- [Narrator] So now, how is urine formed?
Well, it start off when the blood's filtered.
That's glomerular filtration.
Now, this glomerular filtration
is moving protein-free solution.
The reason it's protein-free, is proteins are too large,
they can't simply leave the blood very easily.
So, urine itself, really shouldn't have
a whole lot of protein.
Should have zero, or maybe some traces.
If the protein levels keep increasing and increasing,
well, that can cause some problems.
That could potentially be kidney damage.
Tubular reabsorption.
Tubular reabsorption, you're reabsorbing.
You're returning a lot of the fluid
and solutes back to the blood.
So that 180 liters of filtrate per day,
the vast majority of that is being returned
back to the blood, in the process of tubular reabsorption.
The blood is reabsorbing the fluid.
And then we have tubular secretion.
Now, tubular secretion is how the blood
can get rid of extra, or additional solutes.
Maybe there's extra particles that need to filtered out,
that couldn't pass through in the glomerulus.
So tubular secretion is a way of getting more things
back into the tubule from the blood.
So here we have a nephron.
If you notice, the glomerular capsule, and the glomerulus
are right in the middle.
That's the blood enter the nephron,
and where the filtration first occurs.
That's called glomerular filtration.
That's where you have roughly 180 liters per day
be filtered through.
We're talking water, ions, glucose, amino acids.
There's tons of stuff in here.
It all that enters into the proximal tubule.
Well the proximal tubule
is where tubular reabsorption occurs.
Now, a majority happens in the proximal tubule,
but you can still end up getting a little bit in the distal,
but not as much.
So you're trying to basically put back into the blood
what should not be excreted.
Now, tubular secretion means they're going
in the opposite direction of tubular reabsorption.
Now, instead of the liquid and solutes leaving the nephron,
in tubular secretion those liquids and solutes
are going into the nephron.
They're leaving the blood.
So glomerular filtration, right when the blood
is being filtered into the nephron.
Tubular reabsorption is when you have liquid and solute
leaving the nephron.
Tubular secretion is where you're pulling fluid and solute
back into the nephron.
So now, let's talk about
this glomerular filtration for a second.
Protein-free plasma.
Sure, no problem, proteins are too large.
A lot of filtration occurred.
But even with that 180 liters a day, it's very selective.
Anything that's too large, so proteins and other cells,
like red blood cells, white blood cells, can't get through.
They're physically too large
to pass through the filtration process.
Now, filtration is driven by blood pressure.
Basically, any filtration has to have a pressure pushing it.
In this case, it's blood pressure
pushing the blood into that capillary, into the glomerulus.
It's that pressure that can push small solutes and liquid
through the semipermeable membrane.
Well, the rate of filtration
is under a chemical control mechanism,
at least for resting.
So you're sitting down, lying down, you're calm, relaxed,
there are local chemicals that are gonna be telling
to increase or decrease filtration rate.
So here you can see the afferent material brings blood in,
goes into the glomerulus, those capillaries,
and the pressure of the blood pressure
is going to force filtration to occur.
In the top left, you can see that microscopic view.
You can see those pink structures
are going to be covered by the purple, the podocyte.
Well, a podocyte is a cell that covers the capillary,
helps to control the filtration,
makes these little things called filtration slits.
So basically if something can fit through a filtration slit,
it's good to go.
If it can't fit through it, it stays in the blood.
So whatever gets filtered out goes in the proximal tubule
and travels on down the nephron.
Now, tubular reabsorption it pretty much returns
all that filtered water and solutes to the blood.
All your glucose is filtered out, all your amino acids,
all your bicarbonates, which is a buffer.
100% of that is going to be reabsorbed.
You need glucose, it's sugar, it's energy.
You need amino acids, that's your building blocks.
Bicarbonate buffers your pH.
But, only half, or 50% of the urea is reabsorbed.
So majority as I mentioned, happens in that proximal tubule.
But sodium is where this whole thing really starts off.
Sodium is where it begins.
Because, when sodium starts to move,
it uses active transport.
That means you put energy into the system.
By putting energy in to the system, it creates a gradient,
which helps other ions and particles also move.
So, it's this movement of sodium providing energy
for glucose and amino acids to move around.
But if you are also reabsorbing salts,
wherever there's salt, there has to be water.
You can kind of think of it, if you ever went to a ballgame,
and you had one of those big soft pretzels,
they're loaded with salt.
Now, what's usually the first thing you want
to do when you finish that?
You want something to drink, you're thirsty.
That's because you have more salt in your body.
Your body's trying to increase the amount of water
to balance it out.
So, by simply bringing salt in,
now you can bring more water in.
So, bring more sodium, brings more energy,
which brings more glucose, more amino acids.
Really, sodium is the starting piece.
Now if you look at this thing, hold on.
This is really complex.
Just look at the two red arrows.
In the top red arrow, you see sodium
is going to go across the membrane, use that ATP.
By doing this, you help glucose and amino acids move.
Now, the lower arrow is water.
Water itself is going to go straight across,
right across that cell membrane,
right back into the capillary,
your body's way of trying to maintain fluid levels.
So, the third part, that tubular secretion.
Tubular secretion is a way
of getting other components out of the blood.
Things that weren't filtered out the first time through.
It moves from these capillaries,
these peritubular capillaries, or even the vasa recta,
back into the nephron.
So, back into that tubule.
Why?
Helps to regulate chemical levels.
If, also, chemical levels seem off,
this is one way of removing excess, or high amounts.
Or it's another way of just getting rid harmful chemicals.
Your liver does a great job of detoxifying the blood,
but it can't get everything.
So, tubular secretion helps put more
of these harmful chemicals back into the nephron.
So, what's being secreted?
Penicillin, cocaine, marijuana, pesticides, preservatives,
basic old hydrogen ions, just hydrogen, ammonium, potassium.
Hold on if you're thinking this doesn't make a lot of sense,
penicillin you want in your body.
That's a antibiotic, it helps you.
True, but most medicines immediately
are broken down or moved once they enter your body.
That's why they only last for a certain period of time.
Cocaine, marijuana, pesticides, preservatives,
all chemicals that are not healthy to the body in them.
Hydrogen ions, now these are just H-plus, simple hydrogen.
Problem here, too many of those can turn your blood acidic.
Ammonium, really don't want that much ammonium in your body.
Potassium, might think, hey, potassium's good.
It is good to a point, you can have too much potassium.