Water Balance Flashcards
What are the normal ranges for plasma osmolarity and urine osmolarity?
Plasma - 285-295 mosmol/L
Urine - 50-1200 mosmol/L
What would happen if we didn’t get rid of the excess volume, excess water and excess salt?
Excess Volume - oedema + increase in blood pressure
Excess Water - cells swell
Excess Salt - cells shrink
What determines ECF volume?
Salt concentration (because water follows salt)
What is the main fluid compartment?
Intracellular - 65%
What are the different routes of getting rid of water? How much is removed via each of these methods?
Skin/sweat - 450 mL/day
Faeces - 100 ml/day
Respiration - 350 ml/day
URINE - 1500 ml/day - CONTROLLABLE and variable
Which part of the nephron is impermeable to water?
Ascending limb of the loop of Henle
Describe how much water is reabsorbed in each of the different parts of the nephron.
70% in the PCT
10% in the loop of Henle
Amount reabsorbed in the collecting duct varies
What needs to be created for water to be drawn out of the tubules and into the interstitium?
Hyperosmolar Region
Describe the osmolarity gradient in the interstitium.
Lowest concentration (around the same as plasma) in the cortex Highest concentration in the inner medulla
What is the approximate gradient between the ascending loop of Henle and the interstitium?
200 mosmol/L
What else accounts for the hyperosmolarity of the interstitial space?
UREA
Which parts of the nephron are permeable to urea? How does this set up a circulation of urea?
The bottom part of the collecting duct (inner medullary collecting duct)
The bottom of the descending limb of the loop of Henle
The upper part of the collecting duct is permeable to water so the urea gets concentrated as it passes down the collecting duct. The urea moves out into interstitial space (down the concentration gradient) and then it passes into the loop of Henle and recirculates.
Where are the urea transporters: A1, A2, A3 and B1 found?
UT-A1 - tubular side of the collecting duct cells
UT-A2 - thin descending limb of the loop of Henle
UT-A3 - basolateral membrane of collecting duct cells
UT-B1 - vasa recta
Now that a hyperosmolar region has been created, what regulates the amount of water that is drawn out by this region?
ADH - changes the number of aquaporin 2 on the apical membrane of the collecting duct cells thus regulating water reabsorption
How do you prevent the vasa recta from washing out the countercurrent gradient?
The vasa recta passes along the same path as the loop of Henle so as it moves down towards the bottom of the loop of Henle it loses water and gains salt as it.
As it goes back up it gains water and loses salt as it travels down the osmolarity gradient meaning that it doesn’t change the countercurrent gradient.
This way it can deliver oxygen and nutrients without disrupting the countercurrent gradient.
