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8/2/2019 Excretion I
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Renal
vein
Inferior
vena cava
Urinary
bladder
Urethra
Renal
artery
Kidney
Aorta
Ureter
Renal
pyramid
Renal
cortex
Renal
medulla
Renal
pelvis
Ureter
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Figure 26.4a, b
Structure of the Kidney
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Nephron
Afferent arteriole
Efferent arteriole
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Blood Supply to Kidney
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Two Capillary system
Glomerular capillary system - high
pressure, filtration system
Peritubular capillaries - low pressure,
absorptive system
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The nephron consists of a renal
corpuscle and renal tubule
The renal corpuscle is composed of
Bowmans capsule and the glomerulus
The renal tubule consists of
Proximal convoluted tubule (PCT)
Loop of Henle: Thin descending limb, Thin ascending limband Thick ascending limb
Distal convoluted tubule (DCT)
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Cortical nephrons
~85% of all nephrons
Located in the cortex Juxtamedullary nephrons
Closer to renal medulla
Loops of Henle extenddeep into renal pyramids
Two types of nephron
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Urine Formation
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Figure 14.8 (1)
Page 518
Afferent arteriole Efferent arteriole
Glomerulus
Bowmans
capsule
Lumen of
Bowmans
capsule
Outer layer of
Bowmans capsule
Inner layer
of Bowmans capsule
(podocytes)
Proximal convoluted tubule
Lumen of
glomerular
capillary
Endothelial
cell
Basement
membrane
Podocyte
foot process
(see
nextslide)
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Podocyte
foot process
Filtrationslit
Basement
membrane
Capillarypore
(see next slide)3 Layers of Glomerular Capillary Membrane
Endothelial layer of capillaries
Basement membrane
Capsular layer with podocytes
Glomerular ultrafiltration Membrane
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Podocyte
foot process
Filtration
slit
Basement
membrane
Capillary
pore
Endothelial
cell
Lumen of glomerular
capillary
Lumen of
Bowmans capsule
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Mesangial Cells
Intra-mesangial cells liebetween capillary tuft and
provide support for
glomeruli. They secrete a
substance similar to basement
mebrane.
Extra mesnagial cells have
contractile properties in
response to neurohormonal
substance which regulateblood flow in glomerulus.
They are also phagocytic in
nature.
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Glomerular filtration Movement of fluid through the glomerular capillaries is determined
by capillary pressure (60 mm Hg), colloidal osmotic pressure, andcapillary permeability.
125 ml of filtrate is formed each minute - (GFR) which can vary
from a few milliliters per minute to as high as 200 ml/minute.
Constriction of the efferent arteriole increases resistance to outflowfrom the glomeruli and increases the glomerular pressure and the
GFR. Constriction of the afferent arteriole causes a reduction in the
renal blood flow, glomerular filtration pressure, and GFR.
Both, afferent and the efferent arterioles are innervated by thesympathetic nervous system and are sensitive to vasoactive
hormones, such as angiotensin II.
Strong sympathetic stimulation, such as shock, constriction of the
afferent arteriole causes a marked decrease in renal blood flow and
thus glomerular filtration pressure & urine output can be zero.
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Facilitated diffusion and Passive transport
Primary active transport
Secondary active transport
Cotransport (Symporter)
Countertransport (Antiporter)
Reabsorption in the kidneys
occurs by different mechanisms
R b i i PCT
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Reabsorption in PCT
65% of reabsorption and
secretion occurs in PCT.
Glucose amino acids, lactateand water soluble vitamins,
ions such Na+, Cl-, K+,
HCO3- completely
reabsorbed.
As these solutes move into the
tubular cells, their
concentration in the tubular
lumen decreases, providing a
concentration gradient for theosmotic reabsorption of water
and urea.
PCT secretes H+ and organic
compounds such as penicillin,
aspirin, morphine.
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Reabsorption of Bicarbonate & Na+ &
Secretion of H + Ions
Na+ antiporters reabsorb Na+
and secrete H+
PCT cells produce the H+ &
release bicarbonate ion to theperitubular capillaries
important buffering system
For every H+ secreted into the
tubular fluid, one filtered
bicarbonate eventually returns
to the blood
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Reabsorption in the PCT Na+ symporters help reabsorb
materials from the tubular filtrateand each type of symporter has an
upper limit on how fast it can
work, called the transport
maximum (Tm).
The maximum amount of
substance that these transport
systems can reabsorb per unit
time is called the transport
maximum.
Tm related elated to the number
of carrier proteins that are
available for transport.
Reabsorption of Nutrients
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Tm determines renal threshold for reabsorption of substances in
tubular fluid
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Symporters in the Loop of Henle
Thin descending limb is
highly permeable to waterand moderately permeable to
urea, sodium, and other ions
Thick ascending limb is
impermeable to water & hasNa+ K- Cl- symporters that
reabsorb these ions.
About 20% to 25% of the
filtered load of sodium,potassium, and chloride is
reabsorbed in loop of Henle.
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Reabsorption in the DCT & Collecting Duct
DCT is relatively impermeable to water
but removal of Na+ and Cl- (5%)
continues in the DCT by means of Na+
Cl- symporters
Ca++ actively reabsorbed under the
influence of parathyroid hormone and
vitamin D. ADH exerts its action on DCT.
Late part of DCT and collecting duct are
the sites for aldosterone action .
Two types of cells:
- principal cells reabsorb Na+ and secrete
K+ under the influence of aldosterone
- intercalated cells reabsorb HCO3- ions in
exchange for H+
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Formation of Concentrated Urine
Urine can be up to 4 times greater osmolarity than plasma It is possible for principal cells & ADH to remove water from urine
to that extent, if interstitial fluid surrounding the loop of Henle has
high osmolarity
Long loop juxtamedullary nephrons and Na+/K+/Cl- makethat possible
Two factors contribute to building and maintaining the osmotic
gradient:
Difference in solute & water reabsorption in differentsections of the tubule
Countercurrent flow
Urea recycling causes a buildup of urea in the renal medulla
F i f C U i ADH
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Formation of Con. Urine: ADH
Increases water permeability ofprincipal cells so regulatesfacultative water reabsorption
Stimulates the insertion of
aquaporin-2 channels into themembrane
water molecules move morerapidly
When osmolarity of plasma &interstitial fluid increases, moreADH is secreted and facultativewater reabsorption increases.
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Tubular
lumen
filtrate Distal tubular cell
Peritubular
capillary
plasma
Water
channel
Increases permeability of
luminal membrane to H2O
by inserting newwater channels
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Countercurrent Mechanism Descending limb is very permeable to water
higher osmolarity of interstitial fluid outside thedescending limb causes water to mover out of the tubuleby osmosis
at hairpin turn, osmolarity can reach 1200 mOsm/liter
Ascending limb is impermeable to water, but symportersremove Na+ and Cl- so osmolarity drops to 100mOsm/liter, but less urine is left
Vasa recta blood flowing in opposite directions than theloop of Henle -- provides nutrients & O2 without affectingosmolarity of interstitial fluid
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Reabsorption within Loop of Henle
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