Renal 3. Water Balance: Water Reabsorption Medullary Osmotic Gradient (Countercurrent Multiplier) Flashcards
Water input and output is ~
2.5L / day, 2.5L go in and 2.5L go out.
What’s are the roles of the kidneys in water balance
They reabsorb a lot of water, increase reabsoption when we are dehydrated, or stop reabsroption when we are fully hydrated.
Do we secreted water?
We never secrete water. No active water movement, all done by concentration gradients (osmolarity), following NaCl and protein.
How os water balance done, how is it moved?
Transporters?
Pumps?
Gradients?
Channels?
all done by concentration gradients (osmolarity), following NaCl and protein.
Countercurrent multiplication :
The process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine.
How many litres are lost out of the body in one day, total losses
- Feces = 0.1L
- Other losses 0.9L
- Urine = 1.5L
The Kidney Can Generate a Urine as Dilute as
30 mOsm
The Kidney Can Generate a Urine as concentrated as
1200 mOsm
What’s the formula for osmoles extracted/day?
Osmoles excreted/day (1200 If dehydrate or 30 if hydrated) =
(urin osmolarity Uosm) X ( urin output/day V)
What is the formula for urine osmolarity
Urine osmolarity Uosm =
(Osmol excreted/day) 600 mOsmol/day
——————————— = ————. =
(urin output/day V). 1.5L/day
400 mOsm
Osmolarity gives indication of how
many solutes dissolved in solution.
Osmolarity can go up either by:
- Adding concentration of. Solute
2. Lower concentration of water
For proper function, regardless of hydration, one requires an osmoles level of ______/day
600 /day.
If not 600, serious problem
Normal urine osmolarity should be
400 mOsm
Known urine osmolarity is slightly
hyper osmotic compared to blood plasma which is around 300
What’s the formula for urine volume output /day?
Osmol excreted /day
——————————- V (urine output)
Uosm.
For extreme water dieresis, what’s the volume of urine excreted / day ?
Osmol excreted /day 600mOsm/day
——————————- = ——————. =
Uosm. 30mOsm
20L/day
For extreme water dieresis, what’s the volume of urine excreted / day ?
Osmol excreted /day 600mOsm/day
——————————- = ——————. =
Uosm. 1200mOsm
0.5 L/day
What are the absolute maximums for urine volume excretion?
Max hydrated —> 20 L/day
Max dehydrated —> 0.5 L/day
Medullary Osmotic Gradient (Countercurrent Multiplier)
This how the kidney creates the environment in the medulla required so that when we do need to reabsorb more or less water, via the countercurrent multiplier system.
In the proximal tubule, Tubule Fluid (TF) Is
Isosmotic
By the time water reaches the end of the loop of henle,
Become diluted
Once water passes through the loop of henle, it goes through the collecting tubules (CCT, OMCD, IMCD), where it will
Either remain Diluted or becomes concentrated.
How does the distal tubule know when to make it concentrated or diluted?
Tells the Kidneys/nephrons (at the distal tubule) to reabsorb more or less water depending on hydration status.
Isosmotic:
Means that concetration of solvent and solute are EQUAL.
What is isosmotic osmolarity # and where’s it found?
300 and found in proximal tubule.
Steps for water passing through the nephron
1) Proximal tubule
2) Loop of henle
3) Distal tubule
4) Collecting tubules (CCT, OMCT,IMCDT)
Steps for water passing through the nephron
1) Proximal tubule:
isosmotic, reabsorb 70% remaining 30% reabsorbed throughout the rest of nephron.
Steps for water passing through the nephron
2) Loop of henle
No matter hydration status, INCREASE in tubular fluid as it does down the DESCENDING limb.
How does the loop of henle cause an increase in tubular fluid ?
Because THIN Ascending limb is ALWAYS permeable to H2O, no matter. No movement of solutes at the ascending.
THIN ascending reabsorb H2O into tubule = Osmolarity increases (HYPERTONIC) [300]
THICK Ascending limb (impermeable to H2O) Adds solutes = DECREASES osmolarity [200]
———->
Goes back down to be HYPOSMOTIC (LESS solutes, osmoles goes from 300 —> 200).
When you add / removed solutes, what happens to osmolarity?
- Add solutes = solutes increase = hypertonic (lower osmolarity)
- remove solutes = solutes decrease = hypotonic (higher osmolarity)
Steps for water passing through the nephron
3) Distal Tubule
At the beginning of the distal tubule, kidneys are signalled to either reabsorb water or not
- becomes permeable to water to do so.
Is the distal tubule permeable to water ?
Usually its not but it can become permeable if its told to reabsorb water (if dehydrated)
When the distal tubule decides to :
A) reabsorb water
B) not reabsorb after
A) reabsorb water: osmolarity increases cuz more fluid
B) not reabsorb after : osmolarity decreases cuz less fluid
Steps for water passing through the nephron
4) Collecting Tubules (CCT,OMCD,IMCD)
Two cases:
A)H2O reabsorbed. B)No reabsorption
A)H2O reabsorbed: distal tubule signals the collecting tubules to add more H2O = increases reabsorption (osmolarity increases)
B) NO reabsorption: distal tubules DOESNT signal the collecting tubules, so NO reabsoption of H2O (osmolarity decreases)
If the collecting tubules were signalled by the distal tubule to reabsorb water, the urine will be
Concentrated (1200)
High solutes and low H2O
If the collecting tubules were NOT signalled by the distal tubule to reabsorb water, the urine will be
Diluted (30)
High H2O
LOW solutes
Depending on the hydration status, the kidneys/nephron will create a
Environment in the medulla nephron that’s favourable for the hydration status,
- Water reabsorption if dehydrated
- NO water reabsorption if hydrated
How is the THIN descending limb permeable to water ?
Through AQP-1 transporter that allows to reabsorb H2O, present in the environment at all times, not matter.
The steps to change environment in medulla
1) . Fluid enters —> active transport of Na+, Cl-, K+ ions into medullary interstitial fluid = increases osmolarity [300]
2) . Water moves out of DESCENDING limb by osmosis
3) . DESCENDING limb is iso-osmotic; osmotic differences between descending [400] and ascending [200]
4) . More fluid enters tubule = pushes fluid via bulk flow. ——> active transport of Na+, Cl-, K+ ion into medullary interstitial fluid (outside tubule) = osmolarity increases in the interstitial space to [400]
5) . Water moves out of DESCENDING limb by osmosis = changes osmolarity of medullary space [300/400—> 350/500]
6) iso-osmotic state in DESCENDING limb; osmotic difference between descending and ascending [350/500 —> 300/150]
7) more water enters tubule, process continues. System is in ready state, the osmotic gradient has been established.
The steps to change environment in medulla
STEP 1)
Fluid enters —> Na+, Cl-, K+ ions moved into medullary space (via active trasport) by the THICK ascending limb
= increases osmolarity [300]
- enter system at this point is isosmotic
[everything is @ 300]
The steps to change environment in medulla
STEP 2)
2). Water moves out of DESCENDING limb by osmosis
- medullary space osmolarity increases
[from 300–> 400] - the removed solutes from the THICK ascending limb causes decrease in osmolarity [from 300 —> 200]
** we now have favourable env for water reabsorption from THIN descending limb to medullary space —-> blood **
Where do all the things in the medullary space end up?
In the blood
The steps to change environment in medulla
STEP 3)
3) . DESCENDING limb is iso-osmotic; theres an osmotic differences between descending [400] and ascending [200]
- THIN descending osmolarity increases cuz of reabsorbed H2O = [400]
- Ascending limbs osmolarity decreases [from 300 —> 200]
The steps to change environment in medulla
STEP 4)
4) . More fluid enters tubule = pushes fluid (via bulk flow). ——> active transport of Na+, Cl-, K+ ions by the THICK ascending limb into medullary interstitial fluid (outside tubule) = osmolarity increases in the interstitial space to [400]
- new fluid causes early THIN descending osmolarity to decrease = [300 —> 300/400]
- new fluid causes early THICK ascending limb causes osmolarity increase =
[200 —> 400/200] (this is the reason why the THICK ascending osmolarity is ALWAYS low) creates H2O reab ravorbale env
How is fluid pushed down the tubule?
Via bulk flow
At what step do we gain an env that’s favourable for water reabsorption at the thin descending limb? Why?
Step 2 because of the bulk flow of fluid pushed down the tubule = osmolarity of THIN dl decreases/
The steps to change environment in medulla
STEP 5)
5) . Water moves out of DESCENDING limb by osmosis = changes osmolarity of medullary space [300/400—> 350/500]
- new fluid being pushed is exposed to hyperosmotic env as it travels down = favours H2O reabsorption out of the THIN descending limb and into medullary space.
- new fluid = osmolarity of THICK ascending limb decreases = [200/400 —> 150/300]
- THIN DL = hyperosmotic
- THICK AL = hypo-osmotic
The steps to change environment in medulla
STEP 6)
6) iso-osmotic state in DESCENDING limb; osmotic difference between descending and ascending [350/500 —> 300/150]
- new fluid pushed down fluid, slowly becoming [350/500] in the THIN DL
- the gradient for water reabsorption from THIN DL to the medullary space is now establish [350/500]
The steps to change environment in medulla
STEP 7)
7) more water enters tubule, process continues. System is in ready state, the osmotic gradient has been established.
- gradient in medullary space changes after maximum number of turns —> [300/500/900/1200/1400]
- osmolarity is at 1200/1400 at the depth of tubule. 1400 = max turns
What’s is the osmolarity of the cortex ?
It’s 300, it’s ALWAYS 300 and it never changes. Unlike the medullary space osmolarity which is responsible for H2O gradient
The size of the cortex compared to the medullary interstitial space determines the
maximum osmolarity as well as length of the tubule.
How does the size of the cortex determine the maximum osmolarity and length of tubule?
As water exits the lumen of tubule, with limited area permeability, luminal [urea] increases
Although the urea gradient opposes water reabsorption, the NaCl gradient is so large that is net effect is water reabsorption
What cause the osmolarity in the medullary space to be 1200-1400?
Half is due to NaCl and half Is due to urea.
The late Distal tubule and collecting duct being impermeable to water is the case if
The body is hydrated.
- Distal tubule always at [300]
- osmolarity of the collecting ducts is [100] throughout the entire thing = no gradient = impermeable to H2O reabsorption
The late Distal tubule and collecting duct being permeable to water is the case if
The body is dehydrated = received signal to become permeable = H2O reabsorption
- Distal tubule is always 100 —> favours H2O reabsorption from distal to cortex (not medullary)
- collecting duct = permeable = osmolarity in both the collecting tubule and medullary space go up = HYPO-OSMOTIC = favours H2O reabsorption = GRADIENT !
How do you tell the distal tubule or the collecting duct to become permeable to water?
Through the release of antidiuretic hormone (ADH) and vasopressin AVP
Antidiuretic Hormone (ADH) (Arginine Vasopressin (AVP)) increases
water permeability = increases H2O reabsorption =
more fluid in blood and less in urine
How ADH increases water permeability
First ADH binds to receptor on basolateral membrane = sets of cascade of events = increase the number of AQP-2 (not AQP-1 or 3 or 4) channels on the apical membrane in the distal tubule and collecting ducts. ——->
More channels = More water that moves from the tubule gets into the principle cell, then AQP-3 on the basolateral will then facilitate water moving from the cell and eventually into the peribular fluid and into the blood to be reabsorbed.
Where is ADH released from?
ADH released in the brain at the hypothalamus, released by the posterior pituitary.
What kind of receptors used for water reabsorption under ADH influence?
- AQP-2 on the apical side (these the ones that are increased by ADH) takes water from distal/collecting to increase cell.
And
- AQP-3 on the basolateral side that takes water from cell into blood
Blood flow in the nephron is
countercurrent
The Vasa Recta’s Countercurrent
Exchange Mechanism and Relatively Low Blood Flow Minimize the
Washout of the Medullary Hypertonicity
KEEP GOING I LOVE YOU AND KEEP LOVING YOUR SELF CUZ
You amazing btich !
How does the vasa recta maintain osmotic gradient and prevent washout?
Vasa recta capillaries run parallel to loops of Henle. Vasa recta slows down the blood flow which helps maintain the osmotic gradient required for water reabsoption
Main function of the vasa recta:
They remove the water and solute that is continuously added to the medullary interstial space by the nephron. Maintains the osmolarity in the blood.
Where is the vasa recta connected to ?
The loops of Henley, both descending and ascending
What’s does counter current mean
The vasa recta and loops of henle
What is countercurrent exchange in kidney?
Countercurrent multiplication in the kidneys is the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine.
As the filtrate I’d entering the descending limb, it progressively is becoming
More concentrated as it losses water into the medullary space.
As the as descending limb is pumping out Na,K,Cl ions inot the medullary space, the filtrate becomes more
hypertonic
Blood in the vasa recta removes water leaving the
Loop of henle
