Urine Concentration and Dilution Flashcards
What is the renal medullary osmotic gradient?
gradient in renal medulla of increasing osmolality from base (corticomedullary junction) to apex (papilla tip)
What does countercurrent multiplication do?
What structures are involved?
establishes/generates medullary osmotic gradient
mechanism involves renal tubule and surrounding interstitium
What does countercurrent exchange do?
What structures are involved?
maintains medullary osmotic gradient
mechanism involves vasa recta (in vasculature of medulla) and surrounding interstitium
What does the osmotic gradient do?
allows for production of hyperosmotic urine
makes urine osmolality > plasma osmolality
tubule can become super concentrated without affecting plasma concentration
Why is the osmotic gradient important?
allows for large excretion of solutes with minimal loss of H2O
if we aren’t able to concentrate our urine to eliminate all the waste products we need to get rid of, we would lose tons of water, and would have to drink water all day long
What is concurrent flow?
- same direction
- best possible exchange with parallel flow is 50%
- gradient driving diffusive transport lessen progressively and is ultimately lost
What is counterconcurrent flow?
- opposite direction
- nearly complete (100%) transfer occurs anti-parallel flow
- donor flow (high temperature or electrochemical gradient) is always entering a region where acceptor flow (low temperature or electrochemical gradient) is lower
- thus, gradient that drives flux never collapses
What is countercurrent multiplication driven by?
active Na+ reabsorption in TAL
main contributor to establishing gradient
What structures does urine concentration and dilution involve?
coordinated function of all segments from loop of Henle (DTL) to collecting duct (IMCD)
What do transport and permeability properties along nephron allow?
- generation of medullary osmotic gradient
- regulation of urine osmolality
Countercurrent Multiplication
What is the TAL important for?
active Na+ reabsorption (transport)
main motor that drives generation of osmotic gradient in medulla
Countercurrent Multiplication
What is the ATL important for?
important for urea handling – changes in handling impact establishment of gradient in deeper region, closer to apex
(absent in short loop nephrons)
Segments of Tubule with Active Na+ Transport
PT (++) DTL (0) ATL (0) TAL (++) DCT (+) CCD (+) IMCD (+)
Segments of Tubule Permeable to H2O?
PT (++) DTL (++) ATL (0) TAL (0) DCT (+AVP) CCD (+AVP) IMCD (+AVP)
Segments of Tubule Permeable to NaCl?
PT (+) DTL (0) ATL (+) TAL (0) DCT (0) CCD (0) IMCD (0)
Segments of Tubule Permeable to Urea?
PT (+) DTL (+) ATL (+) TAL (0) DCT (0) CCD (0) IMCD (+AVP)
Can H2O and Na+ move when descending loop of Henle?
(DTL)
H2O can move but Na+ cannot
Can H2O and Na+ move when ascending loop of Henle?
(ATL)
Na+ can move but H2O cannot
What does active Na+ reabsorption in TAL separate?
separates H2O from solutes
Permeabilities of which segments of the tubule are modifiable by hormones (AVP)?
distal tubule and onwards
- involved in determining how much H2O permeability there is in the tubule region
- determines urea permeability in collecting duct
What is the loop of Henle of a short loop nephron composed of?
thin descending limb – H2O permeable, Na+ impermeable
thick ascending limb – H2O impermeable, Na+ permeable
(ATL absent)
Countercurrent Multiplication – Mechanism
see notes
What does urea handling do?
What structures are involved?
contributes to generation of medullary osmotic gradient in long-loop nephrons
- ATL contributes to increase in osmotic gradient, especially in inner medullary region
- start to dilute tubular fluid as we move out of that region
Ultimately, what are changes we see in osmolality of interstitium due to?
primarily due to Na+ movement occurring as a response to what we see with urea transport
passive Na+ transport contributes to inner medullary gradient that’s occurring in response to urea transport that’s occurring in ATL
NO NA+ ACTIVE TRANSPORT – TAL ONLY
Where is the most concentrated portion of the interstitium?
peak (apex)
Which segments of the tubule are impermeable to urea?
TAL
DCT
CCD
OMCD
Urea Handling – Mechanism
see notes
What are the two structures involved in countercurrent exchange?
descending vasa recta
ascending vasa recta
Countercurrent Exchange
Descending Vasa Recta – What is the gradient?
plasma_osmolality < interstitium_osmolality
gradient for:
- solutes to move from interstitium to plasma
- H2O to move from plasma to interstitium
- (both contribute to concentrating plasma)
loss of H2O and gain of solute contributes to increase in osmolality
Countercurrent Exchange
Ascending Vasa Recta – What is the gradient?
plasma_osmolality > interstitium_osmolality
gradient for:
- solutes to move from plasma to interstitium
- H2O to move from interstitium to plasma
- (both contribute to diluting plasma)
gain of H2O and loss of solute contributes to decrease in osmolality
Countercurrent Exchange
Why is the U-shaped design of the vasa recta important?
critical for preventing washout of medullary osmotic gradient
hypothetical straight tube exchanger:
- loss of H2O from vessel as we move into more concentrated regions
- movement of solute into vessel as we move into more concentrated regions
- problem: plasma leaving would be hyperosmotic – will make you very thirsty
- problem: will dilute concentration we’ve worked hard to establish in this region
