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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 produces
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 the 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.