Measurement of Kidney Function Flashcards
Basic Nephron Processes:
insert slide
Clearance definition
the VOLUME of plasma completely cleared of a given substance in units/time (mL/min)
Clearance equation
(glomerular filtration + tubular secretion) - Tubular rebasorption
Renal clearance is a measurement of
excretion
Provides a summary of 3 basic renal functions:
- glomerular filtration (F)
- tubular re-absorption (R)
- tubular secretion (S)
Glomerular Filtration Rate
the rate at which fluid is filtered at the glomerulus (ml/min)
GFR is a measurement of renal function:
NO
Glomerular function only
GFR vs Clearance:
insert slide
Three Factors Determining Glomerular Filtration Rate:
- filtration pressures
- charge: -ve molecules filtered less
- size: proteins and blood cells too
large less than 7KDa
Effect of Renal Blood Flow: Afferent Arteriolar Constriction:
- fall in glomerular filtration pressure
- GFR falls
Effect of Renal Blood Flow: Efferent Arteriolar Constriction:
- reduces blood flow
- but increases GFR due to increased
filtration pressure - however as renal blood flow further
decreases - GFR falls because filtration pressure
can not be maintained at low levels
of flow
Control of GFR by Macula Densa:
- macula densa sense changes in
sodium chloride delivery to DCT - decreased GFR slows flow rat in
Loop of Henle, causing increased
reabsorption of the % Na+,Cl-
delivered to ascending limb - this:
- decreases resistance to blood
flow in afferent arteriole, which
raises glomerular hydrostatic
pressure, returning GFR to
normal - increases renin release from JGA
of afferent and efferent. Renin
releases results in more
Angiotensin II, increasing
glomerular hydrostatic pressure,
returning GFR toward normal
- decreases resistance to blood
Control of GFR by Macula Densa
insert flowchart
Assessment of GFR (4):
- serum biochem
- measurement of clearance
- calaculated clearance
- renography
Principles of Clearance:
insert slide
Inulin Clearance:
the clearance of inulin is the virtual volume of blood that would be totally cleared of inulin in a given time (ml/min)
Inulin Clearance equation
Urine(inulin)xVol(urine)/Plasma (inulin)
Drawbacks of inulin clearance:
- needs infusion of inulin to achieve
constant plasma conc - blood sample to measure inulin
conc - timed urine collection and
measurement of inulin in urine - inaccurate urine volumes source of
large error - cumbersome for routine clinical use
Creatinine:
- breakdown product of creatine
phosphate in muscle - generally produced at a constant
rate - can be altered with large muscle
mass, cachexia, pregnancy, liver
disease and durgs like trimethoprim - normal range 40-120 micromol/L in
plasma
Creatinine Clearance: Definition:
the virtual volume of blood that would be totally cleared of creatinine in a given time (ml/in)
Creatinine Clearance Equation
insert
Drawbacks of Creatinine Clearance:
- inaccurate urine volumes are a
source of large error - does not tell you about the
proportion of the creatinine filtered
vs secreted - overestimates GFR at low levels of
GFR
What is inulin?
sugar
not produced in the body
experiments: no reabsorption/secretion so useful
Advantage of using creatinine clearance vs inulin clearance:
- does not require constant infusion
- both have urine volume inaccuracies
- due to secretions OVERESTIMATE
GFR because of creatinine secretions
estimation of clearance using plasma creatinine and what does this formula take into account?
CKD-EPI (chronic kidney disease epidemiology) (more accurate because actually used normal pts) (no mass so wastin away/obese not accurate)
