Regulation of Plasma Osmolarity - RM Flashcards
What two ions mainly determine the plasma osmolarity?
Na, Cl
Does water ever move through active transport?
no, water always moves passively across a water permeable barrier from area of low osmolarity (high water concentration) to high osmolarity (low water concentration)
How does solute transport affect water transport?
water follows solute
What is the major route for water loss? How is it regulated?
urine excretion, by changing volume of urine
If the plasma is hypoosmotic, what changes in water excretion and urine formation?
excretes more water, dilute urine of large volume
If plasma is hyperosmotic, what changes in water excretion and urine formation?
retains more water, concentrated urine of small volume
Why can deviations in plasma osmolarity affect CNS function?
changes in transmembrane Na and K gradients can disrupt normal neuronal impulse propagation
If [U/P]osm is > 1, what does this mean about the tonicity of the plasma and urine?
urine hypertonic (concentrated) plasma hyperosmotic (volume depleted)
If [U/P]osm is <1, what does this mean about the tonicity of the plasma and urine?
urine hypotonic (dilute) plasma hypoosmotic (volume expanded)
What is negative free water balance? What kind of urine does it produce?
retaining free water in excess of solutes to minimize plasma hyperosmolarity–>produces hypertonic urine
What is positive free water balance? What kind of urine does it produce?
excreting free water in excess of solutes to minimize plasma hypoosmolarity–>produces hypotonic urine
Does increased ADH promote negative free water balance or positive free water balance?
negative free water balance (excreted when plasma osm. is high to retain water to dilute plasma hyperosmolarity)
What is osmolar excretion? How many mOsm/day are excreted in urine?
Uosm x volume (all of the solutes contributing to urine osmolarity)
600 mOsm/day
What is the minimum urine osmolarity it can be diluted to?
30 mOsm/L (excreting 20L/day to excrete the 600 mOsm/day)
What is the maximum urine osmolarity it can be concentrated to?
1200 mOsm/L (excreting 0.5 L/day to excrete 600 mOsm/day)
How is plasma osmolarity maintained by the kidney?
by excretion of water, not solute
What is free water?
water that is not osmotically obligated to remain in tubular fluid due to presence of salts/osmotic solutes
What is the formula for free water clearance?
Ch20= V- Cosm (where Cosm= (Uosm x V)/ Posm)
How does a diet low in solute affect negative and positive free water clearance?
increases negative free water clearance, decreases positive free water clearance
–amount of water osmotically obligated to stay in tubular fluid is reduced so much more is absorbed
How does a diet high in solute affect negative and positive free water clearance?
decreases negative free water clearance, increases positive free water clearance
–much more water is osmotically obligated to stay in tubular fluid and so less is reabsorbed
How does increasing solute intake affect water excretion?
forces increases in water excretion, even in hyperosmolarity states where you want to retain more water
Why don’t the kidneys correct the decrease in Posm by increasing reabsorption of solute in excess of water from tubular fluid? What does it do instead?
this would lead to hypervolemia and isotonic fluid retention (which isn’t desired)
-instead it excretes water in excess of solute to correct hypoosmolarity without changing plasma volume
Why don’t the kidneys correct the increase in Posm by increasing solute secretion?
this would decrease the plasma volume and cause hypovolemia
What is the relative osmolarity of tubule fluid in proximal tubule?
isoosmotic
What is the relative osmolarity of tubule fluid at end of loop of henle?
hypoosmotic (dilute)
In what part of the nephron can the tubular fluid be diluted or concentrated? What compound does this depend on?
in the collecting duct can be made more dilute or concentrated based on water permeability of CD due to ADH
Is the interstitum still hyperosmotic even when the plasma is hypoosmotic?
Yes, regardless of Posm, the interstitum is always made to be hyperosmotic—doesn’t have any effect because no ADH will be released in hypoosmotic state
Is the fluid made in the distal nephron always hypoosmotic?
Yes, changes occur in the CD afterwards based on presence/absence of ADH
Where is ADH synthesized? stored and released from? When is it released? Where does it act?
- synthesized in hypothalamus
- stored and released from posterior pituitary
- released when osmoreceptors in hypothalamus sense increased Posm.
- acts in distal nephron from late distal tubule to collecting ducts
What is the half-life of ADH, why is this important?
18 minutes, short half life so it can respond rapidly to changes in volume/osmolarity balance so you don’t overcorrect
What turns off ADH signaling?
decreased ADH secretion once hyperosmolarity is corrected, short half life, degraded by enzymes
What is the mechanism of action of ADH?
- increases H20 permeability of principal cells by causing: -fusion of vesicles containing aquaporins with apical membrane (to allow transcellular flow of water driven by osmosis from tubular fluid into hypertonic interstitum)
- synthesis of new aquaporins
What additional effect does ADH have in cases when blood volume is severely reduced, as in hemorrhagic shock?
acts on vascular smooth muscle to cause vasoconstriction to maintain blood pressure and blood flow
Which segment of nephron has the highest water permeability?
proximal tubule and thin descending limb–must be very water permeable to allow massive 67% reabsorption of solute and water
What is the water permeability in the diluting segment (thin ascending loop, thick ascending loop, distal convoluted tubule)?
relatively water impermeable–>allows solute reabsorption in excess of water which is how it dilutes it
What determines the water permeability of the collecting ducts?
presence of ADH
Where are aquaporins constitutively located?
basolateral membrane
Can water diffuse down its osmotic gradient even without aquaporins?
yes, by simple diffusion but with much less volume and much more slowly
How small of a change in Posm causes a detectable increase in ADH?
1% increase in Posm
How is ADH secretion dependent on volume status?
for a given increase in Posm, more ADH is secreted when volume contracted (more gain when hypovolemic) than volume expanded (less gain when hypervolemic) because you don’t want to get even more volume expanded
What is more important: preserving plasma volume or plasma osmolarity? Why?
volume, because blood pressure depends on volume to drive circulation
how does plasma osmolarity signal the osmoreceptors in the hypothalamus?
alters the mechanical stretch of the osmoreceptor cell membrane secondary to cell swelling when Posm decreases or cell shrinking when Posm increases
Does an increase in Posm depolarize or hyperpolarize the osmoreceptor membrane?
depolarizes (sends more APs to posterior pituitary to release ADH)
What does the OVLT osmoreceptor do?
stimulates thirst and desire for salt to increase H20 intake and increase negative free water clearance
