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It has filled countless diapers,
caused discomfort for any number
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of airline passengers,
and it totally ruined the Dude’s rug,
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which really tied the room together, man.
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Anatomists call it micturition,
and I don’t know why
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because the rest of us call it urination,
which seems like a fine word.
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All mammals and most animals
urinate to remove toxins
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and to help maintain water-volume
homeostasis, or blood pressure.
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And while some of us spray it around
to attract mates or mark territory,
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or deter predators; as far as I know,
only humans actually study pee.
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In fact, we’ve been doing it
for thousands of years.
-
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Even before Hippocrates extolled
the diagnostic virtues of pee-sniffing,
-
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early Sumerian and Babylonian physicians
were making urine-related observations.
-
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Medieval doctors as well diagnosed
diseases based on on smelling,
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inspecting, or even tasting urine samples.
-
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And although they were often
totally off-base,
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which makes me feel bad for those guys
who sipped urine for no reason;
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they were kind of on to something.
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Today, urological tests can help detect
a lot of ailments based on
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the color, the smell, the clarity,
and chemical composition of a sample.
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Freshly peed urine is usually about
95% water,
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slightly acidic with a pH of around 6,
a little aromatic,
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and usually somewhere
between clear and dark yellow in color
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depending on your level of hydration.
-
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Urine also contains over
3000 different chemical compounds,
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and their varying levels of concentration
can tell us a lot about
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what’s going on in the body.
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For example,
if you give me a urine sample,
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I will have no idea what to do with it.
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But if you give it to a doctor
and they can see that
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it’s cloudy with white blood cells,
that’s a good sign
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you’ve got a urinary tract infection.
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If it smells sweet
and contains a lot of glucose,
-
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you might have diabetes.
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If it looks pink,
then unless you’ve recently eaten beets,
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you probably have
internal bleeding somewhere.
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And if it is chocked full of proteins,
you could be pregnant,
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or working out too hard,
or have high blood pressure,
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or be headed to heart failure.
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So as you can see, even if the most
thought you’ve given the subject is
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to wonder if you should pee now
or wait until the end of the movie;
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the whole process of producing, storing,
and eliminating pee
-
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is no where near as simple as it may seem.
-
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From osmotic pressure
to stretch receptors to hormones,
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our circulatory, nervous,
and endocrine systems regulate
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how much urine we produce,
what goes into it,
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and when to get rid of it.
-
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So join me
as we journey into the world of pee.
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Wait, can we rephrase that?
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[♪ Intro Music ♪]
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All right, how about:
“let’s look at where your pee comes from.”
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No, actually,
that doesn’t sound good either.
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Let’s begin by looking at
what regulates the production of urine.
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That works.
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Last time, we discussed
how your kidneys filter your blood,
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but the actual production of urine
can be affected
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by a whole host of factors.
-
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One thing that might have
crossed your mind last time
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is that the production of urine
must by its very nature
-
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be influenced by blood.
-
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Specifically, its volume and its pressure.
-
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Because, step one in pee-making
is the process of glomerular filtration
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where blood is filtered
in the little blood-filled balls of yarn
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that are the glomeruli.
-
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So, just like water in a hose,
higher pressure in the blood
-
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must push more plasma
out of the capillaries
-
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and into the glomeruli.
-
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But here’s a problem:
Your kidneys can only handle
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so much filtrate at a time.
-
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So they have to maintain
a constant rate of flow inside of them.
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This is known as
the glomerular filtration rate,
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or how much blood passes through
the glomeruli every minute.
-
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And your kidneys have ways
of regulating this rate
-
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despite changes in blood pressure.
-
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If your blood pressure happens
to increase, for example,
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the higher pressure causes the arterioles
leading to your glomeruli to stretch.
-
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And then the smooth muscle
in the walls of the glomeruli
-
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respond to this stretching stimulus
by constricting,
-
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automatically reducing the amount
of blood flow into the glomeruli
-
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and leaving the flow rate
relatively unchanged.
-
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This kind of intrinsic control,
or autoregulation, is helpful
-
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in controlling the filtration rate
through normal ranges of blood pressures.
-
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But the kidneys mostly regulate
urine concentration
-
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at the other end of the nephron tubules.
-
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This kind of regulation
I’m sure you’re familiar with.
-
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If you’ve ever had too much coffee
or gone on a bit of a bender,
-
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you may have experienced the pleasure
of having to pee every five minutes.
-
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That’s because your endocrine system
has a lot to say
-
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about your bathroom breaks.
-
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So you have some strong hormonal
mechanisms that affect
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when and how often you go.
-
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And as it happens,
both caffeine and alcohol inhibit
-
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the release of one of these hormones
called antidiuretic hormone, or ADH;
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which is secreted by
the posterior pituitary gland
-
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to help the body retain water
and stay hydrated.
-
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How ADH works is kind of complex,
but first let’s remember
-
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that cell membranes are generally
not that permeable to water.
-
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But in the parts of the nephron
that reabsorb water,
-
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like the descending limb of the loop
of Henle,
-
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water has to move easily through cells
from the filtrate to the blood.
-
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This is possible because of
special protein channels
-
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in their membranes called aquaporins
that are on both the apical,
-
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or filtrate-facing side, and the basal
or capillary-facing side of the cells.
-
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By contrast, the cells lining
the collecting duct
-
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only have aquaporins on the basal side,
so not a lot of reabsorption
-
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takes place there usually.
-
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But ADH triggers those cells
to move aquaporins they have in storage
-
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over to the apical side,
which allows more water
-
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to leave the urine.
-
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And since caffeine
and alcohol inhibit ADH,
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that means no moving aquaporins,
which means very little
-
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water reabsorption,
and ultimately tons of peeing,
-
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and dehydration.
-
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So, yeah, lots of factors
affect the production of urine.
-
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But once it’s produced,
it doesn’t just leave the building.
-
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It has to be moved and stored
until the time is right.
-
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Once the urine leaves the kidneys,
it enters the ureters,
-
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a pair of slender tubes that drop down
to the posterior urinary bladder.
-
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Contrary to what you might think,
your ureters aren’t just passive tubes
-
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and your pee doesn’t wind up
in your bladder because of gravity alone.
-
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Rather like the small intestines,
each ureter features a layer
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of smooth muscle that contracts
to move urine using peristalsis.
-
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The frequency and strength
of these peristaltic waves varies
-
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depending on how fast urine
is being produced;
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and a series of valves prevent pee
from backing up
-
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making sure that instead
it reaches the bladder.
-
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The bladder is a hollow, collapsible sac
that temporarily stores urine.
-
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Like the kidneys, it’s retroperitoneal,
located posterior to the pubic bone
-
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and anterior to the rectum.
-
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The bladder wall consists of 3 layers:
an inner mucosa
-
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surrounded by a thick muscular layer
called the detrusor
-
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wrapped in a fibrous,
protective outer membrane.
-
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The inner mucosal layer
consists of transitional epithelium,
-
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which allows the bladder to expand
so it can hold more urine.
-
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A handy feature for social mammals
like us who prefer dry underwear
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and peeing in private.
-
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When it’s empty,
it collapses into a triangular shape,
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folding up on itself
like a deflated balloon.
-
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Then as urine accumulates,
the bladder thins and expands
-
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into a pear-shape,
and all those folds disappear.
-
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A full bladder can comfortably hold
around 500 mL of pee,
-
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though it can usually expand
to hold a maximum of around one liter.
-
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At that point, though,
you’re pushing your luck
-
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because prolonged overdistention
could, in theory, lead to a burst bladder.
-
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Although you’d probably
just pee your pants first.
-
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But let’s assume for the sake
of polite conversation
-
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that you have found an appropriate
location to relieve yourself.
-
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Your urine enters the thin
but muscular urethra
-
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by passing through the internal
urethral sphincter.
-
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Now we don’t actually have voluntary
control over this particular sphincter,
-
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but the autonomic nervous system
keeps it cinched up
-
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whenever you’re not peeing
to prevent leakage.
-
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Once the urine is through the sphincter,
it heads down
-
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through the urogenital diaphragm
which includes the last stop:
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the external urethral sphincter;
which is probably the one
-
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that you’re familiar with
because it’s made of skeletal muscles
-
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and is the one that you control
voluntarily.
-
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Only now are we finally ready to explore
the act of micturition itself,
-
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the actual excretion of urine, urination.
-
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As the pee from your morning coffee
builds up,
-
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it causes the bladder to push out,
activating the stretch receptors
-
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in its walls.
-
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The resulting nerve impulses zip along
afferent fibers
-
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to the sacral region of the spinal cord,
along interneurons, and toward the brain;
-
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eventually exciting
the parasympathetic neurons
-
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and inhibiting the sympathetic system.
-
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This tells the detrusor to contract
while the internal urethral sphincter
-
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simultaneously opens,
and the external sphincter relaxes
-
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so that the pee can flow out.
-
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This, you may or not recall,
is kind of an acquired skill.
-
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When you’re a baby,
those stretch-receptor impulses
-
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trigger a simple spinal reflex
that coordinates this whole process,
-
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and you have no real control over
when you pee.
-
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But within a couple of years of birth,
your brain’s circuits have developed
-
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the ability to override
simple reflexive urination
-
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and to choose a different neural pathway.
-
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So how’s that possible?
-
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Well, an area of your brainstem
called the pons
-
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contains two different centers
that lock down your urination control,
-
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or lack of it.
-
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There’s the pontine storage area,
which inhibits urination;
-
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and the pontine micturition center,
which gives it the green light.
-
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As your bladder fills up,
impulses triggered by stretch receptors
-
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head to the pons
and other higher brain centers
-
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that give you that conscious feeling
that you have to pee.
-
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If your bladder isn’t full
and you’re too busy to find a bathroom,
-
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it mostly activates
the pontine storage area
-
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that keeps you from peeing by
inhibiting your parasympathetic activity
-
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and increasing sympathetic output.
-
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Of course, the longer you hold it,
the more your bladder fills up,
-
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and eventually the need to pee
becomes too strong to ignore.
-
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At which point the pontine
micturition center jumps into action,
-
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overriding the previous orders,
and opening the sphincters
-
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so you can finally tinkle.
-
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And that’s how your own
personal waterworks works.
-
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Whether you’re a baby in diapers,
or a grown-up science student,
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or a guy who was sent to
“leave a message”
-
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on Jeffrey Lebowski’s rug.
-
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Today you learned
how the urinary system regulates
-
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the production of urine by maintaining
a study glomerular flow rate.
-
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We also talked about the anatomy
of storing and excreting urine
-
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from the ureters to the urethra;
and we went over the nervous system’s role
-
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in controlling the act of urination.
-
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Thank you to our Headmaster
of Learning, Linnea Boyev,
-
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and thank you to all of our Patreon
patrons whose monthly contributions
-
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help make Crash Course possible,
not only for themselves, but for everyone.
-
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If you like Crash Course and you want
to help us keep making videos like this,
-
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and you want to get thanked
at the end of every episode
-
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like I just did
for all of our Patreon patrons;
-
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.if that’s you,
then thank you so much.
-
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You can go to patreon.com/crashcourse.
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This episode was filmed
in the Doctor Cheryl C. Kinney
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Crash Course Studio.
-
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It was written by Kathleen Yale,
edited by Blake de Pastino,
-
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and our consultant is Dr. Brandon Jackson.
-
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It was directed by Nicholas Jenkins,
edited by Nicole Sweeney.
-
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Our sound designer is Michael Aranda,
and the Graphics team is Thought Cafe.
-
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[♪ Outro Music ♪]