WEBVTT

00:00:06.828 --> 00:00:11.388
If you lined up all 
the blood vessels in your body,

00:00:11.388 --> 00:00:15.429
they'd be 95,000 kilometers long

00:00:15.429 --> 00:00:21.984
and everyday, they carry the equivalent
of over 7,500 liters of blood,

00:00:21.984 --> 00:00:27.508
though that's actually the same four
or five liters recycled over and over,

00:00:27.508 --> 00:00:30.041
delivering oxygen, and precious nutrients

00:00:30.041 --> 00:00:34.960
like glucose and amino acids
to the body's tissues.

00:00:34.960 --> 00:00:40.240
All that blood exerts a force on
the muscular walls of the blood vessels.

00:00:40.240 --> 00:00:43.066
That force is called blood pressure,

00:00:43.066 --> 00:00:46.999
and it rises and falls
with the phases of the heartbeat.

00:00:46.999 --> 00:00:48.995
It's highest during systole,

00:00:48.995 --> 00:00:52.854
when the heart contracts to force
blood through the arteries.

00:00:52.854 --> 00:00:55.196
This is your systolic blood pressure.

00:00:55.196 --> 00:00:57.694
When the heart is at rest between beats,

00:00:57.694 --> 00:01:02.436
blood pressure falls to its lowest value,
the diastolic pressure.

00:01:02.436 --> 00:01:05.862
A typical healthy individual produces
a systolic pressure

00:01:05.862 --> 00:01:10.348
between 90 and 120 millimeters of mercury,

00:01:10.348 --> 00:01:14.110
and diastolic pressure between 60 and 80.

00:01:14.110 --> 00:01:19.572
Taken together, a normal reading is a bit
less than 120 over 80.

00:01:19.572 --> 00:01:21.891
The blood traverses 
the landscape of the body

00:01:21.891 --> 00:01:24.977
through the pipes 
of the circulatory system.

00:01:24.977 --> 00:01:26.477
In any plumbing system,

00:01:26.477 --> 00:01:29.945
several things can increase the force
on the walls of the pipes:

00:01:29.945 --> 00:01:31.556
the properties of the fluid,

00:01:31.556 --> 00:01:32.708
extra fluid,

00:01:32.708 --> 00:01:34.760
or narrower pipes.

00:01:34.760 --> 00:01:36.190
So if the blood thickens,

00:01:36.190 --> 00:01:41.437
a higher pressure is needed to push it,
so the heart will pump harder.

00:01:41.437 --> 00:01:44.782
A high-salt diet will lead 
to a similar result.

00:01:44.782 --> 00:01:46.933
The salt promotes water retention,

00:01:46.933 --> 00:01:51.397
and the extra fluid increases the blood
volume and blood pressure,

00:01:51.397 --> 00:01:54.456
and stress, 
like the fight or flight response,

00:01:54.456 --> 00:01:58.365
releases hormones, like epinephrine
and norepinephrine

00:01:58.365 --> 00:02:00.550
that constrict key vessels,

00:02:00.550 --> 00:02:05.211
increasing the resistance to flow
and raising the pressure upstream.

00:02:05.211 --> 00:02:08.772
Blood vessels can usually handle
these fluctuations easily.

00:02:08.772 --> 00:02:13.521
Elastic fibers embedded in their walls
make them resilient,

00:02:13.521 --> 00:02:18.606
but if your blood pressure 
regularly rises above about 140 over 90,

00:02:18.606 --> 00:02:21.846
what we call hypertension, 
and stays there,

00:02:21.846 --> 00:02:23.970
it can cause serious problems.

00:02:23.970 --> 00:02:26.694
That's because the extra strain
on the arterial wall

00:02:26.694 --> 00:02:28.456
can produce small tears.

00:02:28.456 --> 00:02:30.619
When the injured tissue swells up,

00:02:30.619 --> 00:02:33.214
substances that respond 
to the inflammation,

00:02:33.214 --> 00:02:36.583
like white blood cells,
collect around the tears.

00:02:36.583 --> 00:02:41.014
Fat and cholesterol floating 
in the blood latch on, too,

00:02:41.014 --> 00:02:43.586
eventually building up to form a plaque

00:02:43.586 --> 00:02:47.694
that stiffens and thickens
the inner arterial wall.

00:02:47.694 --> 00:02:50.498
This condition is called
atherosclerosis,

00:02:50.498 --> 00:02:52.646
and it can have dangerous consequences.

00:02:52.646 --> 00:02:56.692
If the plaque ruptures, a blood clot
forms on top of the tear,

00:02:56.692 --> 00:02:59.605
clogging the already narrowed pipe.

00:02:59.605 --> 00:03:00.922
If the clot is big enough,

00:03:00.922 --> 00:03:06.745
it can completely block the flow of
oxygen and nutrients to cells downstream.

00:03:06.745 --> 00:03:08.374
In vessels that feed the heart,

00:03:08.374 --> 00:03:10.401
that will cause a heart attack,

00:03:10.401 --> 00:03:14.892
when oxygen-deprived cardiac
muscle cells start to die.

00:03:14.892 --> 00:03:17.576
If the clot cuts off 
blood flow to the brain,

00:03:17.576 --> 00:03:20.266
it causes a stroke.

00:03:20.266 --> 00:03:22.991
Dangerously clogged blood vessels
can be widened

00:03:22.991 --> 00:03:25.797
by a procedure called
an angioplasty.

00:03:25.797 --> 00:03:28.870
There, doctors thread a wire
through the vessel

00:03:28.870 --> 00:03:30.829
to the obstructed site,

00:03:30.829 --> 00:03:34.903
and then place a deflated 
balloon catheter over the wire.

00:03:34.903 --> 00:03:38.894
When the balloon is inflated,
it forces the passageway open again.

00:03:38.894 --> 00:03:41.833
Sometimes a rigid tube
called a stent

00:03:41.833 --> 00:03:45.118
is placed in a vessel 
to held hold it open,

00:03:45.118 --> 00:03:46.747
letting the blood flow freely

00:03:46.747 --> 00:03:50.280
to replenish the oxygen-starved 
cells downstream.

00:03:50.280 --> 00:03:53.911
Staying flexible under pressure 
is a tough job for arteries.

00:03:53.911 --> 00:03:56.673
The fluid they pump 
is composed of substances

00:03:56.673 --> 00:03:59.278
that can get sticky and clog them,

00:03:59.278 --> 00:04:02.930
and your typical healthy heart
beats about 70 times a minute,

00:04:02.930 --> 00:04:07.723
and at least 2.5 billion times
during an average lifetime.

00:04:07.723 --> 00:04:10.899
That may sound like an insurmountable
amount of pressure,

00:04:10.899 --> 00:04:15.108
but don't worry, your arteries
are well suited for the challenge.