Chapter 3 Flashcards
How would you define “clearance”?
The volume of plasma per unit time from which all of a specific substance is removed.
Renal clearance: substance removed from the plasma and excreted in the urine.
Allow the measure of GFR.
Quantification of clearance:
How do you calculate the volume of plasma cleared per unit of time (Cx)?
Cx = V * (Ux/Px)
V: product of the urine flow rate
Ux: Urine concentration of X
Px: Plasma concentration of X
What substance is the best to measure the GFR?
The inulin.
The inulin is a polysaccharide that is freely filtered and not secreted or reabsorbed. So when it is filtered, it flows through the nephron and is excreted in the urine.
The volume of plasma cleared of inulin = the volume filtered -> inulin clearance = GFR
What is the routine assessment of GFR in patients?
Creatinine Clearance:
It is freely filtered, not reabsorbed and a small amount is secreted by the proximal tubule.
Basically what appears in the urine is what has been filtered + small amount secreted.
How do you measure Creatinine Clearance?
A patient’s urine is collected during 24h and a blood sample is taken. Blood and urine are assayed for creatinine concentration and the clearance formula is applied (Cx = V * (Ux/Px) to yield the creatinine clearance
Why is urea a less accurate indicator of GFR?
The range of normal plasma urea concentration varies a lot, depending on protein intake, changes in tissue catabolism, and because its excretion is controlled partially by hormonal regulation.
