[00:06] If you lined up all 
the blood vessels in your body,
[00:11] they'd be 95,000 kilometers long
[00:15] and everyday, they carry the equivalent
of over 7,500 liters of blood,
[00:21] though that's actually the same four
or five liters recycled over and over,
[00:27] delivering oxygen, and precious nutrients
[00:30] like glucose and amino acids
to the body's tissues.
[00:34] All that blood exerts a force on
the muscular walls of the blood vessels.
[00:40] That force is called blood pressure,
[00:43] and it rises and falls
with the phases of the heartbeat.
[00:46] It's highest during systole,
[00:48] when the heart contracts to force
blood through the arteries.
[00:52] This is your systolic blood pressure.
[00:55] When the heart is at rest between beats,
[00:57] blood pressure falls to its lowest value,
the diastolic pressure.
[01:02] A typical healthy individual produces
a systolic pressure
[01:05] between 90 and 120 millimeters of mercury,
[01:10] and diastolic pressure between 60 and 80.
[01:14] Taken together, a normal reading is a bit
less than 120 over 80.
[01:19] The blood traverses 
the landscape of the body
[01:21] through the pipes 
of the circulatory system.
[01:24] In any plumbing system,
[01:26] several things can increase the force
on the walls of the pipes:
[01:29] the properties of the fluid,
[01:31] extra fluid,
[01:32] or narrower pipes.
[01:34] So if the blood thickens,
[01:36] a higher pressure is needed to push it,
so the heart will pump harder.
[01:41] A high-salt diet will lead 
to a similar result.
[01:44] The salt promotes water retention,
[01:46] and the extra fluid increases the blood
volume and blood pressure,
[01:51] and stress, 
like the fight or flight response,
[01:54] releases hormones, like epinephrine
and norepinephrine
[01:58] that constrict key vessels,
[02:00] increasing the resistance to flow
and raising the pressure upstream.
[02:05] Blood vessels can usually handle
these fluctuations easily.
[02:08] Elastic fibers embedded in their walls
make them resilient,
[02:13] but if your blood pressure 
regularly rises above about 140 over 90,
[02:18] what we call hypertension, 
and stays there,
[02:21] it can cause serious problems.
[02:23] That's because the extra strain
on the arterial wall
[02:26] can produce small tears.
[02:28] When the injured tissue swells up,
[02:30] substances that respond 
to the inflammation,
[02:33] like white blood cells,
collect around the tears.
[02:36] Fat and cholesterol floating 
in the blood latch on, too,
[02:41] eventually building up to form a plaque
[02:43] that stiffens and thickens
the inner arterial wall.
[02:47] This condition is called
atherosclerosis,
[02:50] and it can have dangerous consequences.
[02:52] If the plaque ruptures, a blood clot
forms on top of the tear,
[02:56] clogging the already narrowed pipe.
[02:59] If the clot is big enough,
[03:00] it can completely block the flow of
oxygen and nutrients to cells downstream.
[03:06] In vessels that feed the heart,
[03:08] that will cause a heart attack,
[03:10] when oxygen-deprived cardiac
muscle cells start to die.
[03:14] If the clot cuts off 
blood flow to the brain,
[03:17] it causes a stroke.
[03:20] Dangerously clogged blood vessels
can be widened
[03:22] by a procedure called
an angioplasty.
[03:25] There, doctors thread a wire
through the vessel
[03:28] to the obstructed site,
[03:30] and then place a deflated 
balloon catheter over the wire.
[03:34] When the balloon is inflated,
it forces the passageway open again.
[03:38] Sometimes a rigid tube
called a stent
[03:41] is placed in a vessel 
to held hold it open,
[03:45] letting the blood flow freely
[03:46] to replenish the oxygen-starved 
cells downstream.
[03:50] Staying flexible under pressure 
is a tough job for arteries.
[03:53] The fluid they pump 
is composed of substances
[03:56] that can get sticky and clog them,
[03:59] and your typical healthy heart
beats about 70 times a minute,
[04:02] and at least 2.5 billion times
during an average lifetime.
[04:07] That may sound like an insurmountable
amount of pressure,
[04:10] but don't worry, your arteries
are well suited for the challenge.