Transcript Renal tubular transp..

Renal tubular
reabsorption/Secretion
Urine Formation Preview
Renal tubular reabsorption/Secretion
Excretion
refers to the removal of solutes and
water from the body in urine
Reabsorption (movement from tubular fluid to
peritubular blood) and,
 Secretion (movement from peritubular blood
to tubular fluid) refer to direction of movement of
solutes and water across the renal tubular
epithelium
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Renal tubular reabsorption through PCT
Solute
reaborption in the proximal tubule is
isosmotic (water follows solute osmotically and
tubular fluid osmolality remains similar to that of
plasma).
65%-70% of water and sodium reabsorption
occurs in the proximal tubule
 90% of bicarbonate, calcium, K+
 100% of glucose & amino acids
Proximal tubules: coarse adjustment
Distal tubules: fine adjustment (hormonal
control).
Loop of Henle
Responsible for producing a concentrated urine
by forming a concentration gradient within the
medulla of kidney.
 When ADH is present, water is reabsorbed and
urine is concentrated.
 Counter-current multiplier

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Loop of Henle:
Acts in manner of
counter current
exchanger. Note that
each limb of loop has
fluid moving in opposite
directions (even though
connected at one end).
Further concentrates
urine.
Also means that salt
concentration will be
highest near bend in the
loop.
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Absorption through loop of Henle:
Descending limb: is water permeable and allow
absorption of 25% of filtered H2O. It is impermeable to
Na-CL.
Thin ascending limb: is impermeable to H2O, but
permeable to Na-Cl, where they are absorbed passively
in this part .
Thick ascending limb: is impermeable to H2O.
Na-K-2Cl co-transport occur in this part.
Distal convoluted tubule and collecting
ducts
What happens here depends on hormonal
control:
 Aldosterone affects Na+ and K+
 ADH – facultative water reabsorption
 Parathyroid hormone – increases Ca++
reabsorption

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Distal convoluted tubule and collecting
ducts
 Tubular secretion to get rid of substances:
K+, H+, urea, ammonia, creatinine and
certain drugs
 Secretion of H+ helps maintain blood pH
(can also reabsorb bicarb and generate
new bicarb)
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Na+ absorption
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Na+ absorbed occurs by many
mechanisms depending on the part
of nephron:
1- active transport mechanism:
E.g. Collecting tubules & ducts.
2- Passive at the thin ascending limb
3- Co-transport: thick ascending
Glucose handling
Glucose absorption
also relies upon the
Na+ gradient.(Naglucose co-transport)
 Most reabsorbed in
proximal tubule.
 At apical membrane,
needs Na+/glucose
cotransporter (SGLT)
 Crosses basolateral
membrane via
glucose transporters
(GLUT’s), which do
not rely upon Na+.

K+ handling
K+ is major cation in cells
and balance is essential for
life.
 Small change from 4 to 5.5
mmoles/l = hyperkalaemia
 K+ is reabsorbed at
proximal tubule.
 Changes in K+ excretion
due to changes in K+
secretion in distal tubule

K+ handling
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K+ reabsorption along the
proximal tubule is largely
passive and follows the
movement of Na+ and fluid (in
collecting tubules, may also
rely active transport).
K+ secretion occurs in
cortical collecting tubule
(principal cells), and relies
upon active transport of K+
across basolateral membrane
and passive exit across apical
membrane into tubular fluid.
Countercurrent Multiplier
Countercurrent
is that, fluid flows down the
descending limb and up the ascending limb.
The critical characteristics of the loops which
make them countercurrent multipliers are:
1. The ascending limb of the loop of Henle
actively co-transports Na+ and Cl- ions out of
the tubule lumen into the interstitium. The
ascending limb is impermeable to H2O.
2. The descending limb is freely permeable
to H2O but relatively impermeable to NaCl.
H2O that moves out of tubule into intersitium is
removed by the blood vessels called vasa recta
– thus gradients maintained and H2O returned
to the circulation.
Mechanisms of tubular transport:
Active transport:
i. Primary active transport: e.g. Na-K-pump, H+-pump
ii. Secondary active transport : e.g. Na-K-2Cl cotransport, glucose-sodium co-transport, amino
acid-sodium co-transport.
Passive transport:
i. Simple diffusion e.g. Cl, HCO3-, urea.
ii. Facilitated diffusion glucose at the basal border.
Osmosis.
Passive transport (simple diffusion): Definition
◦ Movement of a substance
across a membrane as a result
of random molecular motion
down concentration and
electrochemical gradient. No
energy needed.
◦ E.g. bicarbonate, chloride,
Facilitated diffusion: Definition
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Movement of a substance across a membrane down
its electrochemical gradient after binding with a
specific carrier protein in the membrane.
Needs a carrier.
Examples:
Glucose, amino acids: At Basolateral membranes of
proximal tubules
Sodium: luminal membranes of proximal tubules
Primary active transport:
 Movement
of a substance across a
membrane in combination with a carrier
protein but against an electrochemical
gradient.
 Directly requires metabolic energy (i.e.
hydrolysis of ATP)
Primary active transport:
Examples
Na+-K+ ATPase
 H+ ATPase
 H+-K+ ATPase
 Ca+2 ATPase

+
+
Na -K ATPase
In renal tubular cells found only in basolateral
membrane
 When ATP is hydrolyzed, 2 K+ ions are pumped
into the cell and 3 Na+ ions are pumped out
 Maintains favorable electrochemical gradient for
Na+ entry at luminal membrane
 Maintains cell membrane potential difference and
intracellular osmolality

Secondary active transport: Cotransport

Two substances bind
with one specific carrier lumen
in the cell membrane
and both substances are Glucose, Pi
translocated across the amino acids
membrane.
Na+
◦ Co-transport
Na+
Transported
H+
substances move in
the same direction
across the membrane
Basolat
3 Na+
2
K+
K+
Water Reabsorption: (Osmosis)
In the proximal tubule, water follows sodium
passibvely and isosmotically because the proximal
tubule is very permeable to water.
Water moves both transcellularly and paracellularly.
The transcellular movement is facillitated by
“aquaporin” water channels in both the apical and
basalateral membranes.
Concentration of urine
Counter current multiplier:
Counter current exchanger (vasa recta).
Hyperosmolar medulla produced by:
-Passive absorption of NaCl
-Active absorption of Na in thick acending limb
-ADH helping absorption of urea by collecting
tubules.
- slow flow of tubular blood that prevents rapid
wash out of solutes in medulla (vasa recta).
What do the kidneys do?
Thick ascending : Na-K2Cl co-transport
Descending limb: 25%
H2O2 absorption
The glomeruli filter the blood, and the tubules take back what the body needs
leaving the rest as waste to be excreted. Some wastes also can be actively added
to the tubular fluid.