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2 - Measurements of function and clearance


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What is glomerular filtration rate? What is a normal value?
*the amount of fluid that crosses all glomeruli per unit time
What is a normal value for renal blood flow?
What is the equation for renal plasma flow? What is a normal value?
*RPF = RBF (1 - Hct)
*RPF = 1000 (1 - 0.4) = 600mL/min
What is the filtered load of a solute?
The GFR times the concentration of the solute in plasma.
What is the excreted load of a solute?
Urine flow times the concentration of the solute in urine.
Describe reabsorption and secretion in terms of filtered load and excreted load.
*Reabsorption = filtered load - excreted load
*Secretion = excreted load - filtered load
What is the definition of clearance? What is the formula?
*volume of plasma cleared of a solute per unit time
*clearance = (V x urinary conc.)/plasma conc.
*i.e. clearance = excreted load/plasma concentration
How can a solute have a clearance of zero? Name 2 solutes that have a clearance of zero.
*It is not filtered or secreted (albumin)
*It is filtered but completely reabsorbed (glucose)
How can a solute have a clearance equal to GFR? Name two solutes for which this is true.
*The solute is freely filtered and is not reabsorbed, secreted, or metabolized in the tubules
*inulin and creatinine
How can a solute have a clearance equal to RPF? Give an example of a solute for which this is the case.
*Plasma is completely cleared of the solute in one pass of the kidneys
*The solute is freely filtered (120mL/min), any remaining is then secreted (480mL/min)
What is the clearance of a solute whose filtered load is greater than its excreted load? How does this occur?
*clearance < GFR
*the solute is freely filtered and then partially reabsorbed
What is the clearance volume of water?
What are the normal values for GFR for men and women? How do these change with age?
*men = 95mL/min
*women = 120mL/min
*declines with age
For clearance of a substance to provide an accurate representation of GFR, what must be true of that substance?
Its plasma concentration must be relatively stable.
What is a drawback of using inulin to measure clearance?
It is an exogenous substance, thus a long infusion is required to achieve a steady-state in plasma.
How is creatinine produced in the body? Why does its concentration in circulation remain constant?
*A by-product of muscle metabolism
*Production = excretion
Describe what happens to plasma concentration of creatinine when GFR is decreased.
*because production remains constant, the concentration of creatinine in plasma rises
*a new steady-state is achieved, but at a higher level
*this is reflected as increased creatinine
RBF and RPF can be estimated by tracking the excretion of what substance?
About how much PAH is removed in one pass of the kidneys? Thus what is the normal clearance of PAH?
*because clearance of PAH is equal to RPF, normal clearance = 600mL/min x 90% = 540mL/min
What is the filtration fraction? What is a normal value for this?
*normal = 120/600 = 20%

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