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    "segs": [ {
      "utf8": "If you lined up all \nthe blood vessels in your body,"
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  }, {
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    "segs": [ {
      "utf8": "they'd be 95,000 kilometers long"
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  }, {
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    "segs": [ {
      "utf8": "and everyday, they carry the equivalent\nof over 7,500 liters of blood,"
    } ]
  }, {
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    "dDurationMs": 5524,
    "segs": [ {
      "utf8": "though that's actually the same four\nor five liters recycled over and over,"
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  }, {
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    "segs": [ {
      "utf8": "delivering oxygen, and precious nutrients"
    } ]
  }, {
    "tStartMs": 30041,
    "dDurationMs": 4919,
    "segs": [ {
      "utf8": "like glucose and amino acids\nto the body's tissues."
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  }, {
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    "segs": [ {
      "utf8": "All that blood exerts a force on\nthe muscular walls of the blood vessels."
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    "segs": [ {
      "utf8": "That force is called blood pressure,"
    } ]
  }, {
    "tStartMs": 43066,
    "dDurationMs": 3933,
    "segs": [ {
      "utf8": "and it rises and falls\nwith the phases of the heartbeat."
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    "segs": [ {
      "utf8": "It's highest during systole,"
    } ]
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    "dDurationMs": 3859,
    "segs": [ {
      "utf8": "when the heart contracts to force\nblood through the arteries."
    } ]
  }, {
    "tStartMs": 52854,
    "dDurationMs": 2342,
    "segs": [ {
      "utf8": "This is your systolic blood pressure."
    } ]
  }, {
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    "dDurationMs": 2498,
    "segs": [ {
      "utf8": "When the heart is at rest between beats,"
    } ]
  }, {
    "tStartMs": 57694,
    "dDurationMs": 4742,
    "segs": [ {
      "utf8": "blood pressure falls to its lowest value,\nthe diastolic pressure."
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  }, {
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    "segs": [ {
      "utf8": "A typical healthy individual produces\na systolic pressure"
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  }, {
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    "segs": [ {
      "utf8": "between 90 and 120 millimeters of mercury,"
    } ]
  }, {
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    "dDurationMs": 3762,
    "segs": [ {
      "utf8": "and diastolic pressure between 60 and 80."
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  }, {
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    "segs": [ {
      "utf8": "Taken together, a normal reading is a bit\nless than 120 over 80."
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  }, {
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    "segs": [ {
      "utf8": "The blood traverses \nthe landscape of the body"
    } ]
  }, {
    "tStartMs": 81891,
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    "segs": [ {
      "utf8": "through the pipes \nof the circulatory system."
    } ]
  }, {
    "tStartMs": 84977,
    "dDurationMs": 1500,
    "segs": [ {
      "utf8": "In any plumbing system,"
    } ]
  }, {
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    "dDurationMs": 3468,
    "segs": [ {
      "utf8": "several things can increase the force\non the walls of the pipes:"
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  }, {
    "tStartMs": 89945,
    "dDurationMs": 1611,
    "segs": [ {
      "utf8": "the properties of the fluid,"
    } ]
  }, {
    "tStartMs": 91556,
    "dDurationMs": 1152,
    "segs": [ {
      "utf8": "extra fluid,"
    } ]
  }, {
    "tStartMs": 92708,
    "dDurationMs": 2052,
    "segs": [ {
      "utf8": "or narrower pipes."
    } ]
  }, {
    "tStartMs": 94760,
    "dDurationMs": 1430,
    "segs": [ {
      "utf8": "So if the blood thickens,"
    } ]
  }, {
    "tStartMs": 96190,
    "dDurationMs": 5247,
    "segs": [ {
      "utf8": "a higher pressure is needed to push it,\nso the heart will pump harder."
    } ]
  }, {
    "tStartMs": 101437,
    "dDurationMs": 3345,
    "segs": [ {
      "utf8": "A high-salt diet will lead \nto a similar result."
    } ]
  }, {
    "tStartMs": 104782,
    "dDurationMs": 2151,
    "segs": [ {
      "utf8": "The salt promotes water retention,"
    } ]
  }, {
    "tStartMs": 106933,
    "dDurationMs": 4464,
    "segs": [ {
      "utf8": "and the extra fluid increases the blood\nvolume and blood pressure,"
    } ]
  }, {
    "tStartMs": 111397,
    "dDurationMs": 3059,
    "segs": [ {
      "utf8": "and stress, \nlike the fight or flight response,"
    } ]
  }, {
    "tStartMs": 114456,
    "dDurationMs": 3909,
    "segs": [ {
      "utf8": "releases hormones, like epinephrine\nand norepinephrine"
    } ]
  }, {
    "tStartMs": 118365,
    "dDurationMs": 2185,
    "segs": [ {
      "utf8": "that constrict key vessels,"
    } ]
  }, {
    "tStartMs": 120550,
    "dDurationMs": 4661,
    "segs": [ {
      "utf8": "increasing the resistance to flow\nand raising the pressure upstream."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Blood vessels can usually handle\nthese fluctuations easily."
    } ]
  }, {
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    "dDurationMs": 4749,
    "segs": [ {
      "utf8": "Elastic fibers embedded in their walls\nmake them resilient,"
    } ]
  }, {
    "tStartMs": 133521,
    "dDurationMs": 5085,
    "segs": [ {
      "utf8": "but if your blood pressure \nregularly rises above about 140 over 90,"
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  }, {
    "tStartMs": 138606,
    "dDurationMs": 3240,
    "segs": [ {
      "utf8": "what we call hypertension, \nand stays there,"
    } ]
  }, {
    "tStartMs": 141846,
    "dDurationMs": 2124,
    "segs": [ {
      "utf8": "it can cause serious problems."
    } ]
  }, {
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    "dDurationMs": 2724,
    "segs": [ {
      "utf8": "That's because the extra strain\non the arterial wall"
    } ]
  }, {
    "tStartMs": 146694,
    "dDurationMs": 1762,
    "segs": [ {
      "utf8": "can produce small tears."
    } ]
  }, {
    "tStartMs": 148456,
    "dDurationMs": 2163,
    "segs": [ {
      "utf8": "When the injured tissue swells up,"
    } ]
  }, {
    "tStartMs": 150619,
    "dDurationMs": 2595,
    "segs": [ {
      "utf8": "substances that respond \nto the inflammation,"
    } ]
  }, {
    "tStartMs": 153214,
    "dDurationMs": 3369,
    "segs": [ {
      "utf8": "like white blood cells,\ncollect around the tears."
    } ]
  }, {
    "tStartMs": 156583,
    "dDurationMs": 4431,
    "segs": [ {
      "utf8": "Fat and cholesterol floating \nin the blood latch on, too,"
    } ]
  }, {
    "tStartMs": 161014,
    "dDurationMs": 2572,
    "segs": [ {
      "utf8": "eventually building up to form a plaque"
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  }, {
    "tStartMs": 163586,
    "dDurationMs": 4108,
    "segs": [ {
      "utf8": "that stiffens and thickens\nthe inner arterial wall."
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  }, {
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    "segs": [ {
      "utf8": "This condition is called\natherosclerosis,"
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  }, {
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    "dDurationMs": 2148,
    "segs": [ {
      "utf8": "and it can have dangerous consequences."
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  }, {
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    "segs": [ {
      "utf8": "If the plaque ruptures, a blood clot\nforms on top of the tear,"
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  }, {
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    "segs": [ {
      "utf8": "clogging the already narrowed pipe."
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  }, {
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    "segs": [ {
      "utf8": "If the clot is big enough,"
    } ]
  }, {
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    "dDurationMs": 5823,
    "segs": [ {
      "utf8": "it can completely block the flow of\noxygen and nutrients to cells downstream."
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  }, {
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    "segs": [ {
      "utf8": "In vessels that feed the heart,"
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  }, {
    "tStartMs": 188374,
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    "segs": [ {
      "utf8": "that will cause a heart attack,"
    } ]
  }, {
    "tStartMs": 190401,
    "dDurationMs": 4491,
    "segs": [ {
      "utf8": "when oxygen-deprived cardiac\nmuscle cells start to die."
    } ]
  }, {
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    "dDurationMs": 2684,
    "segs": [ {
      "utf8": "If the clot cuts off \nblood flow to the brain,"
    } ]
  }, {
    "tStartMs": 197576,
    "dDurationMs": 2690,
    "segs": [ {
      "utf8": "it causes a stroke."
    } ]
  }, {
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    "dDurationMs": 2725,
    "segs": [ {
      "utf8": "Dangerously clogged blood vessels\ncan be widened"
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  }, {
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    "dDurationMs": 2806,
    "segs": [ {
      "utf8": "by a procedure called\nan angioplasty."
    } ]
  }, {
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    "dDurationMs": 3073,
    "segs": [ {
      "utf8": "There, doctors thread a wire\nthrough the vessel"
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  }, {
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    "dDurationMs": 1959,
    "segs": [ {
      "utf8": "to the obstructed site,"
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  }, {
    "tStartMs": 210829,
    "dDurationMs": 4074,
    "segs": [ {
      "utf8": "and then place a deflated \nballoon catheter over the wire."
    } ]
  }, {
    "tStartMs": 214903,
    "dDurationMs": 3991,
    "segs": [ {
      "utf8": "When the balloon is inflated,\nit forces the passageway open again."
    } ]
  }, {
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    "dDurationMs": 2939,
    "segs": [ {
      "utf8": "Sometimes a rigid tube\ncalled a stent"
    } ]
  }, {
    "tStartMs": 221833,
    "dDurationMs": 3285,
    "segs": [ {
      "utf8": "is placed in a vessel \nto held hold it open,"
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  }, {
    "tStartMs": 225118,
    "dDurationMs": 1629,
    "segs": [ {
      "utf8": "letting the blood flow freely"
    } ]
  }, {
    "tStartMs": 226747,
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    "segs": [ {
      "utf8": "to replenish the oxygen-starved \ncells downstream."
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  }, {
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    "segs": [ {
      "utf8": "Staying flexible under pressure \nis a tough job for arteries."
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  }, {
    "tStartMs": 233911,
    "dDurationMs": 2762,
    "segs": [ {
      "utf8": "The fluid they pump \nis composed of substances"
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  }, {
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    "dDurationMs": 2605,
    "segs": [ {
      "utf8": "that can get sticky and clog them,"
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  }, {
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    "segs": [ {
      "utf8": "and your typical healthy heart\nbeats about 70 times a minute,"
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  }, {
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    "segs": [ {
      "utf8": "and at least 2.5 billion times\nduring an average lifetime."
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    "segs": [ {
      "utf8": "That may sound like an insurmountable\namount of pressure,"
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      "utf8": "but don't worry, your arteries\nare well suited for the challenge."
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