Lecture 3: Quantification Kidney Fxn Flashcards
Define filtration
The process by which water and solutes in the blood leave the vascular system through the filtration barrier and ENTER Bowman’s space
Define secretion
The process of moving a substance from the peritubular capillaries, across the epithelial layer and into the tubular lumen
Define reabsorption
The process of moving a substance from the tubular lumen across the epithelial layer and into the peritubular capillaries
Define excretion
Exit of a substance form the body
Define clearance
Define solute clearance
A measure of the rate of removal of a substance from the body
Volume of plasma (or fluid) that was entirely cleared of solute X in a given time
Why is it important to monitor the glomerular filtration rate (GFR)?
Almost anything that damages the kidney will result in loss of GFR
What is typical renal blood flow?
1.1 L/min
20% of CO
What is renal plasma flow (RPF)?
RBF * plasma content
What is glomerular filtration rate (GFR) equation?
RPF * glomerular filtration fraction
Typically, glomerular filtration fraction is 20%
Characterize inulin’s solubility
Soluble in water
What are characteristics of inulin when injected IV?
Small - freely filtered by glomerulus
Not reabsorbed, secreted
Not made or metabolized by kidney
Doesn’t affect kidney function
Can be measured in blood & urine
Why does renal clearance of inulin estimate GFR?
Inulin filtered = inulin excreted (since it’s not reabsorbed or secreted, it all gets excreted!)
Therefore
Renal clearance of inulin = GFR
What is an equation for GFR using inulin terms?
GFR =
U(In) * Vdot / P(in)
Where
U(In) = urine [inulin]
Vdot = urine flow rate
P(In) = plasma [inulin]
What is the renal clearance equation of any substrate?
Cx =
U(x) * Vdot / P(x)
What are problems with inulin?
Need intravenous catheter
Need to achieve continuous infusion w/ clean SS blood levels
Need complete emptying of bladder before urine collection
Need complete collection of urine for allotted collection time
How does creatinine compare to inulin?
Both are freely filtered and not reabsorbed…
However, there is tubular secretion of creatinine!
How does secretion of creatinine vary by GFR?
As GFR declines, filtration of Cr decreases proportionally, but % secreted increases
What does creatinine depend on?
Muscle mass
What does timed urine collection for creatinine clearance still require?
Empty bladder at start of collection
Precise collection of all urine produced
Compare the use of inulin clearance and creatinine clearance as measures of GFR
Inulin = good measurement, but not practical
Creatinine = easier, but drawbacks are that it will differ from GFR because of dependence on muscle mass and inclusion of secretion factor
How do serum creatinine and GFR relate?
Higher serum creatinine = lower GFR
What is the Cockcroft-Gault equation?
Takes into account factors that can affect muscle mass to adjust creatinine clearance
C(Cr) =
(140 - age) * (wt [kg]) * (0.85 if female)
/ 72 * P(Cr)
Why does creatinine clearance usually overestimate GFR?
% secreted varies by level of GFR, and as GFR declines the % secreted increases
What are pitfalls of serum creatinine as an indicator of GFR?
Overestimates GFR (b/c of secretion)
Relationship isn’t linear: a lot of loss of GFR is required before serum creatinine increases; but yet large changes in the higher range could actually correspond to little change in GFR
Serum creatinine is influenced by kidney function AND muscle mass
Estimating equations assume serum creatinine is at SS
