Session 2 - measuring kidney function

Question 1

what are the functions of the kidney

Answer 1

• Regulation of:
– BP
– Blood volume
– pH
– Electrolytes
– Osmolality
• Excretion of
– waste products, drugs
• Metabolism
– Drugs, hormones, proteins
• Endocrine
– 1-alpha calcidol, renin, erythropoeitin

Question 2

what is GFR

Answer 2

• Kidney function is measured in terms of glomerular filtration rate
(GFR).
• GFR is the amount of filtrate that is produced from the blood flow per unit time.
• The amount of filtrate is determined by the product of the average filtration of each nephron in each kidney (i.e. normal is about 2 million nephrons)

Question 3

What is normal GFR

Answer 3

90 – 120 mL/min /1.73m2
140 – 180 L /day
Girls have smaller values than boys

Question 4

what does GFR depend on

Answer 4

• Gender
• Age
• Size of individual
• Size of kidneys
• Pregnancy

Question 5

which week of foetal development is neprhon development complete
what age does a babies GFR reach normal values

Answer 5

finished by 35-36th week
premature babies have lower nephron numbers- can lead to hypertension later in lifer
babies start of at GFR of about 20mls/min.1.73m2
reach normal GFR by 18 months

Question 6

at what age does GFR start declining

Answer 6

after 30 years of age
rate of decline is 6-7 mls/min per decade
loss of functioning nephrons
some compensatory hypertrophy
the kidney cortex gets thinner and the medulla gets larger with increasing age

Question 7

what might happen if a kidney is poorly functioning or there is reduced number of neprhons

Answer 7

• Reduced nephron number
• Compensatory hypertrophy
• Existing nephrons get bigger
• Healthy kidney can also get bigger
• Occurs to much greater extent in childhood
• Nephron loss in old age not
associated with compensatory hypertrophy

Question 8

what is the disadvantage of compensatory hypertrophy

Answer 8

Nephrons work harder and are at greater risk of wearing out

cortical sacrs- as poor blood supply to larger kidney

Question 9

what happens to GFR in pregnancy

when does it return to normal

Answer 9

GFR increases by about 50 percent
130-180mL/min
Kidney size increases (~ 1 cm) due to increased fluid volume (vascular & interstitial)
• Nephron number the same just more interstitial fluid
• Back to pre-pregnancy levels ~ 6 months post-partum

Question 10

why might GFR in a person decline

Answer 10

– Decline in number of nephrons
– Decline of GFR within individual nephrons
or both

Question 11

what does a fall in GFR mean

Answer 11

kidney function has gotten worse

Question 12

what does a rise in GFR mean

Answer 12

kidney function has gotten better

Question 13

what happens to GFR if kidney function declines slowly

Answer 13

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

Question 14

what is clearance

Answer 14

Clearance (C) is the volume of plasma cleared of a substance per unit of time where the substance is denoted as ‘x’
clearance = amount of substance eliminated from plasma/plasma concentration of substance
C = clearance
A = amount of substance eliminated from plasma
P = plasma concentration of substrate

Question 15

what is renal clearance

Answer 15

Renal Clearance of a substrate = excretion rate / plasma concentration
Excretion rate = U (amount in urine) x V (urine flow rate)