Unit 2, L16 Renal Concentration and Dilution of Urine Flashcards
Anti-diuresis
Without water, high concentrations of ADH in the plasma, increased reabsorption of water and urea, production of low-volume, high concentration urine
In the cortex, what is the interstitial fluid osmolarity (during antidiuresis with ADH present)
300 mOsm
When we get to the inner parts of the medulla, what is the interstitial osmolality (during antidiuresis with ADH present)
1200
What is Diuresis
When there is over-hydration or aggressive administration of hypotonic solutions, leads to low concentrations of ADH in the plasma, decreased reabsorption of water and urea, and production of high-volume, low concentration urine
When there is water diuresis and no ADH present, what is the osmolality of the cortex
300
When there is water diuresis and no ADH present, what is the osmolality of the medulla (distal medulla)
600
Osmolality of the renal cortex
Isotonic with plasma, so 300 mOsm/L
Osmolality of outer medulla
Mild hyperosmolality, 600-800 mOsm
Osmolality of inner medulla
Strong hyperosmolality, 1200 mOsm/L
Two major contributors to the cortciomedullary osmotic gradient
NaCl (50%)
Urea (50%)
What are the three mechanisms that regulate the medullary hyperosmolality
1) Countercurrent multiplier, which establishes the hyperosmotic gradient
2) Urea cycle, which strengthens the osmotic gradient
3) Countercurrent exchanger, which maintains osmotic gradient
Countercurrent multiplier
Trying to increase osmolarity within the kidney, specifically in the medulla, and multiply that concentration to generate high concentration in the urine
Thin descending loop of Henle, in terms of permeability and water movement
High water permeability and low salt permeability, so water moves out of tubule and leaves salt behind
Thin ascending loop of Henle, in terms of permeability and water movement
Low water permeability and high salt permeability, salt moves out of tubule, leaving water behind
Thick ascending loop of Henle, in terms of permeability and water movement
SIte of most active salt pumping in kidney, it is water-impermeable to the tubular fluid becomes hyposmotic, its the diluting segment
Distal tubule, in terms of permeability and water movement
Increases H2O permeability and salt transport (reabsorption)
Upper collecting duct, in terms of permeability and water movement
Active salt reabsorption and passive water reabsorption under ADH control
Lower collecting duct, in terms of permeability and water movement
Active salt reabsorption and passive water and urea reabsorption under ADH control
The three repeating steps of the countercurrent multiplication
1) shift tubular fluid
2) Pump salts
3) Equilibrate osmolarity
Then move the fluids
Step 1 of the countercurrent multiplication
Downward arm, coming out of proximal tubule, looping around the loop of henle
Everything should be isosmotic so everything is 300, all the way around and on the inside
Step 2 of the countercurrent multiplication
The thick ascending limb is the diluting segment and can pump salt, so moving salt into the interstitium. The gradient is starting to build up, and the thin ascending limb is passive, can move salt and water. Because of this, the descending limb is all 300, the interstitium is 400, and the thick ascending limb goes from 200-300 (top to bottom)
Step 3 of the countercurrent multiplication
Now we need to balance out the salts. On the thin descending limb, we aren’t moving salts but we are moving water. We then need to equilibrate on the left side. Fluid is still coming in from the glomeruli, so its coming down from the proximal tubule. This means that the descending limb is now 400, the interstitium is 400, and the ascending limb is all 200
Step 4 of the countercurrent multiplication
The concentration of the fluid that is flowing down the descending limb is 300. Because of that and the permeability of the thin ascending limb, you get a new gradient set up. Will also push higher concentration down and gets stuffed into the deeper parts of the medulla. So now the descending limb ranges from 300-400, the interstitium ranges from 300-400, and the ascending limb ranges from 200-400
Step 5 of the countercurrent multiplication
Repeat this process over and over again, the salts get shifted, can get the thin descending limb at the lowest part to be high in osmolarity, at 500. The difference you can generate with the active salt pumping as a limit in the osmolarity build up, so the differential is set to 200 at the max (in step 2). So in the descending limb, you have a range of 300-400. In the interstitium, you have a range of 350-500, and in the ascending limb, you have a range of 150-300
Step 7 of the countercurrent multiplication
Keep repeating this and osmolarity, as a whole, will get to 1200
So finally, the descending limb ranges from 300-1200, the interstitium ranges from 300-1200, and the ascending limb ranges from 100-1000
Urea
End product of protein metabolsim that must be removed from the body.
Urea permeability
Urea is impermeable along the entire upper collecting duct and permeable only in the lower collecting duct if and only if ADH is present
Movement of urine and its concentration
As urine flows down the collecting ducts, water is first removed, concentrating the urea within the tubular lumen. As it reaches the lower collecting ducts, the highly concentrated urine diffuses out into the medullary interstitium
After concentrated urea diffuses out into the medullary interstitium, what happens
Urea is partly picked up by the ascending vasa recta capillaries, and the remainder stays in the medullary region. Urea gets concentrated within the medulla nephron in order to pack the low-volume urine with highly concentrated urea for excretion
What are the three purposes for a high medullary urea concentration
1) Does not set up an osmotic gradient for the reabsorption of H2O
2) Protects vasa recta RBCs against crenation in a hyperosmotic environment
3) Sets up a gradient for urea to be excreted in low-volume urine
Relationship between clearance of urea and urinary flow
Clearance of urea varies directly, nonlinearly, and passively with urinary flow
At low urine flow, what happens to water and urea
Tubule reabsorbs lots of water and much urea, and the kidneys excrete only 15% of filtered urea
At high urine flow, what happens to water and urea
Tubules will reabsorb relatively less water and urea, and the kidneys may excrete as much as 70% of filtered urea
Vasa recta capillaries are permeable to what
Water and solutes
The descending vasa recta
Water moves out of the capillary down osmotic gradient, and salt moves into capillary down concentration gradient
The ascending vasa recta
Water moves into the capillary down the osmotic gradient, and salt moves out of capillary down concentration gradient
Countercurrent exchanger net effect
1) Vasa recta exits medulla with slightly more solutes than water
2) Vasa recta flow is relatively slow, so deep medullary gradient is maintained
3) Water shunt, excess water is kept out of deep medulla
4) Solute trapping, excess solutes are kept in the lower medulla
Requirements for renal medullary hyperosmolality
1) Unique renal micro-anatomy to have long loops of Henle
2) Convections of fluids (blood flow and urine flow)
3) Active salt pumping (TAL, DT, and CD)
4) Differential permeabilities to salt and water in the thin descending and thin ascending limbs of the loop of Hendle
What does it mean if U osm / P osm > 1
Concentrated urine, so ADH antidiuresisq
What does it mean if U osm / P osm < 1
Dilute urine, so water diuresis
Positive free water clearance
Expresses the amount of pure (solute-free) water the kidney adds to the urine, diluting the urine below the osmolality of blood, where U osm < P osm
