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Flashcards in Measuring kidney function Deck (28):
1

What is renal clearance?

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

2

Which substances can be used to measure renal clearance?

Any substance

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

3

What is the maximum possible renal clearance?

Plasma flow 650 mL/min

4

What molecule is often used to measure GFR?

Inulin

5

How does the kidney handle inulin?

Freely filtered at the glomerulus

Not reabsorbed by the tubule

Not secreted by the tubule

6

What is GFR?

The rate at which the two kidneys form the filtrate

7

What is the average value of the GFR?

125 mL/min

8

Renal clearance of inulin = GFR of inulin TRUE or FALSE

TRUE

9

What is renal clearance of inulin?

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

10

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

TRUE

11

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

Net reabsorption

Not feely filtered

12

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

Net secretion

13

In which cases if the clearance of a solute = 0

If solute is not filtered at all

 

Completely reabsorbed

14

Which equation do we use to calculate GFR?

Renal clearance equation

 

Substance must have GFR = renal clearance

15

Which two subtances are used clinically to measure GFR?

Inulin Serum creatinine

16

What are the disadvantages of using inulin to measure GFR?

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

17

What are the advantages of using creatinine clearance?

Endogenous by-product of metabolism

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

Only need one blood sample

18

What are the disadvatages of using creatinine?

Some secretion of creatinine from the tubules

Clearance overestimates GFR by around 10-20%

Ease of using compensates for overestimation

19

What is the normal range of GFR?

90 - 140 ml/min

20

What conditions change GFR value between individuals?

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

21

What are symptoms of CKD?

Hyperaemia

Anaemia

Calcification of vasculature

22

How can we calculate renal plasma flow?

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

23

Renal clearance of PAH = renal plasma flow TRUE or FALSE

True

24

Why is PAH injected in low concentration?

Needed to prevent oversaturation of the secretory processes

25

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

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)

26

What is the average RBF?

1 L/min

27

What is the average haematocrit %?

40 %

28

How does kidney stone affect creatinine levels?

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