GFR and Renal Clearance Problem Solving

Question 1

What happens when the afferent arteriole is constricted?

Answer 1

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.

Question 2

What happens when the efferent arteriole is constricted?

Answer 2

PGC increases, GFR increases, and RBF decreases. Since GFR increased and RBF decreased, FF increases.

Question 3

What happens when the afferent arteriole is dilated?

Answer 3

PGC increases, GFR increases, and RBF increases. Since both GFR and RBF increased, FF stays the same.

Question 4

What happens when the efferent arteriole is dilated?

Answer 4

PGC decreases, GFR decreases, and RBF increases. Since GFR decreased and RBF increased, FF decreases.

Question 5

What does clearance indicate?

Answer 5

The volume of plasma cleared of a substance per unit of time.

Question 6

What are the units of clearance?

Answer 6

mL/min or mL/day

Question 7

What is the equation for clearance?

Answer 7

C = U X V / P

Where C is clearance, U is urine concentration, V is urine flow, and P is plasma concentration.

Question 8

What does a clearance greater than GFR indicate?

Answer 8

That the substance is secreted as well as filtered.

Question 9

What does a clearance less than GFR indicate?

Answer 9

That the substance is reabsorbed.

Question 10

How is effective RPF calculated?

Answer 10

By calculating the clearance of PAH (since PAH is filtered and secreted it can be used to calculate RPF).

Question 11

What is the equation for calculating RBF?

Answer 11

RPF/1-hematocrit

Question 12

How is GFR calculated?

Answer 12

By calculating the clearance of inulin (it is freely filtered but neither secreted or reabsorbed).

Question 13

What is the equation for filtration fraction?

Answer 13

FF = GFR/RPF

Question 14

What is the equation for filtered load?

Answer 14

FL = GFR X [P]

Where P is plasma concentration

Question 15

What is the equation for excretion rate?

Answer 15

ER = [U] X V

Notice how this is the top part of the clearance equation.

Question 16

How is re-absorption rate calculated?

Answer 16

Reabsorption rate = filtered load - excretion rate

Question 17

How is secretion rate calculated?

Answer 17

Secretion rate = excretion rate - filtered load

Question 18

Can any concentration unit be used for [P] and [U] when calculating clearance?

Answer 18

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.

Question 19

What is the strategy to follow concerning solving for units?

Answer 19

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.

Question 20

What is the equation for fractional excretion?

Answer 20

(Excretion rate/Filtered load) X 100

Give you the percentage of a substance (most commonly sodium) that is excreted.

Question 21

What is the equation for net acid excretion? (this is an acid/base question)

Answer 21

Net acid excretion = NH4+ excretion + (titratable acid - HCO3- excretion)

Where titratable acid is phosphate or any other bound H+.

Question 22

What is the equation for new HCO3- creation in the plasma? (acid/base question)

Answer 22

New plasma HCO3- = Urinary NH4+ + titratable acid

New HCO3- must equal the amount of non-volatile acid produced.