Renal Plasma Clearance Flashcards
What type of substance do you need to measure GFR?
Only filtered not reabsorbed or secreted
What is inulin?
Inert polysaccharide that filters freely through the glomerular membrane
Why is inulin good for measuring GFR?
Not naturally produced so you can inject a specific amount and know exactly how much is in there
What is the formula for rate of inulin filtration?
[plasma inulin] x GFR
What is renal clearance?
Volume of plasma completely cleared of the substance by kidney per unit time
What are the cons of the inulin method?
Prolonged infusion and repeated plasma samples which make it hard for routine clinical use
What are the advantages of the creatinine method?
It’s intrinsic and inert, released at a relatively steady level in plasma from skeletal muscle, no infusion needed, freely filtered and not reabsorbed in the tubule
What are the disadvantages of the creatinine method?
Some is secreted in the tubule so it’s slightly wrong
How is creatinine made?
Starts as creatine in the liver or diet and then is metabolised into creatinine in the muscle
How does trimethoprim work?
Competes with creatinine for transporters that secrete it from blood into urine
What does trimethoprim do?
Increase serum levels of creatinine
What causes GFR decline?
Age and sex
What are the two equations for estimating GFR?
MDRD and CRD-EPI
Why doesnt CKD-EPI require weight?
Results are normalised to any surface area
Why is the CDK-EPI more accurate than the MDRD?
Less biased as it accepts its limitations at the higher GFR
How many stages of chronic kidney disease are there?
6- 1, 2, 3a, 3b, 4, 5
What group of people is the eGFR inaccurate for?
People with extreme body types (malnourished, obese, amputees)
When is the eGFR not valid?
Pregnant women or children
What groups of people was the MDRD originally validated on?
US white and black patients
What happens to substances with clearance < inulin?
Either not filtered freely or reabsorbed from tubule
What happens to substances with clearance> inulin?
Secreted into tubule
What happens when the clearance < GFR?
It’s not freely filtered and reabsorbed
What happens when clearance > GFR?
Substances that are secreted
What are some examples of clearance > GFR molecules?
Endogenous and exogenous
Give an example of an endogenous substance
Weak organic acids and bases, adrenaline, dopamine and steroids
Give an example of an exogenous substance
Penicillin, probenecia, para-aminohippuric acid
What is renal plasma flow?
Rate at which plasma flows through the kidney
What does renal plasma flow allow us to estimate?
The total blood flow through the kidneys
What % of blood is plasma?
55
What is para-aminohippuric acid?
Weak acidic metabolite of glycine
Where and how is para-aminohippuric acid secreted?
Active transport occurs in the basolateral membrane and passive transport across luminal membrane into tubule
What happens if plasma [PAH] is low enough?
It is completely cleared in a single pass thought the kidney
What does PAH clearance = ?
Renal plasma flow
What can be used to calculate the filtration fraction?
GFR and RPF
What is the filtration fraction?
Fraction of plasma filtered through the glomerulus
