urine concentration and diluting mechanisms Flashcards
week 8
Urine osmolarity (value)
50mOsm/kg –> 1200-1400 Mosm/Kg
how much urine is formed each day?
0.5-20L
Where in the Nephron is it:
a) isoosmotic
b) hyperosmotic
c) hypoosmotic
a) PCT
b) tDL and bottom loop
c) TAL and DCT
How does urine osmolarity vary in the presence of ADH?
PCT, tDL, tAL and TAL all same
early DCT: no ADH = further dilution
ADH= large volume of water reabsorbed
ADH= small V of urine with high osmolarity
No ADH= large V pf urine with low osmolarity
What stimulates ADH secretion?
1% increase in plasma osmolarity
10% increase in Blood Volume
Actions of ADH
Binds to V2 = cAMP of TAL, late DCT and CD = increased water reabs in CD and increased urea reabs in medullary CD.
Stimulates reabs of NaCl in TAL, DCT and CD
Where are the AQP2 located?
Principal cells of late DCT and CD
ADH-dependant
Where are the AQP1 located?
PCT and tDL (basolateral
Where are the AQP3 and 4 located?
Principal cells of late DCT and CD (basolateral)
Water diuresis
Increased urinary output following excessive intake of water or hypotonic solution
In absence of ADH in plasma
Osmotic diuresis
Increased urinary output due to osmotic effect
Due to large amounts of unabsorbed solutes in the PCT.
How does ADH form concentrated urine?
Augments urea cycling from medullary CD into medullary interstitial fluid
Stimulates NaCl reabsorption in TAL
increases size of medullary gradients
Increases water permeability of principal cells of late DCT and CD
Obligatory reabsorption
Water movement that cant be prevented
85% of filtrate
PCT and tDL
Facultative reabsorption
ADH dependent water movement
15% of filtrate
Late DCT and CD
Obligatory Urine Value
= min urine V that excreted solute can be dissolved in
= 500ml
What processes are required to allow kidneys to vary urine concentration?
Adequate glomerular filtration
Na reabsorption w/o water in AL
Variable water permeability in CD
- how does a high protein diet affect urine concentrating ability?
increases the ability of the kidneys o concentrate the urine as more urea is formed
What is free water clearance and what does it indicate?
Amount of free water excreted each day
Measures renal water regulation (tubular dilution or concentration)
Where in the nephron is free water generated?
the diluting segments (TAL and DCT) where solute is reabsorbed without water
Low ADH vs High ADH free water handling
Charge
- Low = +ve water clearance
-high = -ve water clearance
What
-low = excreted
-high= reabsorbed in late DCT and CD
osmolarity
-low = hypo
-high= hyper
Mechanisms that contribute to medullary hyperosmolarity
Active transport ions from TAL
Active transport of ions from CD
Facilitated diffusion of Urea
What is the counter current mechanism?
LoH creates a osmotic gradient in medulla via active reabs of solutes in one limb and passive water movement in other.
Only in JM Nephrons as needs long and thin LoH and vasa recta
What is the countercurrent multipler system?
+ve feedback cycle in which the flow is used to multiply interstitial osmolarity
Steps of Multiplier system
Single effect
Fluid FLow
Gradient multiplication
Steady State Gradient
