Clearance Flashcards
Definition of clearance
the volume of blood or plasma that can be freed of a specified constituent in a specified time (usually one minute)
Name some clinical situations in which clearance measures may be necessary to estimate GFR
- extremes of age and body size
- severe malnutrition or obesity
- disease of skeletal muscle
- paraplegia
- vegetarian diet
- rapidly changing kidney function
- prior to dosing drugs with significant toxicity
Definition of renal clearance
the ability of the kidneys to remove a substances from the blood and excrete it into the final urine
Equation for renal clearance
Cx = (Ux/Px) x V
What is inulin used for?
used to measure clearance and GFR (b/c inulin is freely filtered, i.e. not reabsorbed, secreted, or metabolized by kidneys)
How do the clearances of insulin and inulin compare?
A. C (ins) > C (in)
B. C (ins) = C (in)
C. C (ins) < C (in)
C. C (ins) < C (in)
What are the units of clearance?
volume/time
How can filtrate produced be calculated?
by converting final urine into the volume of filtrate it is generated from
What is the number of mL of plasma filtered by the glomerulus in 1 minute?
Plasma concentration of creatinine is 1mg/dL
Urine produced in 1 minute is 20mL (0.2dL)
Urine creatinine concentration is 50mg/dL
Cx = (Ux/Px) x V
Cx = (50mg/dL / 1mg/dL) x (0.2dL/min)
Cx = 10dL/min
What is filtered load?
the amount of substance filtered into Bowman’s space per unit time (an amount, not a volume)
A (glomerulus)
Filtered load is measured as:
A. weight
B. weight per unit time
C. volume
D. volume per unit time
E. weight per unit volume
B. weight per unit time
When can GFR be measured as clearance for a substance X?
only when all of X delivered as filtrate is excreted in urine
Can creatinine be used instead of inulin?
yes, but some of it is secreted in male dogs
(no secretion with inulin)
What happens to serum creatinine concentration when GFR drops in half?
serum creatinine doubles
Day 1: Dog has a serum creatinine concentration of 1.0 mg/dL
Day 100: Dog has a serum creatinine concentration of 4.0 mg/dL
Which best expresses GFR at day 100 compared to day 1?
A. 150%
B. 125%
C. 50%
D. 33%
E. 25%
E. 25%
Why are Urea and BUN less accurate than using serum creatinine to measure GFR?
- urea production varies
- urea is actively and passively reabsorbed by the kidneys
What is azotemia?
increased levels of nitrogenous wastes in blood
What can cauze azotemia?
- reduced perfusion of the kidneys (prerenal)
- primary kidney disease (renal)
- obstruction to the flow of urine (postrenal)
How can you tell the difference betwen prerenal, renal, and postrenal azotemia?
- prerenal: concentrated urine (>1.030)
- renal: inability to concentrate adequately
- postrenal: unable to void or anuria (no urine)
12 year old cat, PU/PD for 3 months
- serum creatinine = 4.4mg/dL
- urine specific gravity = 1.010
A. Prerenal
B. Renal
C. Postrenal
B. Renal
3 year old cat unable to void, big bladder
- serum creatinine = 4.4mg/dL
- urine specific gravity = 1.028
A. Prerenal
B. Renal
C. Postrenal
C. Postrenal
1 year old cat, 3 day vomiting and diarrhea
- serum creatinine = 4.4mg/dL
- urine specific gravity = 1.064
A. Prerenal
B. Renal
C. Postrenal
A. prerenal
What are the characteristics of a marker (X) that would allow RPF measurement?
- clearance of X must equal RPF
- completely (or nearly so) extracted from plasma by filtration plus secretion
- neither metabolized nor degraded by the kidneys
- no organ other than the kidneys extracts X
- cannot alter RPF
How is effected renal plasma flow (ERPF) measured
measured as clearance of para-aminohippuric acid (PAH)
(PAH completely extracted from plasma during passage through nephron)
What is hematocrit?
the fraciton of blood containing red blood cells
What is the fraction of blood occupied by plasma?
1 - hematocrit
(1- Hct)
Equation for ERBF
ERBF = ERPF / (1 - Hct)
What do the following clearance ratios mean?
Cx / CGFR = 1.0
Cx / CGFR < 1.0
Cx / CGFR > 1.0
Cx / CGFR = 1.0 means substance X is cleared only by glomerular filtration
Cx / CGFR < 1.0 means substance X has limited filtration and/or reabsorption
Cx / CGFR > 1.0 means substance X is secreted
Calculate clearance:
Plasma concentration: 4mg/dL
Urine concentration: 2.7mg/dL
Urine flow rate: 3mL/min
A. ~0.2 mL/min
B. ~0.45 mL/min
C. ~2.0 mL/min
D. ~4.5 mL/min
C. ~2.0 mL/min
