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Urinary > Lecture 6 - GFR and clearance > Flashcards

Lecture 6 - GFR and clearance Flashcards

1
Q

GFR

A

Glomerular filtration rate

Amount of filtrate that is produced from the blood flow per unit time

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2
Q

What determines the amount of filtrate produced

A

Product of the average filtration of each nephron

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3
Q

Normal GFR

A

90 - 120 ml/min/1.73m^2

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4
Q

Total glomerular filtrate per day

A

140 - 180 L/day

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5
Q

What does GFR depend on?

A 
Age 
Gender
Size of kidney 
Size of individual
Pregnant
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6
Q

When does nephron development finish

A

35 - 36 weeks of fetal development

- premature babies often have lower nephron no.

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7
Q

GFR at birth

A

20 ml/min

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8
Q

When does GFR become normal in babies

A

18 months (90- 100ml/min)

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9
Q

When does GFR decline?

A

30 years old

Rate of decline = 6-7ml/min/decade

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10
Q

Why does GFR decline with age

A

Loss of functioning nephrons

In 60 - 70 yr olds :

  • Cortex decreases therefore glomerulus decreases and GFR decreases
  • Medulla increases
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11
Q

Compensatory hypertrophy

A

Increase in cortex and medulla size therefore increases in glomerulus and GFR

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12
Q

Pregnancy

A

GFR increases by 50% (130 - 180ml/min)

Kidney size increases but nephron no. stays the same

GFR increases due to an increase in interstitial and vascular volume increasing ECF

Returns to normal 6 months postpartum

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13
Q

Size

A

Males are generally larger
Larger size have larger kidneys generally
Larger kidneys have more functioning nephrons

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14
Q

Measuring GFR

A

Large variability in GFR therefore monitor the difference in GFR of a patient over time

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15
Q

Why would GFR decline?

A

Less nephrons

Decline in GFR within a nephron - abnormal

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16
Q

Why would GFR increase?

A

Kidney function recovers

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17
Q

Why is GFR not a good marker for CKD?

A

When kidney function declines slowly, individual nephrons may hypertrophy to compensate

Therefore GFR may not fall until significant kidney damage has occured

18
Q

Why can’t actual GFR be measure?

A

Kidney actively reabsorbs and secretes substances

19
Q

Clearance

A

The volume of plasma cleared of a substance per unit time.

20
Q

Equation

A

Clearance = amount of substance eliminated per unit time/ plasma concentration of substance

Cx = Ax/ Pxa

21
Q

Renal clearance

A

Renal clearance = Excretion rate / arterial plasma concentration of substance

UV/Pxa

22
Q

Excretion rate

A

Excretion rate = amount of substance in urine x urine flow rate

= UV

23
Q

What happens to clearance when flow increases?

A

Increases

24
Q

What happens to clearance when plasma concentration increases?

A

Decreases

25
Q

What are the 4 factors for renal clearance markers?

A

Produced by body at a constant rate
Freely filtered across the glomerulus
Not reabsorbed in the nephron
Not secreted into the nephron

26
Q

Exogenous markers

A

Inulin - plant based polysaccharide
51 Cr- EDTA - radioactively labelled marker
Lohexol (contrast agent in CT scans)

27
Q

Endogenous markers

A

Creatinine - end product oof muscle break down

28
Q

Disadvantages of inulin

A

Not produced by body
Requires continuous IV to maintain steady state
Requires catheter and timed urine collections

29
Q

Advantages and disadvantages of 51 - Cr EDTA

A

Disadvantages - not produced by the body therefore requires times injections

Advantages - Cleared exclusively by renal filtration

30
Q

When is 51 - Cr EDTA used?

A

In children with cancer as cancer drugs are nephrotoxic need to know GFR before given

Kidney transplants

31
Q

Creatinine

A

End product of muscle breakdown

The higher the creatinine in blood the lower the GFR as not being cleared

32
Q

Disadvantages of creatinine

A

Inaccurate
Have to carry bottle of urine
Overestimation
Varies greatly between individuals

33
Q

When is creatinine used?

A

In pregnant ladies as not radioactive and safe

34
Q

Normal creatininin

A

70- 150 micromol /L

35
Q

Factors that increase creatinine

A 
High protein intake 
High muscle mass
Young
Male 
Black 
Creatinine supplements
Drugs - trimethoprim
36
Q

Factors that decrease creatinine

A 
Reduced muscle mass
Old
Female
Hispanic
Vegetarian 
Baby once born
37
Q

Why is creatinine an overestimation of GFR

A

It is secreted into the tubule after it has been filtered therefore lower than actual filtrate as more lost.

38
Q

eGFR

A

estimated GFR

Takes into account :

  • Age
  • Sex
  • Ethnicity for black and caucasian
  • Serum creatinine

Standardised to body SA

39
Q

In which people is eGFR inaccurate for?

A 
Children
Pregnancy 
Old age
Other ethnicities 
Amputees - reduced muscle mass 
When true GFR changes quickly - AKI
Patients with high kidney function
40
Q

CKD - EPI

A

More accurate for patients with high kidney function

41
Q

Why is GFR not a good marker for AKI

A

Reduced glomerular SA increases blood flow
Reduced nephron no. can cause nephron hypertrophy
Can get increased creatinine secretion into tubule

Urinary (23 decks)

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