Renal Mechanisms for Concentration and Dilution of Urine Flashcards
Is water excretion dependent on solute secretion?
No. Normally excretion of water is regulated separately from excretion of solutes.
In the PT, are the water and solutes reabsorbed separately?
No. The reabsorption occurs together in the PT.
Where is the major site of water and solute separation?
Loop of Henle
What will vary depending on the state of hydration?
Composition of urine
What should remain relatively constant in the face of changes in the state of hydration?
Plasma osmolality should remain at around 300 mOsm/L
What are the two main factors that regulate the plasma osmolality?
- Functional arrangement of the renal tubules and vasa recta capillaries
- Hormone influences on the kidney
What is antidiuresis?
State of dehydration
In a state of antidiuresis, what is found at a high concentration in the plasma?
ADH
How does the kidney respond in a state of antidiuresis?
- Increases reabsorption of water and urea
- Produces low volume of concentrated urine
What is water diuresis?
State of over hydration
In a state of antidiuresis, what concentration is ADH found at in the plasma?
Low
How does the kidney respond in a state of water diuresis?
- Decreases reabsorption of water and urea
- Produces high volume, dilute urine
Describe the osmolality of the kidney from the cortex to the outer medulla to the inner medulla.
Cortex - isotonic with plasma at 300 mOsm
Outer Medulla - mild hyperosmolality at 300-480 mOsm
Inner Medulla - hyperosmolarity at 480-1200 mOsm
What are the major species that contribute to the renal hyperosmolality and their relative contributions?
Na - 25%
Ca - 25%
Urea - 50%
1200 mOsm total
What happens to the kidney and the nephron during water diuresis?
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.
What happens to the kidney and the nephron during antidiuresis?
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.
What are the mechanisms that generate and regulate medullary hyperosmolality?
- Countercurrent Multiplier
- Countercurrent Exchanger
- Urea Cycle
What is the function of the countercurrent multiplier?
Establishes initial osmotic gradient
What is the function of the countercurrent exchanger?
Maintains osmotic gradient
What is the function of the urea cycle?
Strengthens osmotic gradient
Where does passive NaCl transport occur?
Ascending Thin Limb of Loop of Henle
Where does passive water transports occur in the Loop of Henle?
Descending Thin Limb of Loop of Henle
Where does active NaCl transport occur?
Thick Ascending Limb of Loop of Henle
Where is the urea permeability the highest?
The second half of the collecting duct in the medullary region
Describe the permeability in the thin descending Loop of Henle.
-high water permeability
-low salt permeability
Water moves out of tubule leaving salt behind
Describe the permeability in the thin ascending Loop of Henle.
-low water permeability
-high salt permeability
Salt moves out of tubule leaving water behind
Describe the permeability in the thick ascending Loop of Henle.
- water is impermeable
- active salt pumping
Site of most active salt pumping in kidney (Na+, Cl–, K+) and this leads fluid to become hyposmotic
Describe the absorption in the DT.
Increased H2O permeability in the late portion only and increased salt reabsorption
Describe the absorption in the upper collecting duct.
- active salt reabsorption
- passive water reabsorption under ADH control
Describe the absorption in the lower collecting duct.
- active salt reabsorption
- passive water and urea reabsorption under ADH control
How does the countercurrent multiplier generate the hyper osmotic gradient?
- Active transport of salt in the ascending thin loop generates hyperosmolality in the interstitial area
- H2O from the descending loop will move into the interstitial area as a result
- This generates intratubule osmolality differences from the “fresh” fluid coming from the PT to the “old” fluid near the end of the descending loop
What is the urea cycle?
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.
What is the high medullary concentration not used for?
Does not work as an osmotic gradient for H2O reabsorption
What is the function of the high medullary concentration?
- 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
What happens in the countercurrent exchanger?
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.
Describe the substance movements in the descending vasa recta.
- water moves out of capillary down osmotic gradient
- salt and urea move into capillary down concentration gradient
Describe the substance movements in the ascending vasa recta.
- water moves into capillary down osmotic gradient
- salt and urea move out of capillary down concentration gradient
What can disturb the medullary gradient?
Increasing the vasa recta flow as it depends on its slow flow to maintain the countercurrent exchange
What do medullary gradient disruptions lead to?
Increased urine flow
What are the requirements for medullary hyperosmolarity?
- Long loops of Henle
- Active salt pumping
- Differential permeabilities
What is the positive free water clearance?
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)
What is negative free water clearance?
–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)
