What is clearance?
Renal clearance refers to the volume of
that is completely cleared of a particular
substance by the
kidneys in a given period of time, and is
usually measured in
milliliters per minute.
It can help diagnose conditions such as
chronic kidney disease short CKD.
It assists the efficiency of drug excretion
kidneys, and is often calculated for
substances such as
creatinine and certain drugs or toxins.
The formula for calculating renal clearance
is as follows.
Renal clearance equals urine concentration
of substance X
times urine flow rate divided by plasma
of substance X.
Urine concentration of substance X,
abbreviated as UX, refers
to the concentration of the substance X in
The urine flow rate U is the rate at which
urine is produced in the
kidneys. The plasma concentration of
substance X, short
Px, is the concentration of the substance X
in the blood
plasma. Let's look at an example.
What is the clearance of substance X?
If the plasma concentration is 1mg/ml,
the urine flow rate is one milliliter per
minute and the urine
concentration is 1mg/ml.
Here is what it looks like substituted into
Clearance equals 1mg/ml
times one milliliter per minute divided by
We get a clearance of one milliliter per
In other words, this means that one
milliliter of plasma passing through the
kidneys is cleared of the substance every
We have a few other relationships that we
need to bring
out, and that is there are a number of
factors that we need to
try to calculate, and we can use various
calculations in the
renal system by using this clearance
Now the clearance equation is looking at the
concentration of a
substance in the urine times the urine
formation rate, which is v
over the plasma concentration of the
And that clearance equation won't change
our various substances that we're going to
Interestingly though, you can pick a certain
substance to look at that will give you
insight into the function of the kidney.
Inulin is one of these substances.
We can use inulin as an index to measure
And how you do that is take glomerular
filtration rate times the
concentration of inulin in the urine, times
the urine formation
rate divided by the plasma concentration of
Creatinine is another way to measure
glomerular filtration rate.
We utilize the very same equation.
The only thing we're doing is substituting
creatinine for the x.
So here we have the urine concentration of
creatinine times the urine
formation rate divided by the plasma
concentration of creatinine.
So both of these two substances can be
utilized to measure glomerular
Inulin is a little bit more accurate.
But creatinine is made in the body and
therefore is more functional.
So you can measure someone's creatinine much
easier than you can induce or
introduce inulin into someone.
The last substance that we can utilize in
this kind of clearance equation
PAH is utilized to help us for renal plasma
We use the same equation.
All we're doing is substituting PAH for the
So here we have renal plasma flow equals the
formation concentration of PAH times the
urine formation rate
divided by the plasma concentration of PAH.
A creatinine clearance measurement needs a
24 hour urine
collection. If that's not possible, valuable
about the changes in the glomerular
filtration rate can be obtained by
measuring the plasma creatine concentration,
which is inversely
proportional to the GFR.
The graph on the right shows the approximate
relationship between the glomerular
filtration rate and the plasma creatinine
Note that decreasing the GFR by 50% of
will double the plasma creatinine
concentration, and decreasing
the GFR to 25% of normal will
increase the plasma creatinine concentration
four times normal.
So let's now walk through these in a cartoon
In case you're one of those individuals that
you struggle a little bit with equations.
So let's go through this in our picture form
So what happens to inulin.
Inulin as it travels through the afferent
arteriole into the glomerular
capillary is what we call freely filtered.
And what freely filtered means is it travels
through that filtration
The filtration barrier doesn't block it or
say, hold back.
It allows it to travel through.
Interestingly, inulin is not reabsorbed.
There are no specific transporters for
Inulin is also not secreted.
So all the inulin that is filtered is
excreted into the urine.
There's no addition or subtraction to that
Creatinine looks very similar.
It is also freely filtered.
It is not reabsorbed.
However, there is a tiny amount that is
So in this case that part is different than
Interestingly, though, since you know about
transporters are there, people oftentimes
use a fudge factor for that small
amount of secretion.
That's why you can still use creatinine as a
marker for glomerular
It is just a little less accurate because
the secretion rate
is not fully accounted for.
Properties of PAH.
It is also freely filtered.
Interestingly, it's still not reabsorbed.
And finally it is secreted fully, meaning
that it is going to
secrete as much or based upon a transport
across from the blood through the renal
tubule cells into the tubule
fluid. So in this case, you have both the
freely filtered amount
and the secreted amount that are measured in
The final components that we need to do is
convert renal plasma flow
into renal blood flow.
And how you do that is by taking into
account the hematocrit.
Remember from the cardiovascular section
that hematocrit is the amount of red
blood cells in the blood.
So with this you take the renal plasma flow.
You divide it by one minus the hematocrit.
And that will yield renal blood flow.
Other important equations that we have in
the kidney in
terms of the system is the filtered load of
and what we mean by a filtered load, because
this is kind of hard to think about, is the
glomerular filtration rate multiplied by the
plasma concentration of a
substance. So if you take something like
sodium, if you know the plasma
concentration of sodium, you know the
glomerular filtration rate, you can figure
out how much, in terms of an absolute
load that was delivered to the Bowman's
The last item that's important in renal
physiology is the filtration
fraction. The filtration fraction is the
filtration rate divided by the renal plasma
So what this is in practical terms is the
amount of flow going in
the afferent arteriole through the Bowman's
and then out the efferent arterial.
How much of that flow was filtered.
So that is glomerular filtration rate.
You divide it by renal plasma flow.
I know we went through a lot of different
equations, but these are
important renal functions.
And if you understand these kinds of
equations you'll be able to understand the
concept that is behind them.