PSW 2 - Assessment of Tubular Function Flashcards
What is a way to quantitate the renal contribution to the regulation of body fluid osmolarity? Equation?
Determine the amount of solute-free water that is excreted/reabsorbed by the kidney = amount of pure (or solute-free) water that would have to be added to or removed from the urine to make the urine isosmotic to plasma
V = Cosm + CH2O
V = urine flow rate Cosm = solute clearance CH2O = free water clearance
Cosm = Urine osmolarity.V/Plasma osmolarity
=> CH2O = V - Uosm.V/Posm
If a HYPOsmotic urine is formed, then is free water clearance positive or negative?
Positive
If a HYPERosmotic urine is formed, then is free water clearance positive or negative? What does this mean? Symbol?
Negative => solute-free water is being reabsorbed, not excreted, in order to form urine more concentrated than plasma
Symbol = TcH2O = the volume of free water transported
If the clearance of a substance is above the GFR, what does this mean?
Means it is also secreted by the kidney
If the clearance of a substance is above the clearance of another, what does this mean?
- Secreted MORE by the kidney
2. Better filtered at the glomerulus
How to compare clearances quantitatively?
If Ca>Cb, then Cb/Ca x 100 = x% =>
- Cb is being excreted at x% of the rate at which Ca is being excreted
- Cb is being reabsorbed at 100-x% of the rate at which Ca is being reabsorbed
How to relate excretion fraction, FE, and GFR?
FE = Cx/GFR
How can you calculate FE only with urine and plasma concentrations of creatinine and the substance X? What is this called?
FE = Cx/GFR = (Ux/Px).V/(Ucr./Pcr).V = (Ux/Px)/(Ucr./Pcr)
Called the double ratio
How to calculate clearance when a substance does not readily filter at the glomerulus?
C = U.V / (FF.P)
FF = fraction filtered
What % Mg2+ is bound by plasma proteins?
50%
What is the free water clearance if the urine is isosmotic?
0
CH2O = V - [Uosm.V/Posm] = V-V=0
Why is the ability of the kidney to excrete solute free water important?
Because when we drink a lot, we are diluting plasma osmolarity so we need to be able to excrete it
Unit of CH2O, free water clearance?
mL/min
What factors influence the formation of either positive or negative CH2O?
Factors that allow for the formation of dilute or concentrated urine
What drug would cause the free water clearance to go from positive to negative?
ADH analog that only affects the V2 receptors on the kidney (not the vascular ones that cause vasoconstriction)
Does changing the dietary sodium intake change the sodium plasma concentration? Explain.
NOPE, would just cause ECF/ICF water shifts as sodium controls plasma osmolarity
Plasma sodium levels are not an indication of sodium, but an indication of plasma water volume
How to calculate ERBF when you have ERPF?
ERBF = ERPF/(1-hematocrit)
How can you calculate the total amount of water reabsorbed in a day?
GFR - V over 24 hours
Average daily Na+ excreted?
150 mmol/day
How to calculate the fractional excretion of water? What is important to note?
Equivalent to % GFR excreted
FE=V/GFRx100
Note: this is NOT solute free water
How to relate FE of water to plasma and urine inulin/creatinine?
FE = Pin/Uin
OR
FE = Pcr/Ucr
The higher Uin/Pin…
The more water was reabsorbed by the kidney
How to calculate the GFR of a single nephron? What to note?
SNGFR = TFin.TFR/Pin = 100 mL/min/2M nephrons = average GFR of 50 nl/min => STAYS CONSTANT THROUGHOUT TUBULE because as TFin/Pin goes up, TFR goes down
TFin = tubular fluid inulin concentration TFR = tubular flow rate
What is the micropuncture technique? What is it used for?
Puncture a tubule, block it distally, and use a pipette to collect the incoming tubular fluid => used to measure tubular flow and concentrations throughout a nephron
What would the TFin/Pin ratio be in Bowman’s space?
1
What would the TFin/Pin ratio be mid-proximal tubule?
> 1
What is fractional delivery of water? Include overall and in a single nephron.
Fractional excretion inside 1 tubule = fraction of GFR still present inside the tubule = V/GFR = V / Cin = 1/TFin/Pin = Pin/TFin
What can we assume if the tubular fluid inulin to plasma inulin ratio is increasing?
Flow rate in tubule is proportionally decreasing
How to calculate fractional reabsorption in a single nephron?
1 - Pin/TFin
What is fractional delivery of a substance?
Fraction of substance still present in tubule at a particular point = (TFx/Px)/(TFin./Pin)
Log(x) = 0… x=>
1
At what site in the nephron is transepithelial potential difference of greatest magnitude?
End of DCT/beginning of CD
When given a problem where a patient is urinating at different times, how to calculate V?
You are calculating urine production essentially, so pick at time at which she urinated at a starting point and use the volume peed at the next time and the time difference between the 2 (DO NOT ADD VOLUME)
What is the % TBW of an adult?
60% of weight
What is the % ECF of an adult?
1/3rd of TBW
What is the % plasma of an adult?
1/4th of ECF
What does the “insensible waterloss” mean?
The amount of fluid lost on a daily basis from the lungs and skin
How to calculate minimum volume of urine based on osm intake and urine concentration?
osm intake/urine osm
