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If you lined up all
the blood vessels in your body,
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they'd be 95,000 kilometers long
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and everyday, they carry the equivalent
of over 7,500 liters of blood,
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though that's actually the same four
or five liters recycled over and over,
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delivering oxygen, and precious nutrients
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like glucose and amino acids
to the body's tissues.
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All that blood exerts a force on
the muscular walls of the blood vessels.
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That force is called blood pressure,
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and it rises and falls
with the phases of the heartbeat.
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It's highest during systole,
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when the heart contracts to force
blood through the arteries.
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This is your systolic blood pressure.
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When the heart is at rest between beats,
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blood pressure falls to its lowest value,
the diastolic pressure.
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A typical healthy individual produes
a systolic pressure
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between 90 and 120 millimeters of mercury,
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and diastolic pressure between 60 and 80.
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Taken together, a normal reading is a bit
less than 120 over 80.
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The blood traverses
the landscape of the body
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through the pipes
of the circulatory system.
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In any plumbing system,
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several things can increase the force
on the walls of the pipes:
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the properties of the fluid,
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extra fluid,
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or narrower pipes.
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So if the blood thickens,
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a higher pressure is needed to push it,
so the heart will pump harder.
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A high-salt diet will lead
to a similar result.
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The salt promotes water retention,
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and the extra fluid increases the blood
volume and blood pressure,
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and stress,
like the fight or flight response,
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releases hormones, like epinephrine
and norepinephrine
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that constrict key vessels,
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increasing the resistance to flow
and raising the pressure upstream.
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Blood vessels can usually handle
these fluctuations easily.
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Elastic fibers embedded in their walls
make them resilient,
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but if your blood pressure
regularly rises above about 140 over 90,
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what we call hypertension,
and stays there,
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it can cause serious problems.
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That's because the extra strain
on the arterial wall
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can produce small tears.
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When the injured tissue swells up,
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substances that respond
to the inflammation,
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like white blood cells,
collect around the tears.
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Fat and cholesterol floating
in the blood latch on, too,
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eventually building up to form a plaque
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that stiffens and thickens
the inner arterial wall.
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This condition is called
atherosclerosis,
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and it can have dangerous consequences.
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If the plaque ruptures, a blood clot
forms on top of the tear,
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clogging the already narrowed pipe.
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If the clot is big enough,
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it can completely block the flow of
oxygen and nutrients to cells downstream.
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In vessels that feed the heart,
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that will cause a heart attack,
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when oxygen-deprived cardiac
muscle cells start to die.
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If the clot cuts off
blood flow to the brain,
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it causes a stroke.
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Dangerously clogged blood vessels
can be widened
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by a procedure called
an angioplasty.
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There, doctors thread a wire
through the vessel
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to the obstructed site,
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and then place a deflated
balloon catheter over the wire.
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When the balloon is inflated,
it forces the passageway open again.
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Sometimes a rigid tube
called a stent
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is placed in a vessel
to held hold it open,
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letting the blood flow freely
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to replenish oxygen-starved
cells downstream.
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Staying flexible under pressure
is a tough job for arteries.
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The fluid they pump
is composed of substances
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that can get sticky and clog them,
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and your typical healthy heart
beats about 70 times a minute,
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and at least 2.5 billion times
during an average lifetime.
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That may sound like an insurmountable
amount of pressure,
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but don't worry, your arteries
are well suited for the challenge.