Quantitative Renal Physiology or Evaluation of Renal Function Flashcards

1
Q

What is mass flow?

A

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

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2
Q

In solution, how is mass flow expressed?

A

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

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3
Q

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

A

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

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4
Q

How is urea excreted?

A

urine only

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5
Q

What is the eqn for mass flow in kidney?

A

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

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6
Q

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

A

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

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7
Q

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

A

T. 690 mL/min

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8
Q

Mass Balance along the Nephron

A

Filtration + Secretion= Excretion + Reabsorption

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9
Q

What is the eqn for rate of filtration or excretion?

A

solute conc. x volume flow rate

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10
Q

What is the eqn for filtration rate?

A

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

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11
Q

What is the eqn for excretion rate?

A

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

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12
Q

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

A
  • GFR
  • Plasma clearance of solutes
  • Renal Blood Flow
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13
Q

What is inulin?

A

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

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14
Q

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

A

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

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15
Q

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

A

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

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16
Q

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

A

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)

17
Q

What is clearance?

A

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

18
Q

What is the eqn. for clearance?

A

Cin= (UF*Uin)/Pin

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

19
Q

Clearance rates of various substances

A
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

20
Q

What is the eqn. for extraction ratio?

A

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
21
Q

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

A

renal plasma flow

22
Q

What is the ER of PAH?

A

0.9-9.95, thus

Cpah =~ effective renal plasma flow (RPF)

Cpah= (UF*Upah)/Ppah

23
Q

What is the clearance value for PAH?

A

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

24
Q

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

A

Cpah/Extraction ratio of PAH= RPF

Ex. 585/0.9= 650 mL/min

25
Q

How can you calculate renal blood flow from RPF?

A

RPF/(1-hematocrit)

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

26
Q

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

A

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

27
Q

What is creatinine?

A

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

28
Q

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

A

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

29
Q

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

A

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

30
Q

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

A

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

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

31
Q

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

A

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

32
Q

Why does Pcr not change significantly with slightly lowered GFR?

A

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.

33
Q

Why is inulin clearance the same as GFR?

A

Nor reabsorbed not secreted in the tubule