GFR + clearance Flashcards

1
Q

What is glomerular filtration rate?

A

The amount of filtrate produced from the blood flow per minute (ml/min)

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2
Q

What could cause a decrease in GFR?

A

Worsened kidney function:
- decline in number of nephrons
- decline in GFR within individual nephrons

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3
Q

What is the normal range of GFR?

A

60-120ml/min

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4
Q

What would the GFR be in a patient with kidney failure?

A

<15ml/min
(Stage 5)

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5
Q

What is used to measure GFR?

A

Renal clearance

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6
Q

What is renal clearance?

A

The amount of plasma that is cleared of a substance per minute

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7
Q

Clearance equation

A

Cx = ([U]x x V) / [P]x

Cx - clearance of x ml/min
[U]x - urine conc. of x mg/ml
V - urine flow rate ml/min
[P]x- plasma conc.of x mg/ml

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8
Q

When can clearance be used as a marker for GFR?

A

Provided the substance ‘follows’ the filtrate without being altered by the kidney:
- not reabsorbed
- not secreted
- not metabolised
^ by the kidney

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9
Q

When is nephron development finished during fetal development?

A

35/36th week

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10
Q

What 4 factors of a substance must be true for excretion rate = GFR?

A
  • be produced at a constant rate
  • be freely filtered across the glomerulus
  • not be reabsorbed in the nephron
  • not be secreted into the nephron
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11
Q

What is the best substance to measure GFR in theory?
Why is it not actually used?

A

Inulin
- it is freely filtered, not reabsorbed + not secreted

  • required continuous IV to maintain a steady state
  • requires catheter and timed urine collections
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12
Q

What substance is used instead of inulin to measure GFR?

A

51 Cr-EDTA
or
creatinine

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13
Q

What is creatinine?

A

End product of muscle breakdown

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14
Q

Disadvantages of using creatinine as a marked for GFR

A
  • patient has to carry bottle of urine around
  • overestimates GFR often as some is secreted from the nephron
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15
Q

When could creatinine not be used to measure GFR?

A

When its not produced at a constant rate

  • e.g. diet or training changes
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16
Q

What is normal serum creatinine?

A

70-150 umol/L

17
Q

What factors can increase serum creatinine?

A
  • large muscle bulk
  • young
  • male
  • creatinine supplements
  • high meat intake
  • certain drugs e.g. trimethoprim
18
Q

What factors can reduce serum creatinine?

A
  • reduced muscle mass
  • old
  • female
  • vegetarian
19
Q

Problems with estimating GFR

A

Inaccurate in mild kidney disease:
- reduction in GFR > increased blood flow
- reduced nephron number > nephron hypertrophy so no change in GFR

20
Q

What happens in GFR in pregnancy?
Why?

A
  • GFR increases: 130-180ml/min
  • kidney side increases - increased fluid volume
  • nephron number remains the same