Urine concentration

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Lecture 7

• Urine concentration

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Urine Concentration

• The main function of the loop of Henle is to
  remove NaCl from the lumen and deposit this NaCl
  in the interstitium of the medulla.
• Interstitial osmolality rises from the cortex to
  the tip of the medulla (corticomedullary
  osmolality gradient).
• The maximal interstitial medullary osmolality
  occurs during antidiuresis (1200mOsmol).

p. 833 Ch. 37
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PLASMA OSMOL 290-300 mOsm.
HIGH ADH
LOW ADH
Fig. 37-1 p. 831
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WATER RESTRICTION
Increased water permeability
HIGH
1200 mOsm
HIGH WATER INTAKE
ADH Control
LOW
No water permeability
Fig. 37-2 p. 832
60 mOsm
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WATER RESTRICTION
TAL/DCT Impermeable to Water. ONLY Pumps NaCl out
fig 14-15
CONCENTRATED URINE/LOW VOLUME
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Hyperosmolality of Medulla

• Depends on net transport of NaCl across TAL (part
  of single effect)
• Limit of tubule-interstitium gradient at any one
  point is 200 mOsm.
• Countercurrent multiplier system
• multiplies the single effect of the 200 mOsm
  gradient
• After 39 cycles, the interstitial fluid
  osmolality would be 1200 mOsm.

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Countercurrent multiplier system
-TAL pumps NaCL -equilibrium forms -fluid shifts
thru tubule -TAL pumps more NaCL
Ch. 14 in Vanders Physiology or Fig 37-3 BB
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Urea Recycling on Interstitial Medullary
Osmolality

• Urea cycle
• Absorption of urea from IMCD (intermedullary CD)
• Secretion of urea from interstitium into the tALH
• Carriage of urea up into the cortex and back down
  through the nephron.

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UREA RECYCLING
IMCD is made Permeable to UREA by ADH
ONLY!
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Medullary osmotic gradient
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WATER RESTRICTION
Increased water permeability
HIGH
1200 mOsm
ADH Control
HIGH WATER INTAKE
LOW
No water permeability
Fig. 37-2 p. 832
60 mOsm
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• Potassium Excretion

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POTASSIUM IS HIGH IN INTRACELLULAR FLUID
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Like Sodium and Water, Ingested Potassium is
Largely Excreted
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POTASSIUM DISTRIBUTION

• 98 of total body potassium is in the ICF.
• 2 of potassium is in ECF.
• Normal plasma range for K is (3.5-5.0mM)
• Low ECF K is necessary to maintain steep K
  gradient across membranes (cellular
  excitability).
• Changes in ECF K can cause severe disturbances
  in excitation and contraction.
• Cardiac rhythmicity and muscle paralysis
• Hypo and hyperkalemia
• Diarrhea and vomiting are major causes for
  hypokalemia (dehydration)

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Potassium Excretion
Reabsorption from proximal tubule
(70) Henles ascending limb (20) Collecting
duct (10) Intercalated cells Secretion
into Collecting duct (initial collecting
duct) principal cells of CD (variable) Potassium
excretion depends on K secretion from the
principal cells.
fig 14-26 Vanders Phys.
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HORMONAL REGULATION OF POTASSIUM UPTAKE INTO CELL
Insulin Epinephrine Aldosterone
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Acidosis and Potassium uptake
Na-K pump
Co-transporter
note, opposite for alkalosis
High plasma H (low ph acidosis) Decreases K
uptake into cell by inhibiting pumps and
co-transporters, therefore you get reduced ICF K
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Regulation of potassium excretion
Same principal as sodium, the more you consume,
the more you excrete to keep in potassium
balance.
fig 14-27 Vanders phys.
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Actions of Aldosterone
4.
5.
1.
2.
3.
fig 14-13
Aldosterone actions ? Na channel activity
(luminal side reabsorption), ? Na/K ATPase
pump, ? K channel activity and
secretion/excretion
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Regulation of K excretion aldosterone
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K excretion loop thiazide diuretic stimulation
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K excretion K sparing diuretics inhibition