GFR and Renal Clearance Problem Solving Flashcards
What happens when the afferent arteriole is constricted?
Pressure in the glomerular capillaries (PGC) is decreased, GFR is decreased, and RBF is decreased. Since both GFR and RBF decreases the filtration fraction (FF) stays the same.
What happens when the efferent arteriole is constricted?
PGC increases, GFR increases, and RBF decreases. Since GFR increased and RBF decreased, FF increases.
What happens when the afferent arteriole is dilated?
PGC increases, GFR increases, and RBF increases. Since both GFR and RBF increased, FF stays the same.
What happens when the efferent arteriole is dilated?
PGC decreases, GFR decreases, and RBF increases. Since GFR decreased and RBF increased, FF decreases.
What does clearance indicate?
The volume of plasma cleared of a substance per unit of time.
What are the units of clearance?
mL/min or mL/day
What is the equation for clearance?
C = U X V / P
Where C is clearance, U is urine concentration, V is urine flow, and P is plasma concentration.
What does a clearance greater than GFR indicate?
That the substance is secreted as well as filtered.
What does a clearance less than GFR indicate?
That the substance is reabsorbed.
How is effective RPF calculated?
By calculating the clearance of PAH (since PAH is filtered and secreted it can be used to calculate RPF).
What is the equation for calculating RBF?
RPF/1-hematocrit
How is GFR calculated?
By calculating the clearance of inulin (it is freely filtered but neither secreted or reabsorbed).
What is the equation for filtration fraction?
FF = GFR/RPF
What is the equation for filtered load?
FL = GFR X [P]
Where P is plasma concentration
What is the equation for excretion rate?
ER = [U] X V
Notice how this is the top part of the clearance equation.
How is re-absorption rate calculated?
Reabsorption rate = filtered load - excretion rate
How is secretion rate calculated?
Secretion rate = excretion rate - filtered load
Can any concentration unit be used for [P] and [U] when calculating clearance?
Yes, as long as they are both the same (since the units will cancel out and the answer will be in unit/time from the urine flow). If they are different then a unit conversion must be calculated.
What is the strategy to follow concerning solving for units?
Figure out what equation(s) to use (i.e. what the question is asking). Then put in the given values and figure out from there how to make them match up with the units of the answer choices.
What is the equation for fractional excretion?
(Excretion rate/Filtered load) X 100
Give you the percentage of a substance (most commonly sodium) that is excreted.
What is the equation for net acid excretion? (this is an acid/base question)
Net acid excretion = NH4+ excretion + (titratable acid - HCO3- excretion)
Where titratable acid is phosphate or any other bound H+.
What is the equation for new HCO3- creation in the plasma? (acid/base question)
New plasma HCO3- = Urinary NH4+ + titratable acid
New HCO3- must equal the amount of non-volatile acid produced.
