058 Renal Clearance

Question 1

What are the properties of inulin that allow it to be a marker for glomerular filtration?

Answer 1

It is freely filtered
It is not reabsorbed
It is not secreted

Question 2

What is the formula for gloremular filtration rate

Answer 2

(Urine inulin x urine flow rate)/ plasma inulin

Question 3

What is the definition of renal clearance?

Answer 3

The volume of plasma that would be required to supply that amount of the substance excreted per unit time. (mL/min)

Question 4

What is the drawback of the clearance method?

Answer 4

It provides no information about whether a substance is filtered or the precise sites and mechanisms of transport.

Question 5

The clearance of inulin is 125 ml/min. if substance has lower clearance value, what does this mean?

Answer 5

There is either net absorption or the substance is not freely filtered (e.g. bound to plasma proteins).

Question 6

The clearance of inulin is 125 ml/min. if substance has higher clearance value, what does this mean?

Answer 6

There is net secretion

Question 7

What does a clearance of 0 mean?

Answer 7

It is not filtered at all or it is totally reabsorbed

Question 8

Why would you prefer using creatinine clearance?

Answer 8

Using inuline is invasive as you need to infuse it intravenously and continuous measurements must be taken.

Question 9

What is one of the downsides of using creatinine?

Answer 9

It overestimates GFR by 10-20%

Question 10

What is the benefits of using creatinine?

Answer 10

Plasma concentration is stable, it is freely filtered, it is naturally produced

Question 11

What are normal GFR ranges?

Answer 11

90-140ml/min, depending on age, sex, racial group and body size.

Question 12

What can be used to measure renal plasma flow?

Answer 12

Para-amino hippurate

Question 13

When is using plasma creatinine concentration to estimate GFR useful?

Answer 13

When GFR levels are low

Question 14

What does Transport Maximum (Tm) mean?

Answer 14

The point at which an increase in concentration does not result in an increase in movement of a substance across the membrane.

Question 15

Discuss properities of para-amino hippurate

Answer 15

It is an organic anion not usually present in the body. It is freely filtered and secreted from peritubular capillaries by a Tm dependent mechanism into proximal tubule lumen.

Question 16

What occurs to PAH when it enters renal artery with low plasma concentrations?

Answer 16

PAH enters artery and 1/5 is filtered in the glomerulus. The remaining 4/5 is secreted into proximal tubule lumen, so that there is none left in the renal vein.

Question 17

How does para-amino hippurate measure the renal plasma flow?

Answer 17

Volume of plasma cleared of PAH = volume of plasma entering the kidneys = renal blood flow

Question 18

What is the formula for Renal blood flow (RBF)

Answer 18

RPF x (1 – haematocrit) = renal blood flow.

Question 19

What is the formula for renal plasma flow?

Answer 19

[PAH]urine x vol urine / [PAH] arterial

Question 20

What is filtration fraction? What is the normal value?

Answer 20

Ratio of GFR to RPF; inulin clearance/PAH clearance normally at 20%

Question 21

What occurs when filtration fraction increases?

Answer 21

There will be a decrease in hydrostatic pressure and an increase in oncotic pressure in peritubular capillaries, allowing increased reabsorption.

Question 22

What occurs when filtration fraction decreases?

Answer 22

There will be an increase in the hydrostatic pressure and a decrease in oncotic pressure in peritubular capillaries. This will reduce reabsorption.

Question 23

How do we classify chronic kidney injury?

Answer 23

Stage 1 - stage 5 (CKD stage)
RIFLE criteria:
Risk – GFR decreases by over 25%
Injury – GFR decreases by over 50% 
Failure – GFR decreases by over 75% 
Loss – AKI persists for over 4 weeks. 
ESRD (End Stage Renal Disease) – renal replacement therapy above 3 months.

Question 24

How do you measure the transport maximum?

Answer 24

Filtered load - excretion rate

You can work out how much was able to be absorbed before they are excreted. If Tm = -ve, it is indication of secretion.