Unit 2, L16 Renal Concentration and Dilution of Urine

Question 1

Anti-diuresis

Answer 1

Without water, high concentrations of ADH in the plasma, increased reabsorption of water and urea, production of low-volume, high concentration urine

Question 2

In the cortex, what is the interstitial fluid osmolarity (during antidiuresis with ADH present)

Answer 2

300 mOsm

Question 3

When we get to the inner parts of the medulla, what is the interstitial osmolality (during antidiuresis with ADH present)

Answer 3

1200

Question 4

What is Diuresis

Answer 4

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

Question 5

When there is water diuresis and no ADH present, what is the osmolality of the cortex

Answer 5

300

Question 6

When there is water diuresis and no ADH present, what is the osmolality of the medulla (distal medulla)

Answer 6

600

Question 7

Osmolality of the renal cortex

Answer 7

Isotonic with plasma, so 300 mOsm/L

Question 8

Osmolality of outer medulla

Answer 8

Mild hyperosmolality, 600-800 mOsm

Question 9

Osmolality of inner medulla

Answer 9

Strong hyperosmolality, 1200 mOsm/L

Question 10

Two major contributors to the cortciomedullary osmotic gradient

Answer 10

NaCl (50%)

Urea (50%)

Question 11

What are the three mechanisms that regulate the medullary hyperosmolality

Answer 11

1) Countercurrent multiplier, which establishes the hyperosmotic gradient
2) Urea cycle, which strengthens the osmotic gradient
3) Countercurrent exchanger, which maintains osmotic gradient

Question 12

Countercurrent multiplier

Answer 12

Trying to increase osmolarity within the kidney, specifically in the medulla, and multiply that concentration to generate high concentration in the urine

Question 13

Thin descending loop of Henle, in terms of permeability and water movement

Answer 13

High water permeability and low salt permeability, so water moves out of tubule and leaves salt behind

Question 14

Thin ascending loop of Henle, in terms of permeability and water movement

Answer 14

Low water permeability and high salt permeability, salt moves out of tubule, leaving water behind

Question 15

Thick ascending loop of Henle, in terms of permeability and water movement

Answer 15

SIte of most active salt pumping in kidney, it is water-impermeable to the tubular fluid becomes hyposmotic, its the diluting segment

Question 16

Distal tubule, in terms of permeability and water movement

Answer 16

Increases H2O permeability and salt transport (reabsorption)

Question 17

Upper collecting duct, in terms of permeability and water movement

Answer 17

Active salt reabsorption and passive water reabsorption under ADH control

Question 18

Lower collecting duct, in terms of permeability and water movement

Answer 18

Active salt reabsorption and passive water and urea reabsorption under ADH control

Question 19

The three repeating steps of the countercurrent multiplication

Answer 19

1) shift tubular fluid
2) Pump salts
3) Equilibrate osmolarity

Then move the fluids

Question 20

Step 1 of the countercurrent multiplication

Answer 20

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

Question 21

Step 2 of the countercurrent multiplication

Answer 21

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)

Question 22

Step 3 of the countercurrent multiplication

Answer 22

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

Question 23

Step 4 of the countercurrent multiplication

Answer 23

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

Question 24

Step 5 of the countercurrent multiplication

Answer 24

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

Question 25

Step 7 of the countercurrent multiplication

Answer 25

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

Question 26

Urea

Answer 26

End product of protein metabolsim that must be removed from the body.

Question 27

Urea permeability

Answer 27

Urea is impermeable along the entire upper collecting duct and permeable only in the lower collecting duct if and only if ADH is present

Question 28

Movement of urine and its concentration

Answer 28

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

Question 29

After concentrated urea diffuses out into the medullary interstitium, what happens

Answer 29

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

Question 30

What are the three purposes for a high medullary urea concentration

Answer 30

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

Question 31

Relationship between clearance of urea and urinary flow

Answer 31

Clearance of urea varies directly, nonlinearly, and passively with urinary flow

Question 32

At low urine flow, what happens to water and urea

Answer 32

Tubule reabsorbs lots of water and much urea, and the kidneys excrete only 15% of filtered urea

Question 33

At high urine flow, what happens to water and urea

Answer 33

Tubules will reabsorb relatively less water and urea, and the kidneys may excrete as much as 70% of filtered urea

Question 34

Vasa recta capillaries are permeable to what

Answer 34

Water and solutes

Question 35

The descending vasa recta

Answer 35

Water moves out of the capillary down osmotic gradient, and salt moves into capillary down concentration gradient

Question 36

The ascending vasa recta

Answer 36

Water moves into the capillary down the osmotic gradient, and salt moves out of capillary down concentration gradient

Question 37

Countercurrent exchanger net effect

Answer 37

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

Question 38

Requirements for renal medullary hyperosmolality

Answer 38

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

Question 39

What does it mean if U osm / P osm > 1

Answer 39

Concentrated urine, so ADH antidiuresisq

Question 40

What does it mean if U osm / P osm < 1

Answer 40

Dilute urine, so water diuresis

Question 41

Positive free water clearance

Answer 41

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