Measuring Kidney Function Flashcards
What is the role of the glomerulus?
Glomerulus and its surrounding Bowman’s capsule constitute a renal corpuscle, the basic filtration unit of the kidney
How is kidney function measured?
GFR = amount of filtrate prod from blood flow per unit time
Amount determined by the product of the average filtration of each nephron
What is normal GFR?
90-120
What factors does GFR depend on?
Gender Age Size Size of kidney Preg
Outline GFR in babies
Nephron development finishes 35th week of foetal devel
Premature often have lower nephron numbers
Birth GFR ~20
Normal GFR at ~18months
Outline GFR in advancing age
GFR starts declining after 30
Decline by 6-7ml per decade
How does age affect the kidney size?
Cortex volume and overall kidney volume falls
When do you get compensatory hypertrophy in the kidney?
Small nephrons/reduced number = compensatory growth
Occurs to a greater extent in childhood
Working hard = at greater risk of wearing out
How does GFR vary due to preg?
Increased = 130-180
Nephron number the same
But increased fluid vol = higher filtration rate
Back to normal ~6months post-partum
Broadly speaking if GFR decline what is this due to?
Decline in nephron number
Decline in GFR within individual nephrons
Why may GFR not fall until significant kidney damage has occurred?
Hypertrophy of nephrons
Define clearance
the volume of plasma cleared of a substance per unit of time where the substance is denoted as ‘x’
How is renal Cl calculated?
Renal Clearance of a substrate = excretion rate / plasma concentration
Excretion rate = U (amount in urine) x V (urine flow rate)
What factors are required of a substance in order for it to be used as a measure of GFR?
Be produced at a constant rate
Be freely filtered across the glomerulus
Not be reabsorbed in the nephron
Not be secreted into the nephron
What is the best substance to measure renal Cl?
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