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- [Narrator] So now, how is urine formed?

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Well, it start off when
the blood's filtered.

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That's glomerular filtration.

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Now, this glomerular filtration

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is moving protein-free solution.

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The reason it's protein-free,
is proteins are too large,

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they can't simply leave
the blood very easily.

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So, urine itself, really shouldn't have

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a whole lot of protein.

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Should have zero, or maybe some traces.

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If the protein levels keep
increasing and increasing,

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well, that can cause some problems.

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That could potentially be kidney damage.

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Tubular reabsorption.

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Tubular reabsorption, you're reabsorbing.

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You're returning a lot of the fluid

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and solutes back to the blood.

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So that 180 liters of filtrate per day,

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the vast majority of
that is being returned

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back to the blood, in the
process of tubular reabsorption.

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The blood is reabsorbing the fluid.

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And then we have tubular secretion.

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Now, tubular secretion is how the blood

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can get rid of extra,
or additional solutes.

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Maybe there's extra particles
that need to filtered out,

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that couldn't pass
through in the glomerulus.

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So tubular secretion is a
way of getting more things

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back into the tubule from the blood.

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So here we have a nephron.

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If you notice, the glomerular
capsule, and the glomerulus

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are right in the middle.

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That's the blood enter the nephron,

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and where the filtration first occurs.

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That's called glomerular filtration.

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That's where you have
roughly 180 liters per day

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be filtered through.

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We're talking water, ions,
glucose, amino acids.

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There's tons of stuff in here.

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It all that enters into
the proximal tubule.

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Well the proximal tubule

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is where tubular reabsorption occurs.

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Now, a majority happens
in the proximal tubule,

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but you can still end up getting
a little bit in the distal,

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but not as much.

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So you're trying to basically
put back into the blood

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what should not be excreted.

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Now, tubular secretion means they're going

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in the opposite direction
of tubular reabsorption.

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Now, instead of the liquid and
solutes leaving the nephron,

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in tubular secretion
those liquids and solutes

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are going into the nephron.

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They're leaving the blood.

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So glomerular filtration,
right when the blood

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is being filtered into the nephron.

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Tubular reabsorption is when
you have liquid and solute

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leaving the nephron.

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Tubular secretion is where
you're pulling fluid and solute

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back into the nephron.

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So now, let's talk about

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this glomerular filtration for a second.

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Protein-free plasma.

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Sure, no problem, proteins are too large.

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A lot of filtration occurred.

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But even with that 180 liters
a day, it's very selective.

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Anything that's too large,
so proteins and other cells,

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like red blood cells, white
blood cells, can't get through.

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They're physically too large

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to pass through the filtration process.

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Now, filtration is
driven by blood pressure.

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Basically, any filtration has
to have a pressure pushing it.

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In this case, it's blood pressure

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pushing the blood into that
capillary, into the glomerulus.

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It's that pressure that can
push small solutes and liquid

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through the semipermeable membrane.

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Well, the rate of filtration

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is under a chemical control mechanism,

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at least for resting.

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So you're sitting down, lying
down, you're calm, relaxed,

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there are local chemicals
that are gonna be telling

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to increase or decrease filtration rate.

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So here you can see the afferent
material brings blood in,

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goes into the glomerulus,
those capillaries,

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and the pressure of the blood pressure

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is going to force filtration to occur.

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In the top left, you can
see that microscopic view.

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You can see those pink structures

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are going to be covered by
the purple, the podocyte.

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Well, a podocyte is a cell
that covers the capillary,

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helps to control the filtration,

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makes these little things
called filtration slits.

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So basically if something can
fit through a filtration slit,

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it's good to go.

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If it can't fit through
it, it stays in the blood.

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So whatever gets filtered out
goes in the proximal tubule

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and travels on down the nephron.

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Now, tubular reabsorption
it pretty much returns

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all that filtered water
and solutes to the blood.

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All your glucose is filtered
out, all your amino acids,

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all your bicarbonates, which is a buffer.

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100% of that is going to be reabsorbed.

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You need glucose, it's sugar, it's energy.

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You need amino acids,
that's your building blocks.

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Bicarbonate buffers your pH.

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But, only half, or 50% of
the urea is reabsorbed.

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So majority as I mentioned,
happens in that proximal tubule.

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But sodium is where this
whole thing really starts off.

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Sodium is where it begins.

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Because, when sodium starts to move,

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it uses active transport.

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That means you put energy into the system.

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By putting energy in to the
system, it creates a gradient,

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which helps other ions
and particles also move.

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So, it's this movement of
sodium providing energy

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for glucose and amino
acids to move around.

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But if you are also reabsorbing salts,

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wherever there's salt,
there has to be water.

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You can kind of think of it,
if you ever went to a ballgame,

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and you had one of
those big soft pretzels,

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they're loaded with salt.

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Now, what's usually the
first thing you want

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to do when you finish that?

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You want something to
drink, you're thirsty.

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That's because you have
more salt in your body.

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Your body's trying to
increase the amount of water

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to balance it out.

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So, by simply bringing salt in,

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now you can bring more water in.

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So, bring more sodium, brings more energy,

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which brings more
glucose, more amino acids.

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Really, sodium is the starting piece.

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Now if you look at this thing, hold on.

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This is really complex.

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Just look at the two red arrows.

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In the top red arrow, you see sodium

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is going to go across the
membrane, use that ATP.

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By doing this, you help
glucose and amino acids move.

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Now, the lower arrow is water.

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Water itself is going
to go straight across,

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right across that cell membrane,

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right back into the capillary,

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your body's way of trying
to maintain fluid levels.

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So, the third part,
that tubular secretion.

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Tubular secretion is a way

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of getting other components
out of the blood.

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Things that weren't filtered
out the first time through.

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It moves from these capillaries,

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these peritubular capillaries,
or even the vasa recta,

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back into the nephron.

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So, back into that tubule.

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Why?

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Helps to regulate chemical levels.

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If, also, chemical levels seem off,

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this is one way of removing
excess, or high amounts.

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Or it's another way of just
getting rid harmful chemicals.

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Your liver does a great job
of detoxifying the blood,

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but it can't get everything.

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So, tubular secretion helps put more

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of these harmful chemicals
back into the nephron.

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So, what's being secreted?

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Penicillin, cocaine, marijuana,
pesticides, preservatives,

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basic old hydrogen ions, just
hydrogen, ammonium, potassium.

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Hold on if you're thinking this
doesn't make a lot of sense,

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penicillin you want in your body.

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That's a antibiotic, it helps you.

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True, but most medicines immediately

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are broken down or moved
once they enter your body.

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That's why they only last
for a certain period of time.

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Cocaine, marijuana,
pesticides, preservatives,

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all chemicals that are not
healthy to the body in them.

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Hydrogen ions, now these are
just H-plus, simple hydrogen.

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Problem here, too many of those
can turn your blood acidic.

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Ammonium, really don't want
that much ammonium in your body.

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Potassium, might think,
hey, potassium's good.

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It is good to a point, you
can have too much potassium.