- [Instructor] When producing urine,
it can either be produced in dilute form,
or concentrated form.
So let's start with the dilute form.
When your urine is dilute,
you're trying to get rid of excess water.
Your body has too much of it.
You could actually cause problems by having too much water.
So the way you get rid of it,
the way excrete your excess water
it's obviously the kidneys,
but it's the distal tubule is not helping at all.
The distal tubule is impermeable to water.
That means it's the proximal tubule esom
in loop of henle.
What is happening is salt, NaCl sodium chloride,
is reabsorbed without the concurrent reabsorption
of water.
So you end up reabsorbing, so pulling more salt back in
but not pulling more water.
That means you're adding more sodium into the mix.
More sodium, more solutes
which means you're causing the water to have
a higher solute concentration.
There's more stuff in it basically.
You can see in this figure how urine
is formed in dilute nature.
You start in the top left.
The blood filtered out of the glomerulus
into the glamour capsule.
It goes to the proximal tubule,
those little twists and turns on the top left figure
but then heads down into the doula of the kidney.
Goes down loop of henle and water's pulled out.
It goes up the loop of henle
and salt's pulled out.
Well as you pull salt out,
you're making it more and more dilute.
As you keep going up the proximal to the loop of henle,
now the distal.
The distal tubule, more salt pulled out.
The collecting duct, more salt is pulled out.
More and more salt being pulled out
means the urine is more and more dilute.
Well if you want to conserve your water,
you want to keep the water in, concentrate the urine,
you want to make sure that you have as much water stored,
as little leaving.
The reason this is important is
if you have too little water, there's not enough in the body
it gives you low blood volume.
Low blood volume can mean low blood pressure.
Low blood pressure can cause a whole bunch problems
from lightheadness to fainting to even more severe
problems thereafter.
If you dehydrate your body cells,
they can't function normal.
So the kidneys help to counteract this.
They conserve water
and concentrate the urine.
They keep more water in the body.
They try to bring as much in as possible.
How's it do it?
ADH, antidiuretic hormone.
Now this antidiuretic hormone
will increase the permeability
of the collecting ducts to water.
So once it's gone all the way through the nephron,
you end up with your dilute urine
but instead of the water staying in the collecting duct,
ADH or antidiuretic hormone
cause the water to pull out of the collecting duct.
As you're pulling this water out,
it's concentrating the urine.
So the same basic set up here as dilute urine.
You have the filtration current in the glomerulus.
You go down into the proximal tubule
and down the loop of henle water is pulled out.
Up the loop of henle, salt pulled out.
Distal tubule, salt pulled out.
But once you get into the collecting duct,
you can still pull some salt out
which gives you a very dilute urine.
Then look at all the arrows for water coming out.
Tons and tons and tons of water.
That's how you're conserving the water now.
By the time it leaves the collecting duct,
it's a lower volume
but it's very concentrated.
You're forming concentrated urine.
So the only difference of forming dilute
versus concentrated urine
is with the collecting duct.
If ADH or antidiuretic hormone's present,
then you're conserving water.
It's pulled out of the collecting duct.
But if you don't have very much
or you have no antidiuretic hormone,
that water is not being pulled out of the
collecting duct.
So the whole difference is the collecting duct.
But now even if we form the urine,
it still is going to sit in the urine bladder.
Eventually it has to leave the body.
That's where this micturition reflex comes into play.
Micturition is the same thing as urination.
Micturition reflex will be triggered
when your stretch receptors in your urine bladder
are stretched far enough.
So what ends up happening is
when your urine bladder fills up enough,
the internal sphincter just smooth muscle,
that means involuntary,
the internal sphincter is going to open up.
You then have to go to the external sphincter
which is skeletal muscle.
External sphincter is under your voluntary control.
This is why even if you get the urge immediately,
you have to go, you have to urinate,
you don't immediately go at that second
because you still have some partial control
in this micturition reflex.
You control the external urethral sphincter.
But the brain can override that.
There's time when the micturition reflex
becomes completely involuntary.
No matter how much you might not want here at that point,
the brain can say uh uh, it doesn't matter.
It's time now.
That it can do this because of potential damage.
If the urine bladder is stretched out too much,
it could actually tear and damage it.
So the body will override way before
you get to the point of damage.
But now this whole voluntary aspect
gets harder and harder.
The more urine in the urine bladder,
the harder it is to prevent urination.
That's because your body's building up more
and more of these stretch receptors.
There's more and more stimuli
telling your body that you have to urinate now.
It's the time.
So voluntary control gets less and less
until eventually there is no voluntary control.
The body will automatically
excrete the urine.