Measuring kidney function

Question 1

What is renal clearance?

Answer 1

Volume of plasma completely cleared of a substance by the kidneys per unit time

The higher the renal clearance = more plasma is cleared of the substance

Question 2

Which substances can be used to measure renal clearance?

Answer 2

Any substance

Value obtained depends on how the substance is handled by the kidney

Question 3

What is the maximum possible renal clearance?

Answer 3

Plasma flow 650 mL/min

Question 4

What molecule is often used to measure GFR?

Answer 4

Inulin

Question 5

How does the kidney handle inulin?

Answer 5

Freely filtered at the glomerulus

Not reabsorbed by the tubule

Not secreted by the tubule

Question 6

What is GFR?

Answer 6

The rate at which the two kidneys form the filtrate

Question 7

What is the average value of the GFR?

Answer 7

125 mL/min

Question 8

Renal clearance of inulin = GFR of inulin TRUE or FALSE

Answer 8

TRUE

Question 9

What is renal clearance of inulin?

Answer 9

Hypothetical volume of plasma completely cleared of inulin by the kidneys per unit time

Question 10

If a substance is freely filtered, not reabsorbed or secreted renal clearance = GFR TRUE or FALSE

Answer 10

TRUE

Question 11

In which cases will clearance of a solute be less than that of inulin?

Answer 11

Net reabsorption

Not feely filtered

Question 12

In which cases will the clearane of a solute be more than that of inulin

Answer 12

Net secretion

Question 13

In which cases if the clearance of a solute = 0

Answer 13

If solute is not filtered at all

Completely reabsorbed

Question 14

Which equation do we use to calculate GFR?

Answer 14

Renal clearance equation

Substance must have GFR = renal clearance

Question 15

Which two subtances are used clinically to measure GFR?

Answer 15

Inulin Serum creatinine

Question 16

What are the disadvantages of using inulin to measure GFR?

Answer 16

Not made endogenously - has to be injected

Need 45 minutes before inulin concentration remains stable

Hard to keep inulin concentration stable

Continual blood and urine sampling required every 10-20 minutes

Very invasive and time consuming

Chemical analysis - not very accurate

Question 17

What are the advantages of using creatinine clearance?

Answer 17

Endogenous by-product of metabolism

Released into the blood at a relatively constant rate - plasma concentration is stable

Only need one blood sample

Question 18

What are the disadvatages of using creatinine?

Answer 18

Some secretion of creatinine from the tubules

Clearance overestimates GFR by around 10-20%

Ease of using compensates for overestimation

Question 19

What is the normal range of GFR?

Answer 19

90 - 140 ml/min

Question 20

What conditions change GFR value between individuals?

Answer 20

Age - GFR decreases every year

Sex - 10% lower in women

Race - higher muscle mass in afro-carribeans

Body size - increase in body size = increase in kidney size

Question 21

What are symptoms of CKD?

Answer 21

Hyperaemia

Anaemia

Calcification of vasculature

Question 22

How can we calculate renal plasma flow?

Answer 22

Inject PAH in low concentration

20% is filtered across the glomerular capillaries

The remaining 80% is secreted from the peritubular capilaries

Measure the volume of PAH entering the kidney

Use the clearance equation to measure the plasma flow

Since all of it is secreted the volume of plasma cleared of PAH = renal plasma flow

So renal clearance of PAH = renal plasma flow

Question 23

Renal clearance of PAH = renal plasma flow TRUE or FALSE

Answer 23

True

Question 24

Why is PAH injected in low concentration?

Answer 24

Needed to prevent oversaturation of the secretory processes

Question 25

How do you convert the renal plasma flow into the renal blood flow?

Answer 25

Calculate renal plasma flow

Take a blood sample and centrifuge it to calculate haematocrit

Find the volume percentage of the blood ( 1 - % haematocrit )

Plug in renal plasma flow and haematocrit level into the calculation: RPF = RBF x ( 1 - % haematocrit)

Question 26

What is the average RBF?

Answer 26

1 L/min

Question 27

What is the average haematocrit %?

Answer 27

40 %

Question 28

How does kidney stone affect creatinine levels?

Answer 28

Blockage of ureter leads to a drop in GFR

This decreases urine creatinine excretion

Plasma creatinine concentration increases

Leads to an increase in urine creatinine excretion

A steady state will be reached where the creatinine excretion = production

GFR remains low but creatinine exretion remains the same as before.

Plasma creatinine becomes elevated