Renal Mechanisms for Concentration and Dilution of Urine

Question 1

Is water excretion dependent on solute secretion?

Answer 1

No. Normally excretion of water is regulated separately from excretion of solutes.

Question 2

In the PT, are the water and solutes reabsorbed separately?

Answer 2

No. The reabsorption occurs together in the PT.

Question 3

Where is the major site of water and solute separation?

Answer 3

Loop of Henle

Question 4

What will vary depending on the state of hydration?

Answer 4

Composition of urine

Question 5

What should remain relatively constant in the face of changes in the state of hydration?

Answer 5

Plasma osmolality should remain at around 300 mOsm/L

Question 6

What are the two main factors that regulate the plasma osmolality?

Answer 6

• Functional arrangement of the renal tubules and vasa recta capillaries
• Hormone influences on the kidney

Question 7

What is antidiuresis?

Answer 7

State of dehydration

Question 8

In a state of antidiuresis, what is found at a high concentration in the plasma?

Answer 8

ADH

Question 9

How does the kidney respond in a state of antidiuresis?

Answer 9

• Increases reabsorption of water and urea

- Produces low volume of concentrated urine

Question 10

What is water diuresis?

Answer 10

State of over hydration

Question 11

In a state of antidiuresis, what concentration is ADH found at in the plasma?

Answer 11

Low

Question 12

How does the kidney respond in a state of water diuresis?

Answer 12

• Decreases reabsorption of water and urea

- Produces high volume, dilute urine

Question 13

Describe the osmolality of the kidney from the cortex to the outer medulla to the inner medulla.

Answer 13

Cortex - isotonic with plasma at 300 mOsm

Outer Medulla - mild hyperosmolality at 300-480 mOsm

Inner Medulla - hyperosmolarity at 480-1200 mOsm

Question 14

What are the major species that contribute to the renal hyperosmolality and their relative contributions?

Answer 14

Na - 25%
Ca - 25%
Urea - 50%

1200 mOsm total

Question 15

What happens to the kidney and the nephron during water diuresis?

Answer 15

ADH is low and so the CD becomes impermeable to water and dilute urine is released. The hyperosmolarity of the medulla is drastically decreased as well as urea goes into the CD.

Question 16

What happens to the kidney and the nephron during antidiuresis?

Answer 16

ADH is high so the CD is very permeable to water and much water is reabsorbed leading to the production of concentrated urine. The hyperosmolarity of the medulla is maximal as urea goes into the medullary space.

Question 17

What are the mechanisms that generate and regulate medullary hyperosmolality?

Answer 17

• Countercurrent Multiplier
• Countercurrent Exchanger
• Urea Cycle

Question 18

What is the function of the countercurrent multiplier?

Answer 18

Establishes initial osmotic gradient

Question 19

What is the function of the countercurrent exchanger?

Answer 19

Maintains osmotic gradient

Question 20

What is the function of the urea cycle?

Answer 20

Strengthens osmotic gradient

Question 21

Where does passive NaCl transport occur?

Answer 21

Ascending Thin Limb of Loop of Henle

Question 22

Where does passive water transports occur in the Loop of Henle?

Answer 22

Descending Thin Limb of Loop of Henle

Question 23

Where does active NaCl transport occur?

Answer 23

Thick Ascending Limb of Loop of Henle

Question 24

Where is the urea permeability the highest?

Answer 24

The second half of the collecting duct in the medullary region

Question 25

Describe the permeability in the thin descending Loop of Henle.

Answer 25

-high water permeability
-low salt permeability
Water moves out of tubule leaving salt behind

Question 26

Describe the permeability in the thin ascending Loop of Henle.

Answer 26

-low water permeability
-high salt permeability
Salt moves out of tubule leaving water behind

Question 27

Describe the permeability in the thick ascending Loop of Henle.

Answer 27

• water is impermeable
• active salt pumping

Site of most active salt pumping in kidney (Na+, Cl–, K+) and this leads fluid to become hyposmotic

Question 28

Describe the absorption in the DT.

Answer 28

Increased H2O permeability in the late portion only and increased salt reabsorption

Question 29

Describe the absorption in the upper collecting duct.

Answer 29

• active salt reabsorption

- passive water reabsorption under ADH control

Question 30

Describe the absorption in the lower collecting duct.

Answer 30

• active salt reabsorption

- passive water and urea reabsorption under ADH control

Question 31

How does the countercurrent multiplier generate the hyper osmotic gradient?

Answer 31

1. Active transport of salt in the ascending thin loop generates hyperosmolality in the interstitial area
2. H2O from the descending loop will move into the interstitial area as a result
3. This generates intratubule osmolality differences from the “fresh” fluid coming from the PT to the “old” fluid near the end of the descending loop

Question 32

What is the urea cycle?

Answer 32

Urea is concentrated at the upper CD and it moves out of the lower CD passively into the inner medulla where the vasa recta pick it up and it moves into the outer medulla and is recycled back into the descending thin loop of Henle.

It is recycle back to the lower CD and the process is repeated.

Question 33

What is the high medullary concentration not used for?

Answer 33

Does not work as an osmotic gradient for H2O reabsorption

Question 34

What is the function of the high medullary concentration?

Answer 34

• Protects the RBCs in the vasa recta against crenation in a hyper osmotic environment
• Sets up a gradient for the excretion of urea in low volume urine

Question 35

What happens in the countercurrent exchanger?

Answer 35

H2O, salt and urea move passively across vasa recta capillary walls in renal medulla.

The blood exiting the vasa recta is slightly hyper osmotic and what happens is a water shunt that keeps out excess water from the medulla and a salt trap that keeps solutes in the lower medulla.

Question 36

Describe the substance movements in the descending vasa recta.

Answer 36

• water moves out of capillary down osmotic gradient

- salt and urea move into capillary down concentration gradient

Question 37

Describe the substance movements in the ascending vasa recta.

Answer 37

• water moves into capillary down osmotic gradient

- salt and urea move out of capillary down concentration gradient

Question 38

What can disturb the medullary gradient?

Answer 38

Increasing the vasa recta flow as it depends on its slow flow to maintain the countercurrent exchange

Question 39

What do medullary gradient disruptions lead to?

Answer 39

Increased urine flow

Question 40

What are the requirements for medullary hyperosmolarity?

Answer 40

• Long loops of Henle
• Active salt pumping
• Differential permeabilities

Question 41

What is the positive free water clearance?

Answer 41

CH2O expresses the amount of pure (solute-free) water the kidney adds to the urine, diluting the urine below the osmolality of blood.

Amount/time of water “stolen” from the urine by the kidney (mL/min)

Question 42

What is negative free water clearance?

Answer 42

–CH2O expresses the amount of pure (solute-free) water the kidney subtracts from the urine, concentrating the urine above the osmolality of blood.

Amount/time of water “added” to the urine by the kidney (mL/min)