Urine Concentration and Dilution Flashcards
What is osmolality?
A measure of the number of dissolved particles per unit of water in the urine
What is the equation for osmolality?
Osm = [X] x n
What is counter-current multiplication?
The process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine
What is the role of the thick ascending limb in counter current multiplication?
NKCC2, ROMK and CLCK/Barttin actively pump out sodium, potassium and chloride
The thick ascending limb in impermeable to water
This establishes a concentration gradient between the inside of the tubule and the interstitium
On the basolateral membrane of these cells is a sodium-potassium pump that establishes the initial sodium gradient that allows their movement into cells
Increased osmotic pressure in the interstitium
What is the role of the thin descending limb in counter current multiplication?
It is permeable to water and impermeable to solutes (except for a very small NaCl leak)
The concentration of the tubular fluid increases down the descending limb (290 to 1400)
H2O moves out through AQP1
Water moves out and solutes move in
What is the role of the thin ascending limb in counter current multiplication?
Water impermeable and passively permeable to NaCl and urea
Solutes move out of tubular fluid but water is retained
Tubular fluid becomes more dilute as you move up the ascending limb
KO of which channel in the descending limb leads to problems with urine concentration?
Aquaporin I
What are some of the consequences of Bartter’s syndrome?
Salt wasting due to loss of NaCl
Polyuria due to less water reabsorption
Hypokalaemia
What does the transverse and vertical gradient hypothesis state?
That solute that moves out of the ascending limb moves into the descending limb so the osmolality goes down in the ascending limb and goes up in the descending limb
The solute then moves to go into the ascending limb, where fluid will once again move out into the descending limb, further increasing to osmolality
What is the role of urea in the nephron?
There is a low urea permeability in the early part of the collecting duct
The early part of the collecting duct is also permeable to water
Water therefore moves out, increasing the urea concentration down the collecting duct
Inner medullary collecting duct is now permeable to urea - a small amount of urea is now transported back into the thin ascending limb
Describe how urea is transported in the inner medullary collecting duct
Two transport proteins: UT-A1 (apical) and UT-A3 (basolateral)
Concentration of urea on the apical side is higher than on basolateral
Driven by early CD not being permeable to urea
What happens in UT-A1/A3 double KO mice?
These mice are H2O deprived due to minimal urea in the interstitial fluid and therefore less water reabsorption - less of a driving force
Urine osmolality increases
In free access to water the urine osmolality decreases by approx half
When H2O deprived the KO can’t respond and urine osmolality stays the same
Disrupts counter current multiplication
What is counter current exchange?
Prevention of wash out of interstitial fluid by the vasa recta
As the blood vessel moves down into the medulla, solutes move from the interstitial fluid into the plasma, increasing the osmolality of the plasma
As the blood vessel moves back up, solute moves back out of the plasma, therefore decreasing its osmolality
What does loss of function of UT-B lead to?
An inability to concentrate urine
