welcome to part two of acute renal failure
let's little let's review a little bit here about what we were talking about we
were talking about chem seven we have the BUN and we've got the creatinine bu
n stands for the blood urea nitrogen remember it's made in the liver
it is both filtered and reabsorbed whereas the creatinine is the end
product of muscle metabolism and it is filtered and secreted so let's look at
our nephron once again that's the loop of Henle
that's distal convoluted tubule collecting duct and you've got the
toilet down here okay and you've got the
the a Ferentz and the efferent arteriole and remember what we said about the U n
bu n is the blood urea nitrogen it goes through and also gets reabsorbed okay so
that's B u N and you've got the creatinine cratan n' goes does not get
reabsorbed but instead goes out actually a little bit gets secreted here okay so
if you've got acute renal failure
remember that the GFR the glomerular filtration rate is going to go down so
if that's the case then your B UN the UN is going to go up just because of the
fact that the glomerular eighth is lower therefore not as much as being filtered
the B UN is going to go up but because it's going by slower because the GFR is
down then more of it is going to be reabsorbed and
so it's going to go up by even more that's opposed to the creatinine
creatinine is going to go up why because the GFR is low and the GFR is low
because there's a cute renal failure going on and that means that Crichton is
going to go up but it's not going to go up any more than that
because it's not being reabsorbed and so what you see here is that the BU n goes
up more than the Kratt name does when you have a decrease in your GFR just
based on the fact that there's lower flow so lower flow means higher Buick
writing ratio okay so what would be a number here that we could associate with
it so normal would be for instance 15 over 1 so if the B went over cratan een
if the b1 went up by more than that of the crannium we could say for instance
that this would go to a2 and a 30 okay so the B would go up to 30 in the grant
would go up to 2 so you could go up by instead of having a 15 to 1 ratio
you would have more like instead of even 30 would even go up high or go to 40 so
40 to 2 ratio which is the same as a 20 to 1 ratio so here we would see the
normal ratio is 15 to 1 and it would go up to 20 to 1 or in this case 40 over 2
and that would be from a low flow State now what would happen here if for some
reason there was a shutdown a different type of acute renal failure at acute
renal failure not because there's less flow coming to the glomerulus but
instead because the actual tubules which line the nephron something we call renal
renal failure if they weren't working well what would happen is that you would
still get a slow down here because these cells are not working and
the b1 would go up but it would only go up for one reason and that's because the
GFR is low you would not get reabsorption of this the UN and so what
you would have is this would be more of a pre renal what I mean by that is is
that there is a reduction in the flow coming here and because of that you're
getting the BU when increasing because of the low flow and also the vun is
being reabsorbed because these cells are working so that would be a pre renal
situation or in a renal situation in a real situation these cells are not
working and so if these cells are not working the flow is going to be low as a
result of that but you're not going to get bu and reabsorption and so it's
going to go up instead of being 50 into one okay it's going to go to 30 over two
which is still the same 15 to one ratio but they're both going up but this is
not going up more than that it so it still holds the same ratio so in a renal
situation where there's a problem with the kidney itself you hold and maintain
that 15 to one ratio but when there is a pre renal State in other words there's
not enough flow coming to the glomerulus you're going to get a reabsorption of
the BU n preferentially and that's going to increase your bu and ratio to 20 to 1
okay so the bottom line here is that the bu and the correcting ratio can kind of
tell you what kind of renal failure have is it pre renal sot mia or is it renal a
zootie mia pre renal azo to me is where there's not enough blood coming to the
kidney renal azo t Mia is where there's something wrong with the kidney itself
if it's 20 to 1 ratio like this you're thinking of pre renal sot mia if it's
still 15 to 1 ratio its renal sot mia okay another way of looking at that is
just looking at sodium and concentration so let's go ahead and draw
our glomerulus again a little bigger this time here's the glomerulus the
proximal convoluted tubule loop of Henle and then out okay so pretend you're a
nephron and you've got two different situations you've got a situation where
you have a pre renal situation where there is not enough blood coming in to
the kidney and you've got a renal situation where there's a problem with
the actual tubules that are supposed to be transporting fluids and reabsorbing
in a pre renal situation there's not enough blood coming in and you have a
reduction in your GFR well the kidney thinks that there's not enough volume
and the way the kidney regulates volume is by reabsorbing sodium so if there's
not enough volume all the kidney is going to do is just try to reabsorb more
sodium and so what's going to happen to your urine that's coming out you think
it's going to be high in sodium or low in sodium well it's going to be very low
in sodium in a pre renal situation and of course as sodium goes up guess what
else comes with it water and so are you going to have a lot of water in the
filtrate in the urine that's coming out the answer is no so you're going to be
very low in sodium and very low in water so that leaves a high concentration or a
high osmolarity to the urine that's coming out okay let's look at the flip
side of that again here's our Clare Ulis
Oh Mary Alice proximal convoluted be a loop of Henle and collecting tube you'll
going out to the toilet down below okay so in this situation we've got a renal
problem the renal problem is is that these cells are not reabsorbing in fact
none of them are reabsorbing there's a problem all over and so what's the
situation here here we've got plenty of fluid coming to the kidneys but because
these cells are not working all of that sodium water is just going right through
and out so we would expect to see a high sodium concentration a high water
concentration and therefore a low osmolarity to the fluid and that's in
renal when there's a problem with renal
okay so let's summarize once again make sure everyone's on the same page here so
we've got the B UN okay what are some of the points that we learned about the B
UN it's synthesized in the liver okay it's filtered and absorbed okay what are
things that can increase it that have nothing to do with the kidney we've
talked about that we've talked about fever we've talked about GI bleeding
okay there's antibiotics that tip like
tetracycline there is catabolic effects
steroids all that what are some things that can decrease it
well liver failure
okay and then what about proximal convoluted tubule we see there we see
that it parallels the reabsorption of sodium and water and so we know that
it's reabsorbed okay and so at because of that in a low flow
state and a low flow state it's going to be really absorbed it's going to go up
okay now what about granting all right we know that it's the end product of
muscle and that it is filtered and secreted okay so what are some things
that can increase it they have nothing to do with the kidney function we know
that it can be increased by drugs that prevent its secretion in the kidney and
that would be cimetidine and trimethoprim what are some things that
they can decrease it muscle wasting okay and we know that it goes up in renal
failure okay well let's talk about the B UN to creatinine ratio we know that
because the B UN goes up even higher in a low flow state that it's usually 20 to
1 ratio if it's pre renal pre renal again is any reason that causes a
decrease in blood flow to the kidneys and that it's 15 to 1 in renal okay and
in renal remember is where there's a problem with the kidney itself and pre
renal is where there's a problem with the amount of blood flow that's going to
the kidney okay now
join me for part three for the final wrap-up of acute renal failure thanks
for joining me