Lecture 6 - GFR and clearance

Question 1

GFR

Answer 1

Glomerular filtration rate

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

Question 2

What determines the amount of filtrate produced

Answer 2

Product of the average filtration of each nephron

Question 3

Normal GFR

Answer 3

90 - 120 ml/min/1.73m^2

Question 4

Total glomerular filtrate per day

Answer 4

140 - 180 L/day

Question 5

What does GFR depend on?

Answer 5

Age 
Gender
Size of kidney 
Size of individual
Pregnant

Question 6

When does nephron development finish

Answer 6

35 - 36 weeks of fetal development

- premature babies often have lower nephron no.

Question 7

GFR at birth

Answer 7

20 ml/min

Question 8

When does GFR become normal in babies

Answer 8

18 months (90- 100ml/min)

Question 9

When does GFR decline?

Answer 9

30 years old

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

Question 10

Why does GFR decline with age

Answer 10

Loss of functioning nephrons

In 60 - 70 yr olds :

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

Question 11

Compensatory hypertrophy

Answer 11

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

Question 12

Pregnancy

Answer 12

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

Question 13

Size

Answer 13

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

Question 14

Measuring GFR

Answer 14

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

Question 15

Why would GFR decline?

Answer 15

Less nephrons

Decline in GFR within a nephron - abnormal

Question 16

Why would GFR increase?

Answer 16

Kidney function recovers

Question 17

Why is GFR not a good marker for CKD?

Answer 17

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

Therefore GFR may not fall until significant kidney damage has occured

Question 18

Why can’t actual GFR be measure?

Answer 18

Kidney actively reabsorbs and secretes substances

Question 19

Clearance

Answer 19

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

Question 20

Equation

Answer 20

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

Cx = Ax/ Pxa

Question 21

Renal clearance

Answer 21

Renal clearance = Excretion rate / arterial plasma concentration of substance

UV/Pxa

Question 22

Excretion rate

Answer 22

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

= UV

Question 23

What happens to clearance when flow increases?

Answer 23

Increases

Question 24

What happens to clearance when plasma concentration increases?

Answer 24

Decreases

Question 25

What are the 4 factors for renal clearance markers?

Answer 25

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

Question 26

Exogenous markers

Answer 26

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

Question 27

Endogenous markers

Answer 27

Creatinine - end product oof muscle break down

Question 28

Disadvantages of inulin

Answer 28

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

Question 29

Advantages and disadvantages of 51 - Cr EDTA

Answer 29

Disadvantages - not produced by the body therefore requires times injections Advantages - Cleared exclusively by renal filtration

Question 30

When is 51 - Cr EDTA used?

Answer 30

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

Question 31

Creatinine

Answer 31

End product of muscle breakdown The higher the creatinine in blood the lower the GFR as not being cleared

Question 32

Disadvantages of creatinine

Answer 32

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

Question 33

When is creatinine used?

Answer 33

In pregnant ladies as not radioactive and safe

Question 34

Normal creatininin

Answer 34

70- 150 micromol /L

Question 35

Factors that increase creatinine

Answer 35

``` High protein intake High muscle mass Young Male Black Creatinine supplements Drugs - trimethoprim ```

Question 36

Factors that decrease creatinine

Answer 36

``` Reduced muscle mass Old Female Hispanic Vegetarian Baby once born ```

Question 37

Why is creatinine an overestimation of GFR

Answer 37

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

Question 38

eGFR

Answer 38

estimated GFR Takes into account : - Age - Sex - Ethnicity for black and caucasian - Serum creatinine Standardised to body SA

Question 39

In which people is eGFR inaccurate for?

Answer 39

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

Question 40

CKD - EPI

Answer 40

More accurate for patients with high kidney function

Question 41

Why is GFR not a good marker for AKI

Answer 41

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