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
What is the equation for the total urinary excretion of substance X?
Ux Vdot =
P(x) * GFR + S(x) - R(x)
Why is glucose clearance so low (and therefore not a good estimate of GFR)?
Glucose is nearly completely reabsorbed by the proximal tubule - very little is cleared in the urine
Why is is that there will be no PAH in the renal vein?
PAH is freely filtered at the glomerulus
And, @ low serum concentrations,
Whatever isn’t filtered is secreted by the proximal tubules into the tubular fluid
What does the fraction of blood that is plasma equal to?
1 - Hct
What is a freely filtered solute?
Concentrate of solute in the filtrate = concentration in plasma
What are two important properties of freely filtered solutes?
(1) MW <5000 Da
(2) solute not significantly bound to plasma proteins
If a solute is freely filtered and has a plasma concentration Px, how much X is filtered per unit time?
= Px * GFR
What term is used to describe Px * GFR?
Rate of filtration
Distinguish between rate of filtration and glomerular filtration rate
Rate of filtration = amount of solute filtered per time
GFR = volume of plasma filtered per time
How do you calculate the volume of plasma that is needed to supply the excreted amount X?
Volume of plasma per unit time to supply excreted X =
Rate of excretion of X / Px =
Ux Vdot / Px =
Volume of plasma per unit time cleared of X
What is special about the clearance of a solute that is filtered but neither reabsorbed or secreted?
Cx = GFR
Why must inulin be administered by IV infusion?
It is not an endogenous solute
What is the best known example of a solute that is so efficiently secreted that the combination of filtration and secretion removes essentially all of the solute from RPF?
PAH
What does C(PAH) actually measure?
Effective RPF (ERPF): rate of plasma flow to regions of the kidney that can remove PAH
For GFR and RPF, indicate the most accurate and most useful determinations
GFR
Most accurate = Cin (useful = Ccr)
RPF
Most accurate = Fick principle (useful = C(PAH))
What is the equation for the fractional excretion of water?
FE(H20) = amount of water excreted per time/ amount filtered per time = Vdot / GFR
What is an equation for fractional excretion of water that does not require a timed urine collection?
FE(H2O) = 1/ (U/P)In
