Quantitative Renal Physiology or Evaluation of Renal Function

Question 1

What is mass flow?

Answer 1

Expression of solute movements as a function of time expressed as (ex. ml of water/min, mEq of Na+/min, mg of creatinine/min)

Question 2

In solution, how is mass flow expressed?

Answer 2

Product of concentration of substance in urine and rate of urine flow

= (amount/ml)*(ml/min)

Ex. mass flow of Na+; urine flow = 1.5ml/min; Na+ conc in urine = 78 mmol/L= 0.078 mmol/ml

MF= 0.078*1.5= 0.117 mmol/min

Question 3

T or F. NaCl is a substance that is only present in the body from diet

Answer 3

T, It is not produced endogenously.

Thus, Rate of output= rate of input

Urea and uric acid, on the other hand are only produced endogenously as breakdown molecules

Question 4

How is urea excreted?

Answer 4

urine only

Question 5

What is the eqn for mass flow in kidney?

Answer 5

= rate of flow of the 4 different routes x concentration of each flow

Question 6

How can you estimate venous + lymphatic urea output from the kidney?

Answer 6

urea input = arterial urea (BUN)plasma flow into kidney ex. 25 mg/dl6.9dl/min= 172.5 mg/min

If urea output is 20 mg/min: 172.5-20 = 152.5 mg/min of venous +lymphatic urea output from the kidneys

Question 7

T or F. Water input to the kidney is the same as plasma flow into the kidney

Answer 7

T. 690 mL/min

Question 8

Mass Balance along the Nephron

Answer 8

Filtration + Secretion= Excretion + Reabsorption

Question 9

What is the eqn for rate of filtration or excretion?

Answer 9

solute conc. x volume flow rate

Question 10

What is the eqn for filtration rate?

Answer 10

GFR x Px, where Px = plasma conc. of a substance

Question 11

What is the eqn for excretion rate?

Answer 11

UF x Ux, where Ux = urinary conc. of a substance

Question 12

What are some good clinical indicators of progression of renal disease?

Answer 12

• GFR
• Plasma clearance of solutes
• Renal Blood Flow

Question 13

What is inulin?

Answer 13

a polymer of fructose that is not produced in the body, not metabolized, and is only excreted by the kidneys used to analyze kidney function

not secreted or reabsorbed

Question 14

What is the eqn. for P[x], plasma conc of a substance?

Answer 14

Rate of infusion (mg/min) (intake + production)/ Rate of output (excretion + metabolism)

Question 15

If 50% of the kidney is damaged, there will be a ___ decrease in GFR. Why?

Answer 15

25%, with increase in plasma conc. of solutes. There are compensatory mechanisms by the functioning nephrons

Question 16

T or F. For inulin, the rate of filtration is equal to the rate of excretion

Answer 16

T. Because inulin is not reabsorbed or secreted in the kidneys

Rate of filtration= Rate of excretion
GFR x Pin = UF x Uin

or GFR(in)= UF x Uin/Pin (aka clearance)

Question 17

What is clearance?

Answer 17

rate at which different substances are ‘cleared’ from the plasma: a way of quantitating kidneys ability to excrete various substances

Question 18

What is the eqn. for clearance?

Answer 18

Cin= (UF*Uin)/Pin

Rule: The greater the reabsorption of a substance, the less the clearance and vice-versa

Question 19

Clearance rates of various substances

Answer 19

Inulin- 125 ml/min
Glucose- 0
Sodium- 0.9
Chloride- 1.3
K+- 12.0
Phosphate- 25.0
Creatinine- 140 ml/min (secreted)

GFR= 130 ml/min

Question 20

What is the eqn. for extraction ratio?

Answer 20

percentage of substance removed from plasma

= (Ax-Vx(renal venous))/Ax

Ax= arterial conc.
Vx= venous conc. 

reabsorption= low extraction ratio
secretion= high extraction ratio

Question 21

If the substance is nearly completely extracted from plasma, that is the ER is almost 1, then the clearance is equal to what?

Answer 21

renal plasma flow

Question 22

What is the ER of PAH?

Answer 22

0.9-9.95, thus

Cpah =~ effective renal plasma flow (RPF)

Cpah= (UF*Upah)/Ppah

Question 23

What is the clearance value for PAH?

Answer 23

Cpah= (1ml x. 5.85 mg/mL)/0.01 mg/mL= 585 mL/min

Question 24

How can you calculate renal plasma flow from the clearance value?

Answer 24

Cpah/Extraction ratio of PAH= RPF

Ex. 585/0.9= 650 mL/min

Question 25

How can you calculate renal blood flow from RPF?

Answer 25

RPF/(1-hematocrit)

Ex. (PAH)- 650/(1-0.45)= 1182 mL/min

Question 26

What ENDOGENOUS substance could be a good indicator of renal function?

Answer 26

creatinine because although it is slightly secreted, its clearance is very close to GFR (140 compared to 130 mL/min)

Question 27

What is creatinine?

Answer 27

by-porduct of skeletal muscle metabolism that is produced in quantities of about 2g/day and has a relatively constant plasma concentration

Question 28

What happens to UF*Uin (excretion) if GFR drops suddenly?

Answer 28

It will immediately drop in proportion but will rebound to normal levels quickly because when urinary output drops, Pin rises in proportion to reach a new steady state which compensates

Question 29

Can this principle be applied to plasma creatinine in a patient with uni-nephrectomy?

Answer 29

Creatinine is produced at a rate of 1.8g/day and Pcr is maintained at 1 mg/dL

Yes, initially creatinine will drop and a new Pcr will be established

Question 30

What is the new steady state of Pcr achieved? New plasma conc.?

Answer 30

GFR or Ccr= (1.8g/day)*(1mg/dL)= 180L/day

with uninephrectomy: Pcr= (1.8g/day)*(90L/day)= 2 mg/dL

Question 31

What is the nature of relationship between Pcr (y) and GFR(x)?

Answer 31

At normal GFR (130mL/min), creatine is very low. A decrease down to 60 mL/min corresponds with a very small increase in Pcr. However, past this point the change is proportional and by GFR= 20, it is exponentially higher

Question 32

Why does Pcr not change significantly with slightly lowered GFR?

Answer 32

compensation with increased secretion until it reaches its maximum

So, in mild renal disease Pcr is not a very sensitive index and is a bad diagnostic tool.

Question 33

Why is inulin clearance the same as GFR?

Answer 33

Nor reabsorbed not secreted in the tubule