PSW 1 - Renal Clearance, Measurement of RBF and GFR Flashcards
What is the principle of mass balance through the kidney? What to note? Equation for RPF?
Input = output through an organ, so:
Rate of delivery of a substance to renal artery = rate of exit of a substance in renal vein + rate of exit of a substance in ureter
Pa x RPFa = Pv x RPFv + U x V
P = concentration of substance in plasma RPF = renal plasma flow rate U = concentration of substance in urine V = urine flow rate
NOTE: If RPFa ~ RPFv, then: RPF x (Pa-Pv) = U x V => RPF = U x V / (Pa-Pv)
What is the principle of mass balance equivalent to?
Fick’s principle with O2 consumption of an organ
Equation for rate of filtration of a substance? Assumption?
Assumption: if a substance can freely diffuse from plasma through glomerular pores then it’s concentration in Bowman’s space is the same as in plasma
Equation:
F = P x GFR
P = concentration of substance in plasma
Equation for rate of excretion of a substance?
Equation:
E = U x V
U = concentration of substance in urine V = urine flow rate
Equation for rate of reabsorption of a substance?
Equation:
R = F - E
Equation for rate of secretion of a substance?
Equation:
S = E - F
Equation for fractional excretion of a substance? What does it mean?
What fraction of the amount filtered was excreted
Equation:
FE = E/F x 100%
Equation for fractional reabsorption of a substance? What does it mean?
What fraction of the amount filtered was reabsorbed
Equation:
FR = R/F x 100% = 100-FE
How to relate the 6 equations of the rates of a substance through the kidney?
Amount excreted = amount filtered - amount reabsorbed + amount secreted
E = F - R + S U.V = P.GFR - (F-E) + (E-F)
What is clearance? Equation?
Volume of plasma cleared of a substance by the kidney per unit time
C = U.V / P
U = concentration of substance in urine V = urine flow rate P = concentration of substance in plasma
Does the body produce inulin endogenously?
NOPE
What 4 criteria does inulin fulfill? What does this mean?
- Freely filtered at the glomerulus
- Not reabsorbed or secreted by the nephron
- Not metabolized or produced by the kidney
- Does not alter GFR
=> amount of inulin filtered = amount of inulin excreted:
E = F P.GFR = U.V GFR = U.V/P = inulin clearance
U = concentration of substance in urine V = urine flow rate P = concentration of substance in plasma
What is inulin?
Large sugar
What is the average GFR in humans? So in 1 h, how many times does the total plasma volume get filtered?
100 mL/min = 144 L/day
Filtered twice in an hour (we have 3.5 L of plasma)
What is creatinine?
Byproduct of skeletal muscle creatine metabolism, which is normally produced endogenously at a constant rate in an amount proportional to muscle mass
What is creatinine useful for in terms of kidney function?
A very small amount is secreted by the tubule (+10% of amount filtered - E>F), so its clearance is only a slight overestimate of GFR
4 criteria required of a substance to be able to use it to measure GFR? Why?
- Freely filtered at the glomerulus
- Not reabsorbed or secreted by the nephron
- Not metabolized or produced by the kidney
- Does not alter GFR
To ensure that the whole amount of substance filtered is being excreted
Size of inulin?
14 Angstroms
Range of normal plasma creatinine levels?
0.7-1.2 mg/dL ~ 1 mg/dL
What happens to plasma creatinine if GFR is halved? What does mean?
It doubles
Plasma creatine is a great quick assessment of GFR
Is it easy to obtain the renal venous concentration of a substance?
NOPE
What is Para-Amino Hippuric acid (PAH)? What is it used for? Explain.
Organic anion
Filtered at the glomerulus and at low plasma concentrations virtually all the PAH that escapes filtration is secreted, such that the plasma leaving the kidney is completely cleared of PAH => PAH renal concentration is 0 => PAH clearance is an estimate of effective renal plasma flow, which is the portion of plasma which perfuses the secreting portions of the kidney
EQUATION:
Rate of amount excreted = rate of delivery to renal artery
E = P. RPF
RPF = U.V/P
U = concentration of PAH in urine V = urine flow rate P = concentration of PAH in plasma
From where do the secretions into the nephron come from?
Peritubular capillaries
What is the caveat of using PAH to calculate effective RPF? What does this mean?
Not all blood leaving the kidney goes to secretion area; e.g. blood through medulla (represents 5%)
Using PAH to calculate EFFECTIVE RPF will be a slight UNDERestimation of the actual RPF
Normal effective RPF?
500 mL/min
What is the filtration fraction equation? Normal value?
GFR/RPF ~ 20%
What is the renal fraction?
% of CO to kidneys = 20%
Average pee volume?
~300 mL
Average RBF in adult?
1L/min
When you are given diet osmolarity, urine osmolarity, and volume losses, how to calculate the volume of water needed to prevent a rise in plasma osmolarity?
- Calculate urine volume: diet osmolarity/urine osmolarity
2. Add daily insensible losses to that amount
