RENAL 02: CLEARANCE Flashcards
What is the definition of load and what are the units for it?
The product of its concentration * the flow
mg/min
How many input and output loads does the kidney have? What is the relationship between these?
One input (arterial plasma load)
Three outputs (venous, ureter, lymph)
Total input load must equal output loads
Arterial plasma load of substance x = the ureter, venous, and lymphatic loads of that same substance X, summed
We typically ignore lymphatic load because it is so small, and we typically ignore venous load since it turns into the arterial load next cycle
How do you calculate load
Load (mg/min) = concentration (mg/mL) * flow (mL/min)
Therefore, the mL cancel out
Renal clearance
The volume of plasma cleared of any substance in one minute (therefore mL/min)
How do we calculate renal clearnace
Clearance of substance X is equal to the ureter concentration times flow of that substance, over the plasma concentration of it, where X is netiher produced nor consumed by the kidneys
If something is completely cleared once filtered (cannot be secreted nor reabsorbed), then what can we say about its clearance?
It will equal the renal blood flow
Can clearance exceed 660?
NO
Clearance cannot expand into hematocrit, it can only eat into maximum renal plasma flow
What is another way we can think of the loads on the kidney? Think of 4 major loads overall associated with an equation we talked about eralier
Filtered load
Secreted load
Reabsorbed load
excreted load
this still gets back into input load = output load
F+S=R+E
Glomerular filtration rate i what?
The volume of filtrate per unit time in the kidney; it is the rate of volume going through the glomerulus at any point in time (therefore it is NOT the renal plasma flow).
A normal GFR is about 120-125, and down to 15 is renal failure
How do you determine GFR?
You must use something of known concentration and WILL b cleared if present
Substances we can use to measure GFR
Creatinine
Inulin
How is inulin used to measure GFR? What is it?
It is a complicated plant oligosaccharide which will be filtered and excreted but not reabsorbed or secreted, nor can we modify it. Therefore, F=E
What is creatinine?
It is an end-product of muscle metabolism; concentrations in plasma are normally stable.
How much does excreted creatinine compare to filtered creatinine?
F < E because you secrete 10%
How is it possible that we can use blood tests to determine GFR?
there is a relationship between blood plasma and urine
The relationship between GFR and plasma concentration of substance X is _____ (inversely, directly proportional) ____
Inversely proportional
What is the downside to using creatinine levels as a measure of someone’s kidney health
Creatinine is going to be variable depending on someone’s muscle mass
What is BUN?
Blood urea nitrogen (BUN)
This is a marker of muscle catabolism (blood urea, it contains nitrogen and you’re measuring the nitrogen)
What is the downside to using BUN to measure GFR?
BUN values experience fluctuations depending on ingestion of protein during meals
Because of the disadvantages to using BUN or creatinine to measure GFR on their own, how do we measure GFR using those values?
We take the BUN: Creatinine ratio which is generally a 10:1 ratio. If this ratio is high this is an indication that there is volumetric contraction
Why should we have an understanding of renal blood flow?
It can help understand if something is an intra-renal or extra-renal issue
How can we measure renal blood flow?
PAH clearance
It is freely filtered, but not reabsorbed. It is also secreted and most is excreted. Therefore, nearly all of it is cleared out of the plasma.
To calculate true renal plasma flow, it is necessary to sample renal venous blood to measure plasma PAH concentration, which isn’t usually done. What do we measure to cancel out the venous sample and what is this?
Effective renal plasma flow - based on the amount we see in pee pee
What is the filtration fraction?
Ratio between GFR and RPF ; it is the fraction of the RPF that is filtered by the glomerular capillaries. Normal filtration fraction is 20%, meaning 80% of the RPF is not filtered and enters via efferent arterioles into peritubular capillaries.
What is the filtered load of any substance?
GFR x plasma concentration of substance X
