Glossary of 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?
- *FF = GFR/RPF
*normal = 120/600 = 20%
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