Renal Clearance Flashcards
define filtered load and excreted load
- filtered load = rate at which a certain solute is being removed from the glomerular capillaries (mg/ml)
- excreted load = rate at which a certain solute is leavint the kidney (mg/ml)
explain the “principle of mass balance” with respect to the kidney
says that if a substance is not reabsorbed or secreted, then at steady state, the rate at which a solute leaves the plasma (filtered load) must equal the rate at which is enters the urine (excreted load). modeled by this euqation
- VpPx = VuUx
- Vp is the “hypothetical” plasma flow (ml/min)
- Px is the plasma concentration (mg/ml
- Vu is the rate of urine production (ml/min)
- Ux is the urine concentration (mg/ml)
- define clearance
- how to solve for clearance using the relationship between filtered and excreted load
- clearance = the volume of plasma that was cleared of solute “X” in one minute
- i.e., the amount of volume containing solute “X” that leaves the glomerular capillaries per minute
- represented by Vp in filtered load
- clearance = Vp = VuUx/ Px
the use of clearance to measure GFR
- why does this work?
- what are the necessary characteristics of the cleared solute?
GFR is the amount of fluid removed from the glomerular capillaries into bowman’s capsule per minute
- i.e. the amount of fluid “cleared” per minute
- the clearance of a solute that is only filtered, and not reabsorbed/secreted, represents GFR
solute must
- be freely filtered: is the correct size/polarity to move thru the glomerular capillary membrane
- not reabsorbed from tubules
- not secreted into tubules
- insert: not metabolized by the kidney
inulin
- properties
- uses
- a synthetic fructose polymer
- its clearance is used to estimate GFR, since it:
- is freely filtered (molecular weight < 5000)
- not secreted/reabsorbed
- thus, the rate of its filtration = rate of excretion
creatinine
characteristics and uses
- like inulin, it is can be used to calculate GFR (not absorbed/secreted, freely filtered)
- but, it is a byproduct of muscle breakdown, and doesnt need to be injected
relationship between plasma creatinine and GFR
- inverse relationship
- an increased plasma creatinine is due to a smaller creatinine clearance, and thus indicates a smaller GFR
- non-linear relationship:
- normal GFR between 90-120
- plasma creatinined maintained pretty well in this range
- as GFR decreases, smaller drops in GFR lead to higher plasma creatinine
- normal GFR between 90-120
relationship between BUN, plasma creatinine and GFR
- BUN has the same relationship to GFR as creatinine
- BUN normally exists at a 10/1 ratio to creatinine
- 10/1- 20/1 BUN:Plasma creatinine is clinically normal
what demographic factors can decreased GFR?
how is this modelled clinically?
determining GFR would involve a urine sample (to determine excretion of solutes/per minute from the urine) to determine excreted load, and plasma sample of creatinine.
- a model generated based on collected data provides a quicker estimation
- age decreases GFR
- body mass increases GFR
what is PAH and how is it used to measure renal function?
- PAH is freely filtered and completely secreted
- thus, its clearance can be used to model renal plasma flow:
- renal plasma flow = all of the plasma that goes through nephron
- since all of PAH ends up in the tubules (20% filtered at glomerulus, remainder secreted in tubule system) its clearance represents renal plasma flow
define renal blood flow
how it can be calculated using RBF
= entirety of blood that moves through kidney (plasma + cellular components)
- can be calculated using RPF (indicated by PAH clearance)
- accounts for hematocrit (cellular components of blood)
what is filtration fraction and how is it calculated?
- The fraction of the plasma volume entering the glomerulus that is filtered from the glomerular capillaries into Bowman’s capsule
- FF = GFR/RPF = Ccr/Cpah
what is fractional excretion?
how can we calculate it?
- how much of of what is filtered gets excreted
- FE = excreted load/filtered load
- since filtered load = VpPx
- and excreted load = VuUx
- FE = VuUx/VpPx = VuUx/GRFPx
- (since Vp = GFR)
- GFR replaced with Ccr, equation rearranged such that Cx/Ccr
- FE = excreted load/filtered load
what is the fractional excretion (= 1, < 1, > 1) of a solute that is reabsorbed, a solute that is secreted, and inulin?
- FE of inulin = 1.0
- If a solute is reabsorbed, its FE < 1.0 (more is filtered than excreted)
• If a solute is secreted, its FE > 1.0 (more is excreted than filtered)
if solute “Y” if freely filtered and reabsorbed to some extent
- what is the relationship between filtered load and excreted load?
- what can filtered/excreted load be used to determine?
- what is the relationship between filtration rate and excretion rate?
- what can these rates be used to determine?
- Filtered load (GFR×Py) > Excreted load (Vu ×Uy)
- GFR = rate of fluid leaving plasma in glomerulus
- Py = plasma concentration of “y”
- Vu = rate of fluid leaving kidney
- Uy = urine concentration of “y”
- Reabsorption rate = Filtered load - Excreted load
- Filtration rate > Excretion rate (i.e., GFR > Cy)
- Excretion rate = Filtration rate - Reabsorption rate
