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Chapter 17
Physiology of
the Kidneys
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17-1
Chapter 17 Outline
Overview
of Kidney Structure
Nephron
Glomerular
Filtration
Function of Nephron Segments
Renal Clearance
Hormonal Effects
Na+, K+, H+, and HCO3- Relationships
Clinical Aspects
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17-2
Kidney Function
Is
to regulate plasma and interstitial fluid by formation
of urine
In process of urine formation, kidneys regulate:
Volume of blood plasma, which contributes to BP
Waste products in blood
Concentration of electrolytes
Including Na+, K+, HCO3-, and others
Plasma pH
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17-3
Overview of Kidney Structure
17-4
Structure of Urinary System
Paired
kidneys are
on either side of
vertebral column
below diaphragm
About size of fist
Urine flows from
kidneys into ureters
which empty into
bladder
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17-5
Structure of Kidney
 Cortex
contains many capillaries and outer parts of nephrons
 Medulla consists of renal pyramids separated by renal columns
 Pyramid contains minor calyces which unite to form a major
calyx
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17-6
Structure of Kidney continued
 Major
calyces join
to form renal pelvis
which collects urine
 Conducts urine
to ureters which
empty into
bladder
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17-7
Micturition Reflex (Urination)
Bladder
has a smooth muscle wall called the detrussor
muscle
Stretch can cause spontaneous APs and contraction
Also innervated and controlled by parasympathetic
Drugs for overactive bladders target muscarinic
receptors
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17-8
Micturition Reflex (Urination) continued
Actions
of internal and external urethral sphincters are
regulated by reflex center located in sacral part of cord
Filling of bladder activates stretch receptors that send
impulses to micturition reflex center
This activates Parasymp neurons causing
contraction of detrusor muscle that relaxes internal
urethral sphincter creating sense of urgency
There is voluntary control over external urethral
sphincter
When urination is consciously initiated, descending
motor tracts to micturition center inhibit somatic motor
fibers of external urethral sphincter and urine is
expelled
17-9
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Nephron
17-10
Nephron
Is
functional unit of kidney; responsible for forming
urine
>1 million nephrons/kidney
Is a long tube and has associated blood vessels
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17-11
Renal Blood Vessels
Blood
enters kidney
through renal artery
Which divides
into interlobar
arteries
That divide
into arcuate
arteries that
give rise to
interlobular
arteries
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17-12
Renal Blood Vessels continued
Interlobular
arteries give rise to afferent arterioles
which supply glomeruli
Glomeruli are mass of capillaries inside glomerular
capsule that gives rise to filtrate that enters nephron
tubule
Efferent arteriole drains glomerulus and delivers that
blood to peritubular capillaries (vasa recta)
Blood from peritubular capillaries enters veins
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17-13
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17-14
Nephron Tubules
 Tubular
part of nephron begins with glomerular capsule which
transitions into proximal convoluted tubule (PCT), then to
descending and ascending limbs of Loop of Henle (LH), and
distal convoluted tubule (DCT)
 Tubule ends where it empties into collecting duct (CD)
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17-15
Glomerular (Bowman's) Capsule
Surrounds
glomerulus
Together they
form renal
corpuscle
Is where glomerular
filtration occurs
Filtrate passes
into PCT
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17-16
Proximal Convoluted Tubule
Walls
consist of single layer of cuboidal cells with
millions of microvilli
Which increase surface area for reabsorption
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17-17
Type of Nephrons
 Cortical
nephrons
originate in outer 2/3
of cortex
 Juxtamedullary
nephrons originate
in inner 1/3 cortex
 Have long LHs
 Important in
producing
concentrated
urine
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17-18
Glomerular Filtration
17-19
Glomerular Filtration
Glomerular
capillaries and Bowman's capsule form a
filter for blood
Glomerular Caps are fenestrated--have large pores
between its endothelial cells
Big enough to allow any plasma molecule to pass
100-400 times more permeable than other Caps
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17-20
Glomerular Filtration continued
To
enter tubule
filtrate must
pass through
narrow slit
diaphragms
formed
between
pedicels (foot
processes) of
podocytes of
glomerular
capsule
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17-21
Glomerular Filtration continued
Plasma
proteins are mostly excluded from the filtrate
because of large size and negative charge
The slit diaphragms are lined with negative charges
which repel negatively-charged proteins
Some protein (especially albumin) normally enters
the filtrate but most is reabsorbed by receptormediated endocytosis
In some diseases, a lot of protein appears in the
urine (=proteinuria)
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17-22
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17-23
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17-24
Glomerular Ultrafiltrate
Is
fluid that
enters
glomerular
capsule, whose
filtration was
driven by blood
pressure
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17-25
Glomerular Filtration Rate (GFR)
Is
volume of filtrate produced by both kidneys/min
Averages 115 ml/min in women; 125 ml/min in men
Totals about 180L/day (45 gallons)
So most filtered water must be reabsorbed or
death would ensue from water lost through
urination
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17-26
Regulation of GFR
Is
controlled by extrinsic and intrinsic (autoregulation)
mechanisms
Vasoconstriction or dilation of afferent arterioles affects
rate of blood flow to glomeruli and thus GFR
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17-27
Sympathetic Effects
Sympathetic
activity constricts
afferent arteriole
Helps
maintain BP
and shunts
blood to heart
and muscles
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17-28
Renal Autoregulation
Allows
kidney to maintain a constant GFR over wide
range of BPs
Achieved via effects of locally produced chemicals on
afferent arterioles
When average BP drops to 70 mm Hg afferent
arteriole dilates
When average BP increases, afferent arterioles
constrict
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17-29
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17-30
Renal Autoregulation continued
Is
also maintained by negative feedback between
afferent arteriole and volume of filtrate
(tubuloglomerular feedback)
Increased flow of filtrate sensed by macula densa
(part of juxtaglomerular apparatus) in thick
ascending LH
Signals afferent arterioles to constrict
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17-31
Function of Nephron Segments
17-32
Reabsorption of Salt and H2O
In
PCT returns most molecules and H2O from filtrate
back to peritubular capillaries
About 180 L/day of ultrafiltrate produced; only 1–2 L
of urine excreted/24 hours
Urine volume varies according to needs of body
Minimum of 400 ml/day urine necessary to
excrete metabolic wastes (obligatory water loss)
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17-33
Reabsorption of Salt and H2O continued
 Return
of filtered
molecules is called
reabsorption
 Water is never
transported
 Other molecules
are transported
and water follows
by osmosis
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17-34
PCT
 Filtrate
in PCT is
isosmotic to blood (300
mOsm/L)
 Thus reabsorption of H2O
by osmosis cannot occur
without active transport
(AT)
 Is achieved by AT of
Na+ out of filtrate
 Loss of + charges
causes Cl- to
passively follow
Na+
 Water follows salt
by osmosis
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17-35
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17-36
Significance of PCT Reabsorption
Na+, Cl-, and H2O is reabsorbed in PCT and
returned to bloodstream
An additional 20% is reabsorbed in descending loop of
Henle
Thus 85% of filtered H2O and salt are reabsorbed early
in tubule
This is constant and independent of hydration levels
Energy cost is 6% of calories consumed at rest
The remaining 15% is reabsorbed variably,
depending on level of hydration
~65%
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17-37
Concentration Gradient in Kidney
In
order for H2O to be reabsorbed, interstitial fluid must
be hypertonic
Osmolality of medulla interstitial fluid (1200-1400
mOsm) is 4X that of cortex and plasma (300 mOsm)
This concentration gradient results largely from loop
of Henle which allows interaction between
descending and ascending limbs
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17-38
Descending Limb LH
 Is
permeable to H2O
 Is impermeable to, and
does not AT, salt
 Because deep regions
of medulla are 1400
m Osm, H2O diffuses
out of filtrate until it
equilibrates with
interstitial fluid
 This H2O is
reabsorbed by
capillaries
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17-39
Ascending Limb LH
 Has
a thin segment in
depths of medulla and
thick part toward
cortex
 Impermeable to H2O;
permeable to salt;
thick part ATs salt out
of filtrate
 AT of salt causes
filtrate to become
dilute (100
mOsm) by end of
LH
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17-40
AT in Ascending Limb LH
 NaCl
is actively
extruded from thick
ascending limb into
interstitial fluid
 Na+ diffuses into
tubular cell with
secondary active
transport of K+ and
Cl Occurs at a ratio of 1
Na+ and 1 K+ to 2 Cl-
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17-41
AT in Ascending Limb LH continued
 Na+
is AT across
basolateral
membrane by Na+/ K+
pump
 Cl- passively follows
Na+ down electrical
gradient
 K+ passively diffuses
back into filtrate
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17-42
Countercurrent Multiplier System
 Countercurrent
flow and proximity allow descending and
ascending limbs of LH to interact in way that causes osmolality
to build in medulla
 Salt pumping in thick ascending part raises osmolality around
descending limb, causing more H2O to diffuse out of filtrate
 This raises osmolality of filtrate in descending limb which
causes more concentrated filtrate to be delivered to
ascending limb
 As this concentrated filtrate is subjected to AT of salts, it
causes even higher osmolality around descending limb
(positive feedback)
 Process repeats until equilibrium is reached when osmolality
of medulla is 1400
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17-43
Vasa Recta
 Is
important component of
countercurrent multiplier
 Permeable to salt, H2O (via
aquaporins), and urea
 Recirculates salt, trapping
some in medulla interstitial
fluid
 Reabsorbs H2O coming out
of descending limb
 Descending section has
urea transporters
 Ascending section has
fenestrated capillaries
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17-44
Effects of Urea
Urea
contributes
to high osmolality
in medulla
Deep region of
collecting duct
is permeable
to urea and
transports it
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17-45
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17-46
Collecting Duct (CD)
Plays
important role in water conservation
Is impermeable to salt in medulla
Permeability to H2O depends on levels of ADH
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17-47
ADH
 Is
secreted by post
pituitary in response to
dehydration
 Stimulates insertion of
aquaporins (water
channels) into plasma
membrane of CD
 When ADH is high, H2O
is drawn out of CD by
high osmolality of
interstitial fluid
 And reabsorbed by
vasa recta
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17-48
Osmolality of Different Regions of the Kidney
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17-49
Renal Clearance
17-50
Renal Clearance
Refers
to ability of kidney to remove substances from
blood and excrete them in urine
Occurs by filtration and by secretion
Secretion is opposite of reabsorption--substances from
vasa recta are transported into tubule and excreted
Reabsorption decreases renal clearance; secretion
increases clearance
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17-51
Renal Clearance
Excretion
rate =
(filtration rate + secretion rate) - reabsorption rate
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17-52
Secretion of Drugs
Many
drugs, toxins, and metabolites are secreted by
membrane transporters in the PCT
These transport organic anion and cation molecules
And determine the half-life of many therapeutic
drugs
Many foreign molecules (xenobiotics) are eliminated
by this system at a more rapid rate than by glomerular
filtration
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17-53
Inulin Measurement of GFR
 Inulin,
a fructose polymer, is useful for measuring GFR because
is neither reabsorbed or secreted
 Rate at which a substance is filtered by the glomeruli can be
calculated:
 Quantity filtered = GFR x P
 P = inulin concentration in plasma
 Quantity excreted (mg/min) = V x U
 V = rate of urine formation; U = inulin concentration in urine
 Amount filtered = amount excreted
GFR = V x U
P
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17-54
Renal Clearance of Inulin
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17-55
Renal Plasma Clearance (RPC)
Is
volume of plasma from which a substance is
completely removed/min by excretion in urine
If substance is filtered but not reabsorbed then all
filtered will be excreted RPC = GFR
If substance is filtered and reabsorbed then RPC <
GFR
If substance is filtered but also secreted and excreted
then RPC will be > GFR (=120 ml/ min)
RPC = V x U
P
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17-56
Clearance of Urea
 Urea
is freely filtered into glomerular capsule
 Urea clearance calculations demonstrate how kidney handles a
substance: RPC = V X U/P
 V = 2ml/min; U = 7.5 mg/ml of urea; P = 0.2 mg/ml of urea
 RPC = (2ml/min)(7.5mg/ml)/(0.2mg/ml) = 75ml/min
 Urea clearance is 75 ml/min, compared to clearance of inulin
(120 ml/min)
 Thus 40-60% of filtered urea is always reabsorbed
 Is passive process because of presence of carriers for
facilitative diffusion of urea
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17-57
Measurement of Renal Blood Flow
Not
all blood delivered to glomerulus is filtered into
glomerular capsule
20% is filtered; rest passes into efferent arteriole
and back into circulation
Substances that aren't filtered can still be cleared by
active transport (secretion) into tubules
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17-58
Total Renal Blood Flow Using PAH
 PAH
clearance is used to measure total renal blood flow
 Normally averages 625 ml/min
 It is totally cleared by a single pass through a nephron
 So it must be both filtered and secreted
 Filtration and secretion clear only molecules dissolved in
plasma
 To get total renal blood flow, amount of blood occupied by
erythrocytes must be taken into account
 45% blood is RBCs; 55% is plasma
  total renal blood flow = PAH clearance
 = 625/0.55 = 1.1L/min
0.55
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17-59
Total Renal Blood Flow Using PAH continued
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17-60
Glucose and Amino Acid Reabsorption
Filtered
glucose and amino acids are normally 100%
reabsorbed from filtrate
Occurs in PCT by carrier-mediated cotransport with
Na+
Transporter displays saturation if ligand
concentration in filtrate is too high
 Level needed to saturate carriers and achieve
maximum transport rate is transport maximum
(Tm)
Glucose and amino acid transporters don't saturate
under normal conditions
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17-61
Glycosuria
Is
presence of glucose in urine
Occurs when glucose > 180-200mg/100ml plasma
(= renal plasma threshold)
Glucose is normally absent because plasma levels
stay below this value
Hyperglycemia has to exceed renal plasma
threshold
Diabetes mellitus occurs when hyperglycemia
results in glycosuria
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17-62
Hormonal Effects
17-63
Electrolyte Balance
regulate levels of Na+, K+, H+, HCO3-, Cl-, and
PO4-3 by matching excretion to ingestion
Control of plasma Na+ is important in regulation of
blood volume and pressure
Control of plasma of K+ is important in proper function
of cardiac and skeletal muscles
Kidneys
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17-64
Role of Aldosterone in Na+/K+ Balance
filtered Na+ and K+ reabsorbed before DCT
Remaining is variably reabsorbed in DCT and
cortical CD according to bodily needs
Regulated by aldosterone (controls K+ secretion
and Na+ reabsorption)
In the absence of aldosterone, 80% of remaining
Na+ is reabsorbed in DCT and cortical CD
When aldosterone is high all remaining Na+ is
reabsorbed
90%
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17-65
K+ Secretion
only way K+ ends
up in urine
 Is directed by
aldosterone and
occurs in DCT and
cortical CD
 High K+ or low
Na+ will increase
aldosterone and
K+ secretion
 Is
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17-66
Juxtaglomerular Apparatus (JGA)
Is
specialized region in each nephron where afferent
arteriole comes in contact with thick ascending limb LH
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17-67
Renin-Angiotensin-Aldosterone System
Is
activated by release of renin from granular cells
within afferent arteriole
Renin converts angiotensinogen to angiotensin I
Which is converted to Angio II by angiotensinconverting enzyme (ACE) in lungs
Angio II stimulates release of aldosterone
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17-68
Regulation of Renin Secretion
Inadequate
intake of NaCl always causes decreased
blood volume
Because lower osmolality inhibits ADH, causing less
H2O reabsorption
Low blood volume and renal blood flow stimulate
renin release
Via direct effects of BP on granular cells and by
Symp activity initiated by arterial baroreceptor
reflex (see Fig 14.26)
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17-69
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17-70
Macula Densa
 Is
region of
ascending limb in
contact with afferent
arteriole
 Cells respond to
levels of Na+ in
filtrate
 Inhibit renin
secretion when
Na+ levels are
high
 Causing less
aldosterone
secretion, more
Na+ excretion
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17-71
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17-72
Atrial Natriuretic Peptide (ANP)
Is
produced by atria due to stretching of walls
Acts opposite to aldosterone
Stimulates salt and H2O excretion
Acts as an endogenous diuretic
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17-73
Na+, K+, H+, and HCO3- Relationships
17-74
Na+, K+, and H+ Relationship
 Na+
reabsorption in
DCT and CD creates
electrical gradient for
H+ and K+ secretion
 When extracellular H+
increases, H+ moves
into cells causing K+ to
diffuse out and vice
versa
 Hyperkalemia can
cause acidosis
 In severe acidosis, H+ is
secreted at expense of
K+
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17-75
Renal Acid-Base Regulation
help regulate blood pH by excreting H+ and/or
reabsorbing HCO3Most H+ secretion occurs across walls of PCT in
exchange for Na+ (Na+/H+ antiporter)
Normal urine is slightly acidic (pH = 5-7) because
kidneys reabsorb almost all HCO3- and excrete H+
Kidneys
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17-76
Reabsorption of HCO3- in PCT
Is
indirect because apical membranes of PCT cells are
impermeable to HCO3-
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17-77
Reabsorption of HCO3- in PCT continued
urine is acidic, HCO3- combines with H+ to form H2CO3
(catalyzed by CA on apical membrane of PCT cells)
 H2CO3 dissociates into CO2 + H2O
 CO2 diffuses into PCT cell and forms H2CO3 (catalyzed by CA)
 H2CO3 splits into HCO3- and H+ ; HCO3- diffuses into blood
 When
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17-78
Urinary Buffers
Nephron
cannot produce urine with pH < 4.5
Excretes more H+ by buffering H+s with HPO4-2 or NH3
before excretion
Phosphate enters tubule during filtration
Ammonia produced in tubule by deaminating amino
acids
Buffering reactions
 HPO4-2 + H+  H2PO4 NH3 + H+  NH4+ (ammonium ion)
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17-79
Clinical Aspects
17-80
Diuretics
 Are
used to lower blood volume because of hypertension,
congestive heart failure, or edema
 Increase volume of urine by increasing proportion of glomerular
filtrate that is excreted
 Loop diuretics are most powerful; inhibit AT salt in thick
ascending limb of LH
 Thiazide diuretics inhibit NaCl reabsorption in 1st part of DCT
 Carbonic anhydrase inhibitors prevent H2O reabsorption in PCT
when HCOs- is reabsorbed
 Osmotic diuretics increase osmotic pressure of filtrate
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17-81
Sites of Action of Clinical Diuretics
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17-82
Kidney Diseases
In
acute renal failure, ability of kidneys to excrete
wastes and regulate blood volume, pH, and
electrolytes is impaired
Rise in blood creatinine and decrease in renal
plasma clearance of creatinine
Can result from atherosclerosis, inflammation of
tubules, kidney ischemia, or overuse of NSAIDs
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17-83
Kidney Diseases continued
Glomerulonephritis
is inflammation of glomeruli
Autoimmune attack against glomerular capillary
basement membranes
Causes leakage of protein into urine resulting in
decreased colloid osmotic pressure and resulting
edema
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17-84
Kidney Diseases continued
 In
renal insufficiency, nephrons have been destroyed as a result
of a disease
 Clinical manifestations include salt and H2O retention and
uremia (high plasma urea levels)
 Uremia is accompanied by high plasma H+ and K+ which
can cause uremic coma
 Treatment includes hemodialysis
 Patient's blood is passed through a dialysis machine
which separates molecules on basis of ability to diffuse
through selectively permeable membrane
 Urea and other wastes are removed
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