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Urinary System, Part 2: Crash Course Anatomy & Physiology #39

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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, and eventually the
    need to pee becomes too strong to ignore, at which point the pontine micturition center
    jumps into action, overriding the previous orders, and opening the sphincters 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 … or a guy who was sent
    to “leave a message” on Jeffrey Lebowski’s rug.
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    Today you learned how the urinary system regulates 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 -- from the ureters to the urethra -- and we went over the nervous system’s role in
    controlling the act of urination.
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    Thank you to our Headmaster of Learning, Linnea Boyev, and thank you to all of our Patreon
    patrons whose monthly contributions 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, and you want to get thanked at the end of every episode, like I just did
    for all of our Patreon patrons -- if that’s you then thank you so much -- you can go to
    patreon.com/crashcourse.
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    This episode was filmed in the Doctor Cheryl C. Kinney Crash Course Studio, it was written
    by Kathleen Yale, edited by Blake de Pastino, and our consultant is Dr. Brandon Jackson.
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    It was directed by Nicholas Jenkins, edited by Nicole Sweeney, our sound designer is Michael
    Aranda, and the Graphics team is Thought Cafe.
Title:
Urinary System, Part 2: Crash Course Anatomy & Physiology #39
Description:

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Video Language:
English
Duration:
09:51

English subtitles

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