GFR + clearance Flashcards
What is glomerular filtration rate?
The amount of filtrate produced from the blood flow per minute (ml/min)
What could cause a decrease in GFR?
Worsened kidney function:
- decline in number of nephrons
- decline in GFR within individual nephrons
What is the normal range of GFR?
60-120ml/min
What would the GFR be in a patient with kidney failure?
<15ml/min
(Stage 5)
What is used to measure GFR?
Renal clearance
What is renal clearance?
The amount of plasma that is cleared of a substance per minute
Clearance equation
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
When can clearance be used as a marker for GFR?
Provided the substance ‘follows’ the filtrate without being altered by the kidney:
- not reabsorbed
- not secreted
- not metabolised
^ by the kidney
When is nephron development finished during fetal development?
35/36th week
What 4 factors of a substance must be true for excretion rate = GFR?
- be produced at a constant rate
- be freely filtered across the glomerulus
- not be reabsorbed in the nephron
- not be secreted into the nephron
What is the best substance to measure GFR in theory?
Why is it not actually used?
Inulin
- it is freely filtered, not reabsorbed + not secreted
- required continuous IV to maintain a steady state
- requires catheter and timed urine collections
What substance is used instead of inulin to measure GFR?
51 Cr-EDTA
or
creatinine
What is creatinine?
End product of muscle breakdown
Disadvantages of using creatinine as a marked for GFR
- patient has to carry bottle of urine around
- overestimates GFR often as some is secreted from the nephron
When could creatinine not be used to measure GFR?
When its not produced at a constant rate
- e.g. diet or training changes
What is normal serum creatinine?
70-150 umol/L
What factors can increase serum creatinine?
- large muscle bulk
- young
- male
- creatinine supplements
- high meat intake
- certain drugs e.g. trimethoprim
What factors can reduce serum creatinine?
- reduced muscle mass
- old
- female
- vegetarian
Problems with estimating GFR
Inaccurate in mild kidney disease:
- reduction in GFR > increased blood flow
- reduced nephron number > nephron hypertrophy so no change in GFR
What happens in GFR in pregnancy?
Why?
- GFR increases: 130-180ml/min
- kidney side increases - increased fluid volume
- nephron number remains the same
