GFR and Clearance

Question 1

Functions of kidney

Answer 1

.

Question 2

How is kidney function measured?

Answer 2

GFR

Question 3

What is GFR? what is it determined by? what is normal GFR?

Answer 3

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

determined by the product of the average filtration of each nephron in each kidney

Normal GFR is 90 – 120 mL/min

OR

normal total GFR per day is 140 – 180 L /day

Question 4

what factors does GFR depend on?

Answer 4

• Gender
• Age>> babies get a normal GFR at 18 months, as u get older, lose nephrons
• Size of individual>> Bigger people > bigger kidneys > more nephrons
• Size of kidneys
• Pregnancy

Question 5

Describe the relationship between GFR and kidney function

Answer 5

Declining GFR indicates decling kidney function

BUTTT>>>>

when kidney function declines slowly, individual nephrons may hypertrophy so actual GFR may not fall until significant kidney damage has occurred.

Question 6

who has a highr GFR, Boys or girls?

Answer 6

boys

men: 120 mL/min or 180 L /day

women: 90 mL/min or 140 L /day

Question 7

When can GFR increase?

Answer 7

• Pregnant women: GFR increases (130-180ml/minute)
• Kidney size increases (~ 1 cm)
• Increased fluid volume (vascular & interstitial)
• Nephron number the same interstitial)

Question 8

What shoul dbe the characteristic of the substance to be able to use it to measure GFR?

Answer 8

If used to measure kidney clearance a substance should:

• Be produced at a constant rate
• Be freely filtered across the glomerulus
• Not be reabsorbed or secreted in the nephron

If all these are true then excretion rate = GFR

Question 9

What is the best substance to measure GFR? what is it? why?

Answer 9

inulin

an exogenous substance that is injected. (not produced by body)

freely filtered & neither secreted or reabsorbed by the tubule

Question 10

What is Renal Clearance? Formula? what does it depend on?

Answer 10

how much volume of blood plasma is completely cleared of that substance over time by our kidneys!

• ml/min*
• depends on filtration, secretion and reabsorption.*

Question 11

if inulin is goals in life, Why dont we use inulin to measure clearence? what do we use clically instead?

Answer 11

.Measurement of radio-labelled substances is easier

Question 12

The ideal substance to measure renal clearance therefore has the following properties:

Answer 12

• Freely filtered across the glomerulus
• No secretion by renal tubule
• No reabsorption by renal tubule

If these factors are true, then the amount of substance X excreted in the urine / min is equal to the amount filtered by the kidney / min.

Question 13

ok

Answer 13

surrogate= substitution

Question 14

A substance is present in the urine at a concentration of 100 mg/ml.

The urine flow rate (V) is 2 ml /min.

calculate clearence!

Answer 14

Question 15

what is creatinine? why is this used more to estimate GFR clinically?

Answer 15

• endogenous substance,
• End product of muscle breakdown
• Produced at a constant rate
• freely filtered but small amounts r secreted too
• bc it doesnt require intervenous infusion, μmol/L*
• BUT Creatinine Clearance is an overestimate of GFR bc some of it is secreted by the tubules!*

Question 16

how do u measure someones creatinine?

Answer 16

Question 18

Factors affecting serum Creatinine

Answer 18

Question 19

relationship btw GFR (kidney function) and plasma creatinine

Answer 19

if GFR decreases by 50%>> kidneys will filter & excrete only 1/2 as much of creatinine!

this causes accumalation of creatinine in the plasma! & serum creatinine rises!

soooo

GFR decreases>> plasma creatinine rise

Question 20

In an individual patient two things are important

1. GFR
2. Change in GFR

Answer 20

But remember: eGFR is taken from serum creatinine & not all changes in serum creatinine are due to a change in GFR

Question 21

what r the things that affect creatinine levels in an individual?

Answer 21

.

Question 22

Why is eGFR less accurate with mild kidney disease?

(• eGFR is not the same as GFR – it is a ‘best guess’ _

Answer 22

3 contributing factors

1. Reduction in GFR (e.g. if glomerular surface area reduced) causes increases in BF)
2. reduced nephron number leads to nephron hypertrophy so no change in GFR
3. reduced filtration of creatinine (due to reduced GFR) results in increased serum creatinine and increased secretion into the tubule (maintain relatively steady state of serum creatinine)

Question 24

what is this?

Answer 24

Question 25

Is creatinine clearance and GRF the same thing, If not what is the difference between them?

Answer 25

Creatinine Clearnece approximates GFR, but overestimates it due to the fact that creatinine is secreted by the proximal tubules as well as filtered by glomerulus!