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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,
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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 ♪]
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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