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- [Instructor] When producing urine,

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it can either be produced in dilute form,

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or concentrated form.

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So let's start with the dilute form.

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When your urine is dilute,

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you're trying to get rid of excess water.

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Your body has too much of it.

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You could actually cause problems
by having too much water.

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So the way you get rid of it,

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the way excrete your excess water

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it's obviously the kidneys,

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but it's the distal tubule
is not helping at all.

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The distal tubule is impermeable to water.

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That means it's the proximal tubule esom

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in loop of henle.

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What is happening is salt,
NaCl sodium chloride,

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is reabsorbed without the
concurrent reabsorption

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of water.

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So you end up reabsorbing,
so pulling more salt back in

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but not pulling more water.

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That means you're adding
more sodium into the mix.

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More sodium, more solutes

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which means you're
causing the water to have

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a higher solute concentration.

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There's more stuff in it basically.

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You can see in this figure how urine

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is formed in dilute nature.

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You start in the top left.

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The blood filtered out of the glomerulus

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into the glamour capsule.

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It goes to the proximal tubule,

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those little twists and
turns on the top left figure

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but then heads down into
the doula of the kidney.

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Goes down loop of henle
and water's pulled out.

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It goes up the loop of henle

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and salt's pulled out.

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Well as you pull salt out,

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you're making it more and more dilute.

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As you keep going up the
proximal to the loop of henle,

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now the distal.

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The distal tubule, more salt pulled out.

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The collecting duct,
more salt is pulled out.

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More and more salt being pulled out

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means the urine is more and more dilute.

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Well if you want to conserve your water,

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you want to keep the water
in, concentrate the urine,

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you want to make sure that
you have as much water stored,

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as little leaving.

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The reason this is important is

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if you have too little water,
there's not enough in the body

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it gives you low blood volume.

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Low blood volume can
mean low blood pressure.

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Low blood pressure can
cause a whole bunch problems

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from lightheadness to
fainting to even more severe

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problems thereafter.

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If you dehydrate your body cells,

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they can't function normal.

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So the kidneys help to counteract this.

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They conserve water

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and concentrate the urine.

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They keep more water in the body.

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They try to bring as much in as possible.

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How's it do it?

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ADH, antidiuretic hormone.

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Now this antidiuretic hormone

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will increase the permeability

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of the collecting ducts to water.

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So once it's gone all the
way through the nephron,

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you end up with your dilute urine

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but instead of the water
staying in the collecting duct,

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ADH or antidiuretic hormone

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cause the water to pull
out of the collecting duct.

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As you're pulling this water out,

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it's concentrating the urine.

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So the same basic set
up here as dilute urine.

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You have the filtration
current in the glomerulus.

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You go down into the proximal tubule

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and down the loop of
henle water is pulled out.

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Up the loop of henle, salt pulled out.

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Distal tubule, salt pulled out.

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But once you get into the collecting duct,

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you can still pull some salt out

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which gives you a very dilute urine.

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Then look at all the arrows
for water coming out.

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Tons and tons and tons of water.

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That's how you're
conserving the water now.

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By the time it leaves the collecting duct,

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it's a lower volume

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but it's very concentrated.

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You're forming concentrated urine.

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So the only difference of forming dilute

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versus concentrated urine

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is with the collecting duct.

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If ADH or antidiuretic hormone's present,

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then you're conserving water.

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It's pulled out of the collecting duct.

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But if you don't have very much

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or you have no antidiuretic hormone,

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that water is not being pulled out of the

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collecting duct.

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So the whole difference
is the collecting duct.

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But now even if we form the urine,

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it still is going to sit
in the urine bladder.

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Eventually it has to leave the body.

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That's where this micturition
reflex comes into play.

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Micturition is the same
thing as urination.

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Micturition reflex will be triggered

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when your stretch receptors
in your urine bladder

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are stretched far enough.

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So what ends up happening is

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when your urine bladder fills up enough,

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the internal sphincter just smooth muscle,

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that means involuntary,

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the internal sphincter
is going to open up.

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You then have to go to
the external sphincter

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which is skeletal muscle.

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External sphincter is under
your voluntary control.

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This is why even if you
get the urge immediately,

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you have to go, you have to urinate,

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you don't immediately go at that second

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because you still have
some partial control

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in this micturition reflex.

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You control the external
urethral sphincter.

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But the brain can override that.

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There's time when the micturition reflex

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becomes completely involuntary.

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No matter how much you might
not want here at that point,

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the brain can say uh
uh, it doesn't matter.

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It's time now.

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That it can do this because
of potential damage.

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If the urine bladder is
stretched out too much,

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it could actually tear and damage it.

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So the body will override way before

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you get to the point of damage.

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But now this whole voluntary aspect

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gets harder and harder.

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The more urine in the urine bladder,

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the harder it is to prevent urination.

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That's because your
body's building up more

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and more of these stretch receptors.

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There's more and more stimuli

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telling your body that
you have to urinate now.

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It's the time.

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So voluntary control gets less and less

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until eventually there
is no voluntary control.

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The body will automatically

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excrete the urine.