Measuring Kidney Function

Question 1

What is the role of the glomerulus?

Answer 1

Glomerulus and its surrounding Bowman’s capsule constitute a renal corpuscle, the basic filtration unit of the kidney

Question 2

How is kidney function measured?

Answer 2

GFR = amount of filtrate prod from blood flow per unit time

Amount determined by the product of the average filtration of each nephron

Question 3

What is normal GFR?

Answer 3

90-120

Question 4

What factors does GFR depend on?

Answer 4

Gender
Age
Size
Size of kidney
Preg

Question 5

Outline GFR in babies

Answer 5

Nephron development finishes 35th week of foetal devel

Premature often have lower nephron numbers

Birth GFR ~20

Normal GFR at ~18months

Question 6

Outline GFR in advancing age

Answer 6

GFR starts declining after 30

Decline by 6-7ml per decade

Question 7

How does age affect the kidney size?

Answer 7

Cortex volume and overall kidney volume falls

Question 8

When do you get compensatory hypertrophy in the kidney?

Answer 8

Small nephrons/reduced number = compensatory growth

Occurs to a greater extent in childhood

Working hard = at greater risk of wearing out

Question 9

How does GFR vary due to preg?

Answer 9

Increased = 130-180

Nephron number the same

But increased fluid vol = higher filtration rate

Back to normal ~6months post-partum

Question 10

Broadly speaking if GFR decline what is this due to?

Answer 10

Decline in nephron number

Decline in GFR within individual nephrons

Question 11

Why may GFR not fall until significant kidney damage has occurred?

Answer 11

Hypertrophy of nephrons

Question 12

Define clearance

Answer 12

the volume of plasma cleared of a substance per unit of time where the substance is denoted as ‘x’

Question 13

How is renal Cl calculated?

Answer 13

Renal Clearance of a substrate = excretion rate / plasma concentration

Excretion rate = U (amount in urine) x V (urine flow rate)

Question 14

What factors are required of a substance in order for it to be used as a measure of GFR?

Answer 14

Be produced at a constant rate

Be freely filtered across the glomerulus

Not be reabsorbed in the nephron

Not be secreted into the nephron

Question 15

What is the best substance to measure renal Cl?

Answer 15

51 Cr-EDTA = radio-active labelled marker, need to adjust for 10% lower Cl

Creatinine = collect urine over 24hs or serum levels, overestimation of GFR by 10-20%, used in preg

Question 16

What factors affect creatinine levels?

Answer 16

Intake of cooked meat

Metabolism of muscle

Renal excretion

Extra-renal excretion

Question 17

How do creatinine levels change throughout life?

Answer 17

Foetus = same level as mother

After birth = quickly creatine drops

Reach adult levels at 15y

Question 18

What factors are measured in U+Es?

Answer 18

Urea
Sodium
Potassium
Bicarb
H = pH
Creatinine

Question 19

What is eGFR?

Answer 19

Computer model to estimate GFR

Underestimates true GFR = pts being labelled as having CKD

Question 20

When is eGFR inaccurate?

Answer 20

Kidney disease

Children/old age

Preg

Other ethnicities

Amputees/significantly reduced muscle mass

Pts with higher levels of kidney function

Question 21

Why is eGFR less accurate with mild kidney disease?

Answer 21

Reduced nephron number leads to nephron hypertrophy so no change in GFR

Question 22

What is an intravenous urograph?

Answer 22

X-ray examination of the kidneys, ureters and urinary bladder that uses iodinated contrast material injected into veins