Renal Physiology Formulas

Question 1

How do you calculate the volume of a compartment of interest?

Answer 1

V = Q/C

V= volume
Q= quantity of indicator administered
C= measured concentration of indicator after equilibrium has occurred.

Question 2

What are some indicators used to find TBW?

Answer 2

D20 deuterium (same as ^2H20; aka heavy water)
^3H20 tritium
Antipyrine

Question 3

Indicators used to calculate PV?

Answer 3

Evans blue dye

^125 I-albumin (radioiodinated serum albumin)

Question 4

Indicators used to calculate ECF?

Answer 4

Inulin
Mannitol
^22-sodium

Question 5

3 formulas that connect the relationship between ICF, ISF, and BV?

*These can help you indirectly calculate volumes of other compartments

Answer 5

ICF = TBW - ECF

ISF = ECF - PV

Blood = PV/(1 - hematocrit)

Question 6

What is the normal measured value for body fluid osmolarity?

What is the formula to calculate plasma osmolarity?

Answer 6

BFO = 290 mOsm/L

P(osm) = 2 x [Na] + [glucose]/18 + BUN/2.8

*BUN = [urea]

Question 7

Knowing someones body weight, how do you calculate TBW?

Answer 7

TBW = 0.6 * weight in kg

• TBW is 60% of body weight. If given weight in pounds, must convert to kg.
• divide weight in lbs by 2.2

Question 8

Knowing someones body, how do you calculate ICF volume?

Answer 8

ICF volume = 0.4 * weight in kg

*ICF is 2/3 of TBW, which means it is 40% of body weight

Question 9

How do you calculate ECF volume if you know body weight?

Answer 9

ECF volume = 0.2 * weight in kg

*ECF is 1/3 of TBW, making it 20% of body weight

Question 10

If you want to find the osmolarity of a given compartment, how would you calculate it?

Answer 10

Multiply the compartment volume with the body fluid osmolarity.

Ex: TBW * 290 mOsm/L (the normal value for BFO)

Question 11

What is Renal Clearance, C(x)?

How do you calculate it?

Answer 11

It is the volume of plasma cleared of a given substance by the kidneys per unit time.

C(x) = [Ux] * V/[Px]

Ux = urine concentration of substance x in mg/mL
V = the urine flow rate in mL/min
Px = plasma concentration of substance x in mg/mL

Question 12

What is happening physiologically if:
1) C(x) > GFR?

2) C(x) < GFR?
3) C(x) = GFR?

Answer 12

1) Substance x is being filtered and secreted.
2) Substance is being filtered and reabsorbed.
3) Substance is filtered and excreted with no net secretion or reabsorption.

Question 13

What exogenous substance is freely filterable, not secreted or reabsorbed and can be used to determine the clearance ratio?

Answer 13

Inulin.

Clearance ratio = Clearance(x)/Clearance(inulin)

If 1.0, then substance is excreted like inulin.
If > 1.0, then substance is filtered + secreted
If < 1.0, then substance is filtered + reabsorbed

Question 14

What endogenous substance is used to calculate GFR, and as such can also be used to calculate clearance ratio?

Answer 14

Creatinine.

Note that it is 10% secreted, so its a mild overestimate of GFR.

Question 15

What is the normal GFR?

Answer 15

120-130 mL/min

*120 in book, 130 per Dr. Rao

Question 16

How much of the cardiac output do the kidneys receive?

Answer 16

20-25%

Normal cardiac output = 5L/min. So the kidneys get 1.25 L/min or 1800 L/day

Question 17

What is the average pressure in the afferent arteriole?

Efferent arteriole?

Answer 17

Afferent arteriole = 85 mmHg

Efferent arteriole = 60 mmHg

Question 18

What is the filtration fraction, FF?

Calculation?

Answer 18

FF is the fraction of plasma filtered across the glomerular filtration barrier.

FF = GFR/RPF

Question 19

Normal value for FF?

Answer 19

0.20 or 20%

This means that 20% of the RPF enters the renal tubules, while the remaining 80% leaves the glomerulus via the efferent arteriole and becomes the peritubular capillary circulation.

Question 20

What are the extremes of systolic blood pressure when autoregulation can no longer maintain RPF and/or GFR?

Answer 20

When SBP < 80 mmHg and when SBP > 200 mmHg

Question 21

What is renal plasma flow?

What endogenous substance is used to calculate it?

Answer 21

RPF = Measure of the volume of plasma delivered to the kidney in a given amount of time.

para-Aminohippuric acid (PAH); it’s 90% excreted (filtered and secreted), so the amount of PAH in the plasma of the renal artery is approximate to the amount of PAH in the urine.

Question 22

How would you calculate RPF?

Answer 22

Calculate the clearance of PAH, then divide by 0.9.

Ex: C(PAH) = (U*V)/P

Because its only 90% excreted, you then divide by 0.9.

True RPF = C(PAH)/0.9

Question 23

What is renal blood flow?

How do you calculate it?

Answer 23

RBF = volume of blood delivered to the kidney in a given period of time.

*To calculate it, you must know the RPF.

RBF = (RPF)/(1-hematocrit)

Question 24

How is the FF related to GFR?

How is FF related to RPF?

Answer 24

FF is proportional to GFR, indirectly proportional to RPF.

Remember, FF = GFR/RPF

Question 25

An increase in the FF would do what to the osmolarity in the peritubular capillaries?

Answer 25

An increase in FF reduces the amount and increases the osmolarity of the plasma flowing in the peritubular capillaries.

Question 26

What are the Starling forces that contribute to glomerular filtration?

Answer 26

• -Promote filtration:
  1) Hydrostatic pressure of glomerular capillary
  2) Oncotic pressure of urinary space (essentially zero since no protein should be filtered)
• -Against filtration:
  1) Hydrostatic pressure of urinary space
  2) Oncotic pressure of glomerular capillary

Question 27

At what serum concentration of glucose will you start seeing glycosuria?

Concentration when you see clinical glycosuria?

Answer 27

At about 200 mg/dL, small amounts of glucose enter the urine. This is the threshold value, or splay phenomenon. (called splay on boards).

At glucose > 350 mg/dL you see clinical glycosuria since you’ve now exceeded the transport maximum for glucose in the distal tubule.