PSW 1 - Renal Clearance, Measurement of RBF and GFR

Question 1

What is the principle of mass balance through the kidney? What to note? Equation for RPF?

Answer 1

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)

Question 2

What is the principle of mass balance equivalent to?

Answer 2

Fick’s principle with O2 consumption of an organ

Question 3

Equation for rate of filtration of a substance? Assumption?

Answer 3

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

Question 4

Equation for rate of excretion of a substance?

Answer 4

Equation:
E = U x V

U = concentration of substance in urine
V = urine flow rate

Question 5

Equation for rate of reabsorption of a substance?

Answer 5

Equation:

R = F - E

Question 6

Equation for rate of secretion of a substance?

Answer 6

Equation:

S = E - F

Question 7

Equation for fractional excretion of a substance? What does it mean?

Answer 7

What fraction of the amount filtered was excreted

Equation:
FE = E/F x 100%

Question 8

Equation for fractional reabsorption of a substance? What does it mean?

Answer 8

What fraction of the amount filtered was reabsorbed

Equation:
FR = R/F x 100% = 100-FE

Question 9

How to relate the 6 equations of the rates of a substance through the kidney?

Answer 9

Amount excreted = amount filtered - amount reabsorbed + amount secreted

E = F - R + S 
U.V = P.GFR - (F-E) + (E-F)

Question 10

What is clearance? Equation?

Answer 10

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

Question 11

Does the body produce inulin endogenously?

Answer 11

NOPE

Question 12

What 4 criteria does inulin fulfill? What does this mean?

Answer 12

1. Freely filtered at the glomerulus
2. Not reabsorbed or secreted by the nephron
3. Not metabolized or produced by the kidney
4. 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

Question 13

What is inulin?

Answer 13

Large sugar

Question 14

What is the average GFR in humans? So in 1 h, how many times does the total plasma volume get filtered?

Answer 14

100 mL/min = 144 L/day

Filtered twice in an hour (we have 3.5 L of plasma)

Question 15

What is creatinine?

Answer 15

Byproduct of skeletal muscle creatine metabolism, which is normally produced endogenously at a constant rate in an amount proportional to muscle mass

Question 16

What is creatinine useful for in terms of kidney function?

Answer 16

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

Question 17

4 criteria required of a substance to be able to use it to measure GFR? Why?

Answer 17

1. Freely filtered at the glomerulus
2. Not reabsorbed or secreted by the nephron
3. Not metabolized or produced by the kidney
4. Does not alter GFR

To ensure that the whole amount of substance filtered is being excreted

Question 18

Size of inulin?

Answer 18

14 Angstroms

Question 19

Range of normal plasma creatinine levels?

Answer 19

0.7-1.2 mg/dL ~ 1 mg/dL

Question 20

What happens to plasma creatinine if GFR is halved? What does mean?

Answer 20

It doubles

Plasma creatine is a great quick assessment of GFR

Question 21

Is it easy to obtain the renal venous concentration of a substance?

Answer 21

NOPE

Question 22

What is Para-Amino Hippuric acid (PAH)? What is it used for? Explain.

Answer 22

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

Question 23

From where do the secretions into the nephron come from?

Answer 23

Peritubular capillaries

Question 24

What is the caveat of using PAH to calculate effective RPF? What does this mean?

Answer 24

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

Question 25

Normal effective RPF?

Answer 25

500 mL/min

Question 26

What is the filtration fraction equation? Normal value?

Answer 26

GFR/RPF ~ 20%

Question 27

What is the renal fraction?

Answer 27

% of CO to kidneys = 20%

Question 28

Average pee volume?

Answer 28

~300 mL

Question 29

Average RBF in adult?

Answer 29

1L/min

Question 30

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?

Answer 30

1. Calculate urine volume: diet osmolarity/urine osmolarity

2. Add daily insensible losses to that amount