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