminated, or at least
maintained at low concentrations
• Additionally, protein breakdown leaves
nitrogenous wastes
• Excess sodium (Na+), chloride (Cl-),
potassium (K+), sulfate (SO42-), phosphate
(PO42-), and hydrogen ion (H+) must be
regulated
General
• Several organs transport, neutralize, store
and remove wastes
–
–
–
–
–
–
–
–
Body fluid and body fluid buffers
Blood and blood buffers
Liver
Lungs
Sudoriferous (sweat) glands (minor)
Hair and nails (minor)
GI tract and liver
Kidneys
General
• Urinary system
– Maintains fluid homeostasis including:
• regulation of volume and composition by eliminating
certain wastes while conserving needed materials
• regulation of blood pH
• regulation of hydrostatic pressure of blood and,
indirectly, of other body fluids
– Contributions to metabolism
•
•
•
•
helps synthesize calcitriol (active form of Vitamin D)
secretes erythropoietin
performs gluconeogenesis during fasting or starvation
deaminates certain amino acids to eliminate ammonia
Kidneys
• Paired reddish organs,
just above waist on
posterior wall of
abdomen
– partially protected by
11th, 12th ribs
– right kidney sits lower
than the left kidney
– receive 20-25% of the
resting cardiac output
– Consume 20-25% of
the O2 used by the
body at rest
Kidneys (cont.)
•
Retroperitoneal, as are ureters and urinary bladder
Kidney - Internal Gross Anatomy
Know these terms for
lecture and lab exams!
Kidney - Internal Micro Anatomy
•
Nephron – the functional unit of kidney
– Three physiological processes: 1) filtration, 2) reabsorption , and
3) secretion
– These three processes cooperate to achieve the various functions of
the kidney
– Different sites  different primary functions
The Functions of the Kidney
• Nephron forms urine from blood plasma
– 1) formation of a plasma filtrate
– 2) reabsorption of useful molecules from the
filtrate to prevent their loss in urine
– 3) secretion of excess electrolytes and certain
wastes (nitrogenous wastes, H+) in
concentrations greater than their concentration
in plasma
– 4) regulation of water balance by concentrating
or diluting the urine
– 5) minor endocrine function – releasing hormone
erythropoietin to stimulate RBC production
– 6) releasing renin for angiotensinogen activation
Kidney - Internal Micro Anatomy
•
•
•
~A million
nephrons are
located in the
cortex
The filtrate is
carried by the
collecting duct
system through
the medulla
The urine is
collected at the
papillae into the
minor and major
calyxes
Nephron
Papilla
Minor
Calyx
Nephron
• 2 major parts to the
nephron
Renal Corpuscle
Renal Tubule
Nephron
•
Renal corpuscle
– site of plasma filtration
– 2 components
• glomerulus
– tuft of capillary loops
– fed by afferent
arteriole
– drained by efferent
arteriole
• glomerular (Bowman's) capsule
– double walled cup lined by simple squamous epithelium
– outer wall (parietal layer) separated from inner wall (visceral layer =
podocytes) by capsular (Bowman's) space
– as blood flows through capillary tuft – filtration occurs
• water and most dissolved molecules pass into capsular space
• large proteins and formed elements in the blood do not cross
Nephron
•
Renal tubule - where
filtered fluid passes
from capsule
– proximal convoluted
tubule (PCT)
– loop of Henle (nephron
loop)
– distal convoluted tubule
(DCT)
– short connecting tubules
– collecting ducts
– merge to papillary duct
• then to minor calyx
• 30 pap ducts/papillae
DCT
PCT
ducts
Loop
Nephron
• Cortical vs.
juxtamedullary
nephrons
– Location related
to the length of
loop of the
nephron
– 15-20% of the
nephrons have
longer loops and
increased
involvement in the
reabsorption of
water
H2O
Renal Corpuscle Histology
• Each nephron
•
portion has
distinctive features
Histology of the
glomerular
filtration membrane
– Three components
to the filter
– From inside to out,
the layers prevent
movement of
progressively
smaller particles
Histology of Filtration Membrane
1) Endothelium of glomerulus
–
–
Single layer of capillary endothelium with fenestrations
Prevents RBC passage; WBCs use diapedesis to get out
Histology of Filtration Membrane
2) Basement membrane of glomerulus
–
–
Between endothelium and visceral layer of glom. capsule
Prevents passage of large protein molecules
Histology of Filtration Membrane
3) Filtration slits in podocytes
–
Podocytes
•
•
–
specialized epithelium of visceral layer
footlike extensions with filtration slits between extensions
Restricts passage of medium-sized proteins
Histology of Filtration Membrane
Tubule Histology • PCT - cuboidal cells with
•
•
•
apical microvilli
Descending Loop, and
beginning of Ascending Loop
– simple squamous epithelium
– water permeable
Remainder of Ascending
limb of the Loop
– cuboidal to low columnar
epithelial cells
– impermeable to water
– permeable to solute (ions)
DCT, collecting ducts
– cuboidal with specialized cells
– principal cells - sensitive to
ADH (antidiuretic hormone)
– intercalated cells - secrete H+
Renal Blood Supply
• Important
vessels
– Renal arteries
• 20-25% of
resting CO
• 1200 ml/min
– Segmental
arteries
– Interlobar
arteries through columns
– Arcuate arteries
– Interlobular
arteries
Refer to the kidney models in lab.
Renal Blood Supply
•
Important vessels
peritubular
capillaries
– Afferent arterioles each renal corpuscle
receives one
– Glomerular capillaries
– Efferent arterioles drain blood from
glomerulus
cortex
-------medulla
– Peritubular capillaries
- around cortical nephrons
– Vasa recta - long
networks from the
efferent arteriole around
the Loop (juxtamedullary
nephrons)
Vasa
recta
Renal Blood Supply
• Important
vessels
– Interlobular
veins
– Arcuate veins
– Interlobar veins
– Segmental veins
– Renal veins exits hilus
Refer to the kidney models in lab.
Renin-Angiotensin System
• Juxtaglomerular apparatus
(JGA)
– Distal tubule contacts
afferent arteriole at renal
corpuscle
– Juxtaglomerular (JG) cells
• modified smooth muscle cells in
afferent arteriole wall detect
changes in blood pressure (a
stretch reflex)
• Secrete enzyme renin to trigger
Renin-Angiotensin System if
blood pressure falls
JG
Distal
Convoluted
Tubule
Renin-Angiotensin System
• Juxtaglomerular apparatus
(JGA)
– Distal tubule contacts afferent
arteriole at renal corpuscle
– Macula Densa (MD) cells
• special cells in the wall of the distal
tubule in this area monitor the
osmotic potential in the filtrate in the
distal tubule
• stimulate JG cells to release renin if
filtrate is too dilute, indicating
insufficient filtration and/or low
blood pressure/low blood volume
– Both JG and MD cells work
together to regulate blood
pressure and blood volume
JG
MD
Distal
Convoluted
Tubule
Renin-Angiotensin System
• Hepatocytes secrete inactive precursor
Angiotensinogen into the bloodstream
• Juxtaglomerular (JG) cells secrete the
enzyme renin to convert Angiotensinogen to
Angiotensin I in the bloodstream
• Angiotensin I is transported to the lungs
where Angiotensin Converting Enzyme (ACE)
converts Angiotensin I to Angiotensin II
• Both Angiotensin I and Angiotensin II act as
circulating hormones to increase blood
pressure and blood volume; AII is stronger
Renal Nerve Supply
• Nerves from renal plexus of Sympathetic
Division of ANS innervate the kidney
• Vasomotor nerves accompany the renal
arteries and their branches
– What is the role of sympathetic stimulation on
renal blood flow?
– In “Fight or Flight” or muscular exertion:
 decrease renal arterial flow
 decrease urine production
 maintain blood volume
 increase systemic blood pressure
Physiology of Urine Formation
• Glomerular filtration
- first step in urine
formation
– forcing of fluids and
dissolved solutes
through membrane by
hydrostatic pressure
– same process as in
systemic capillaries
– results in a filtrate
– 180 L/day, about 60
times plasma volume
– 178-179 L/day is
reabsorbed (~99%)
1-2 L/day of urine is typical
Glomerular Filtration
•
3 structural features of the renal
corpuscles enhance their filtering capacity:
1) Glomerular capillaries are relatively long which
increases their surface area for filtration
2) Filter (endothelium-capsular membrane) is thin
and porous
 Fenestrated glomerular capillaries are 50 times
more permeable than regular capillaries
 The filtration slits of the basement membrane only
permit passage of small molecules
3) Glomerular Capillary blood pressure is high –
the efferent arteriole diameter is less than
the afferent arteriole diameter -- increasing
filtration pressure in the renal corpuscle
Glomerular Filtration
NFP = GBHP – CHP – BCOP
• Net filtration
10 = 55 - 15 - 30
pressure (NFP)
depends on 3
pressures:
1) glomerular blood
hydrostatic
pressure (GBHP)
2) capsular
hydrostatic
pressure (CHP)
3) blood colloid
osmotic pressure
(BCOP)
1
2
3
Glomerular Filtration Rate (GFR)
• GFR
– Volume of filtrate that forms in all renal
corpuscles in both kidneys/min
– Adult’s GFR  125 mL/min (180 L/day)
• Regulation of GFR
– When more blood flows into glomerulus, GFR 
– GFR depends on systemic blood pressure, and
the diameter of afferent & efferent arterioles
– If glomerular capillary blood pressure falls much
below 45 mm Hg, no filtration occurs  anuria
(no urine output)
Glomerular Filtration Rate (GFR)
•
3 principal regulators of GFR:
1) Renal autoregulation of GFR
•
•
the kidneys are able to maintain a relatively
constant internal blood pressure and GFR despite
changes in systemic arterial pressure
there is negative feedback from the
JuxtaGlomerular Apparatus adjusting blood
pressure and blood volume
Glomerular Filtration Rate (GFR)
•
3 principal regulators of GFR (cont.):
2) Hormonal regulation of GFR
A. Angiotensin I & II
–
–
–
activated by renin released from JG cells and further by
ACE in the lungs
5 important functions
» direct vasoconstriction
»  aldosterone secretion
»  thirst generated at the hypothalamus
»  ADH secretion
»  Na+ reabsorption (H2O follows passively)
Net Effect  increased blood pressure and blood volume
Glomerular Filtration Rate (GFR)
•
3 principal regulators of GFR (cont.):
2) Hormonal regulation of GFR
A. Angiotensin I & II
B. Atrial Natriuretic Peptide (ANP)
–
–
–
–
secreted by cells in atria of heart in response to stretch
 GFR, promotes excretion of H2O, Na+, but retention of K+
suppresses output of ADH, aldosterone, and renin
Net Effect  decreased blood pressure and blood volume
C. Aldosterone
–
–
–
–
secreted by cells in adrenal cortex in response to
angiotensin I & II (and ACTH)
 GFR, promotes retention of H2O, Na+, but excretion of K+
antagonist to Atrial Natriuretic Peptide
Net Effect  increased blood pressure and blood volume
Glomerular Filtration Rate (GFR)
•
3 principal regulators of GFR (cont.):
3) Neural regulation
•
•
kidney’s blood vessels supplied by vasoconstrictor
fibers from Sympathetic Division of ANS which
release Norepinephrine
strong sympathetic stimulation causes JG cells to
secrete renin and the adrenal medulla to secrete
Epinephrine
GFR Control
modest
Tubular Reabsorption
• Movement of water and certain solutes back
into bloodstream from the renal tubule
– Filter 180 L/day of fluid and solutes
• nutrients (Na+, K+, Glucose, etc.) are needed by body
• body will expend ATP to get them back into blood
– about 99% of the filtrate volume is reabsorbed
from the tubule by active transport and osmosis
• Epithelial cells in PCT (microvilli) increase
surface area for tubular reabsorption
• DCT and collecting ducts play a lesser role in
nutrient/solute reabsorption
Reabsorption of Na+ in PCT
•
•
PCT is site of most
electrolyte
reabsorption
Mechanisms which aid
Na+ transport
– Na+/ K+ ATPase on
basolateral side is
fundamental
• Concentration of Na+
inside the tubular
cells is low
• Interior of the cell
negatively charged
– Double gradient for
Na+ movement from
filtrate to tubular cell
– Requires ATP energy
Reabsorption of Nutrients in PCT
•
•
•
~100% of the filtered glucose and other sugars, AA's, lactic
acid, and other useful metabolites are reabsorbed
Na+ symporters power secondary active transport systems
Why secondary? They rely on the Na+/ K+ ATPase pump.
Reabsorption of Na+ in PCT
• Na+ is passively transported from the filtrate in
tubule lumen into tubular cells to replace the Na+
being actively transported into the peritubular
capillaries. Glucose moves with Na+.
Reabsorption of H2O in PCT
• H2O follows Na+ passively by osmosis from the
filtrate through the tubular cells into the
peritubular capillaries
Reabsorption of Nutrients in PCT
• The movement of water back to the bloodstream
concentrates the remaining solutes in the filtrate
[H2O] 
[solutes] 
Reabsorption of Nutrients in PCT
• The new concentration gradients increase the
diffusion of some of the other remaining solutes in
the filtrate from lumen to the blood stream.
Transport Maximums (Tm)’s
• each type of symporter has an upper limit
(maximum) on how fast it can work
• any time a substance is in the filtrate in an
amount greater than its transport maximum,
some of it will be left behind in the urine
• only Na+ has no transport maximum because
Na+ is being actively transported by the
Na+/ K+ ATPase pump at all times.
Renal Thresholds
 The Renal Threshold is the plasma
concentration at which a substance begins to
spill into the urine because its Tm has been
surpassed.
 If the plasma filtrate concentration is too high,
all of the substance cannot be reabsorbed.
 For example, glucose spills into the urine in
untreated diabetics.
 Tm for glucose = 375 mg/min
 If blood glucose > 400 mg/100 mL, large quantities of
glucose will appear in the urine
Reabsorption in the PCT
• By the end of the PCT the following
reabsorption has occurred:
– 100% of filtered nutrients (sugars, albumin,
amino acids, vitamins, etc.)
– 80-90% of filtered HCO3– 65% of Na+ ions and water,
– 50% of Cl- and K+ ions
Reabsorption in Loop of Henle
• Cells in the thin
descending limb are only
permeable to water
• H2O reabsorption is not
coupled to reabsorption
of filtered solutes
(osmosis) in this area as
it had been in the PCT
•
[Note: illustration at right is not
thin descending limb of nephron
loop]
Reabsorption in Loop of Henle
•
•
•
Cells in the thicker
ascending Loop feature
sodium-potassium-chloride
symporters
– reabsorb 1 Na+, 1 K+, 2 Cl– depend on the low cytoplasmic
Na+ concentration to function
– little or no H2O is reabsorbed
from the thick ascending Loop
Loop reabsorbs 30% of K+,
20% of Na+, 35% of Cl-, and
15% of H2O
H2O reabsorption is not
coupled to reabsorption of
filtered solutes (osmosis)
Reabsorption in DCT and
Collecting Ducts
• Filtrate reaching the DCT has already had
~80% of the solutes and H2O reabsorbed
• Fluid now has the characteristics of “urine”
• DCT is the site of final adjustment of urine
composition
– less work to do, so no need for microvilli = brush
border to increase surface area for tranporters
– Na+/K+/Cl- symporter is a major DCT
transporter
– DCT reabsorbs another 10% of filtrate volume
Reabsorption in DCT and
Collecting Duct
• Principal cells are present in the distal DCTs
and collecting ducts
• 3 hormones act on principal cells to modify
ion and fluid reabsorption
– [1] Anti-Diuretic Hormone (ADH) (from
neurohypophysis)
•  H2O reabsorption by increasing permeability to H2O
in the DCT and collecting duct
• details discussed later on
Reabsorption in DCT and
Collecting Duct
• 3 hormones act on principal cells . . .
– [2] Aldosterone (from adrenal cortex)
•  Na+ reabsorption; Cl- and H2O follow passively;  K+
reabsorption
•  numbers of basolateral Na+/K+ ATPases
•  activity and numbers of Na+-K+ transporters and K+
channels
– [3] Atrial Natriuretic Peptide (ANP) is the
antagonist to Aldosterone
•  K+ reabsorption;  Na+ reabsorption; Cl- and H2O
follow passively; adding “salt” and water to urine
Reabsorption Summary
Loop and DCT
are sites for
additional
electrolyte
reabsorption
PCT is the
site for
reabsorption
of all nutrients
and most
electrolytes
Collecting
Ducts complete
electrolyte
reabsorption
Reabsorption in the Nephron
• Note:
reabsorption of electrolytes must
maintain an electrostatic equilibrium.
The Net Charge must remain in balance
in each fluid compartment.
• For every cation (e.g., Na+) which crosses
a membrane in a particular direction, one
of two things must also happen:
– An anion (e.g., Cl-, HCO3-) must cross the
membrane in the same direction, or
– A different cation (e.g., K+) must cross the
membrane in the opposite direction
Reabsorption in the Nephron
• Aldosterone and Atrial Natriuretic
Peptide regulate the rate of tubular
reabsorption of Na+ and Cl- and the
concurrent secretion of K+.
• Parathormone regulates the rate of
tubular reabsorption of Ca++ and Mg++ and
the concurrent secretion of HPO4-.
Fluid Reabsorption in the Nephron
•
Use GFR (mLs/min) values to track reabsorption of filtrate
Start with a GFR
of 125 mLs/min:
PCT reabsorbs
105 mLs/min and
DCT reabsorbs
19 mLs/min
leaving 1 mL/min
as urinary
output. This is
obligatory water
reabsortion.
1440 mLs/day
produced under
these “standard”
conditions.
Tubular Secretion
• Removes substances from the blood, adds
them to the filtrate
– includes H+, K+, NH4+, HPO4-, creatinine, plant
alkaloids (toxins), penicillin and other drugs
• Two primary functions:
– Helps rid body of certain routinely generated
waste substances and toxins
– Regulates blood pH by secretion of H+ (and to a
lesser degree, reabsorption of HCO3-)
Secretion of K+ ions
• Principal cells+ in collecting ducts secrete variable
amount of K in exchange for reabsorbed Na+
• Most animal diets contain excess K+ but scarce Na+
• Na+/K+ ATPases are the “ion pumps”
• Controlled by Aldosterone and Atrial Natriuretic
Peptide
• Aldosterone is released from the Adrenal Cortex in response to
Angiotensin I & II
• With excess K+, Aldosterone secretion predominates: Na+
(and Cl-) are reabsorbed while considerable K+ is secreted
• Atrial Natriuretic Peptide is released from the Atrial walls in the
heart in response to stretching when blood volume or blood
pressure increase
• With excess Na+, Atrial Natriuretic Peptide secretion
predominates: K+ is reabsorbed while considerable Na+ (and
Cl-) are secreted
Secretion of H+ ions
• Cells of the renal tubule can elevate blood
pH in 3 ways:
– Secrete H+ ions into the filtrate
– Reabsorb filtered HCO3– Produce more HCO3-
• The key is the chemical relationship
between H+ ions and HCO3- ions:
–
H2O + CO2  H2CO3  H+ + HCO3– This reaction occurs spontaneously and it is also
catalyzed by the enzyme carbonic anhydrase.
Secretion of H+ ions
• In PCT
– [1] Na+/H+ antiporter puts H+ ions into the filtrate
– H+ ions combine with HCO3- in lumen to form CO2 and H2O
1
H+
1
HCO3-
Secretion of H+ ions
• In PCT
– [2] CO2 from the filtrate or plasma enters the tubular
cell where it combines with H2O to form H2CO3
H+
HCO3-
2
2
Secretion of H+ ions
• In PCT
– [3] H+ is pumped into the lumen
– [4] H2CO3- follows pumped Na+ back to the bloodstream
H+
3
HCO3-
HCO3-
4
Secretion of H+ ions
• Collecting ducts also
secrete H+ ions
– H+ pumps are a primary
active transport process
powered by ATPs
– generate as much as a 1000
fold concentration gradient
 strongly acid urine
– new bicarbonate ions are
reabsorbed by the
basolateral HCO3-/Clantiporter
– adding new HCO3- buffer to
the bloodstream
HCO3H+
Secretion of NH3 and NH4+
• Ammonia is a toxic waste absorbed from
bacterial metabolism in the large intestine
and ammonia is generated from the
deamination of amino acids in the liver
• Liver converts ammonia to urea, a much less
toxic nitrogenous waste
• PCT cells can also deaminate certain amino
acids and secrete additional NH4+ with a
Na+/NH4+ antiporter when blood pH becomes
acidic
Summary of
Nephron
Functions
GFR  125 mL/min
Summary
of
Nephron
Functions
PCT
reabsorbs
nutrients,
electrolytes,
and water
Summary of
Nephron Functions
Loop also
reabsorbs some
electrolytes and
water
Summary
of
Nephron
Functions
DCT and Collecting
Ducts continue the
absorption of water and
electrolyte, especially
Na+ and HCO3-;
DCT and CDs also
secrete K+ and H+ and
ammonia ions into the
filtrate
Summary of
Nephron
Functions
The final
process to
discuss is
regulation of
water balance –
making a dilute
or concentrated
urine.
Nephron Reaborbs ~99% of H2O
• Water balance determines the fate of the last 1%!
Start with a GFR
of 125 mLs/min:
PCT reabsorbs
105 mLs/min and
DCT reabsorbs
19 mLs/min
leaving 1 mL/min
as urinary
output.
1440 mLs/day
produced under
these “standard”
conditions.
~1 mL/Min
is adjusted
as needed
by ADH.
That is
facultative
water
reabsorption.
Adjusting Water Balance
• Distal tubular cells and
•
•
cells in the collecting
ducts expend ATP energy
to create an osmotic
gradient between the
cortex and medulla of the
kidney
The key substances
transported are urea and
NaCl
Countercurrent flow
mechanisms maintain the
osmotic gradient
Countercurrent Flow Mechanisms
• Compare to a system
•
•
•
•
of co-current flow:
two pipes are semipermeable
the fluids flow in the
same direction
solutes will diffuse
along concentration
gradients
solutes will all reach
equilibrium values
Countercurrent Flow Mechanisms
• In a system of
•
•
•
•
•
countercurrent flow:
two pipes are still
semi-permeable
but the fluids flow in
opposite directions
solutes again diffuse
along concentration
gradients
the gradient always
favors transfer
solutes do not reach
equilibrium values
Countercurrent Flow Mechanisms
• Countercurrent flow is
•
•
•
•
seen in a variety of
physiological systems:
How do penguins stand
in freezing water in
their bare feet?
blood flows in opposite
directions
heat is transferred
along the heat gradient
most of the heat
moves from arterial to
venous blood and is not
lost to the water
Countercurrent Flow Mechanisms
• Countercurrent flow is
•
•
•
•
seen in a variety of
physiological systems:
How do fish gills
oxygenate blood?
blood flows in opposite
directions
O2 is transferred along
the O2 gradient
O2 continues to move
from water to the
blood and the gradient
is always favorable
O2
Nephron’s Countercurrents
• Renal tubule has a
more complicated
system of countercurrent flow:
 PCT & descending
Loop vs. ascending
Loop and DCT
 arterial vasa recta
vs. venous vasa recta
 Renal tubule versus
vasa recta
• This system
permits the osmotic
gradient to develop
DCT
PCT
ducts
Loop
Nephron’s Countercurrents
• complex countercurrent
flow between the
juxtamedullary nephrons
and their vasa recta
– [1] the entire flow in the
renal tubule (loop) is
countercurrent to the flow
in the vasa recta
– [2] each U-shaped vessel
also has countercurrent
flow between its descending
and ascending limbs
1
2
Nephron’s Countercurrents
• in the medulla, urea
•
•
and NaCl are actively
transported from the
vessels exiting the
medulla
this increases the
concentration of urea
and NaCl in the medulla
although urea and NaCl
can diffuse into the
vessels entering the
medulla, they do not
carry the solutes away
Nephron’s Countercurrents
• the countercurrent
•
•
•
flow is in a loop
the active transport
pumps work at all times
therefore, the solutes
accumulate at the
bottom of the loop
the vasa recta carry
the water back to the
medulla and, thus, back
to the body
Nephron’s Countercurrents
• the combination of complex countercurrent flow
and the active transport pumping of urea and NaCl
maintain the osmotic gradient between the cortex
and the medulla at all times
Adjusting Water Balance
• water conservation is dependent on ADH
• normal osmotic concentration in the body fluids,
•
•
•
plasma and interstitial fluids, including the kidney’s
cortex, is ~300 mOsm/L
glomerular filtrate is isosmotic to plasma
thick limb of the ascending Loop is impermeable to
water but urea and Na+/Cl- ions are actively
transported out of the filtrate
DCT and collecting ducts are impermeable to water
unless ADH is present
Producing a Dilute Urine
• With adequate H2O,
•
•
the posterior pituitary
releases little ADH
the glomerular filtrate
equilibrates with
medullary conditions
while passing down
through the loop
meanwhile, tubular
reabsorption of solutes
continues
Producing a Dilute Urine
• As the filtrate enters
•
the ascending limb of
the Loop, and the DCT,
and then the collecting
ducts, no water can
diffuse out of the
filtrate
Meanwhile, continuing
tubular reabsorption of
solutes in the DCT &
CDs creates a dilute =
hypo-osmotic
(hypotonic) urine
Producing a Concentrated Urine
• with inadequate H2O,
•
•
the posterior pituitary
releases more ADH
the glomerular filtrate
equilibrates with
medullary conditions
while passing down
through the loop
meanwhile, tubular
reabsorption of solutes
continues
Producing a Concentrated Urine
• the ascending limb of
•
•
the Loop remains
impermeable to H2O
however, as the
filtrate enters the
DCT, and then the
collecting ducts, ADH
causes the tubular
cells to become
permeable to H2O
water can diffuse in or
out of the filtrate
Producing a Concentrated Urine
• the filtrate becomes
•
hypo-osmotic
(hypotonic) in the DCT
while H2O and solutes
are returned to the
bloodstream
however, the filtrate
equilibrates with
medullary conditions
while passing down
through the collecting
ducts
Producing a Concentrated Urine
•
•
•
even though the filtrate is
still losing urea and NaCl to
active transport, the other
solutes cannot leave
the filtrate becomes
hyper-osmotic (hypertonic)
as it equilibrates with the
osmotic gradient
surrounding the collecting
ducts
water is drawn into the
vasa recta and back to the
bloodstream
Producing a Concentrated Urine
• The effect of ADH is to create a concentrated =
hyper-osmotic (hypertonic) urine
The Final Common Pathway
• Ureters
– extensions of the
renal pelvis
– enter the bladder
medially from the
posterior
• Histology - 3 layers
– inner mucosa lined
with transitional
epithelium
– muscularis – smooth
muscle in circular and
longitudinal layers
– retroperitoneal
(serosa or adventitia)
• Physiology
– transport urine to the
bladder
– peristalsis primarily, but
hydrostatic pressure of
gravity helps in humans
The Final Common Pathway
• Urinary bladder
– hollow muscular organ
– generally smaller in females due to presence of a uterus
– retroperitoneal in the pelvic cavity, posterior to the
pelvic symphysis
– freely movable
• Structure - trigone
The Final Common Pathway
• Bladder histology
– inner mucosa lined with
transitional epithelium
– muscularis – smooth
muscle in three layers
– Sphincters control
entry from ureters and
exit at the urethra
• circular smooth muscle
fibers form internal
urethral sphincter
• lower is the external
urethral sphincter with
skeletal muscle for
voluntary control
– retroperitoneal (serosa
or adventitia)
The Final Common Pathway
Urethra routed
differently in males
and females – see
chapter 28
The Final Common Pathway
• Urethra
– small tube from floor of bladder to exterior of body
• females -- fairly straight path exits anterior to vagina
• males -- passes through the prostate gland and exits through
the penis
– histology
• female: three coats
– inner mucosa, intermediate thin layer of spongy tissue with plexus
of veins
– outer muscular coat continuous with the bladder
• male two layers
– inner mucous membrane and a muscularis
– outer submucosa tissue with various accessory structures which
connect to it
• both genders have a stratified squamous epithelial lining
The Final Common Pathway
• Urethra
– Physiology - terminal portion of urinary tract, in
males the urethra also serves as the duct
through which semen is discharged from the
body
• Urine
– Volume
• 1000-2000 ml/day
• influenced by blood pressure, blood osmotic pressure,
temperature, mental state, general health, diet,
diuretics, other drugs
– Chemical Composition - 95% water, 5% solutes
Micturition
A
•
B
2
Voluntary and involuntary
(ANS) nerve impulses
control the process
1. 700-800 mL capacity
2. when volume > 200-400 mL,
stretch receptors fire
3. processed in cortex
a) micturition reflex
b) initiates a conscious desire
to expel urine
1
3
4. parasympathetic commands
coordinate the process
5. contraction of detrusor
(bladder), relaxation of
internal sphincter
End Chapter 25