they'd be 95,000 kilometers long

and everyday, they carry the equivalent of over 7,500 liters of blood,

though that's actually the same four or five liters recycled over and over,

delivering oxygen, and precious nutrients

like glucose and amino acids to the body's tissues.

All that blood exerts a force on the muscular walls of the blood vessels.

That force is called blood pressure,

and it rises and falls with the phases of the heartbeat.

It's highest during systole,

when the heart contracts to force blood through the arteries.

This is your systolic blood pressure.

When the heart is at rest between beats,

blood pressure falls to its lowest value, the diastolic pressure.

A typical healthy individual produces a systolic pressure

between 90 and 120 millimeters of mercury,

and diastolic pressure between 60 and 80.

Taken together, a normal reading is a bit less than 120 over 80.

The blood traverses the landscape of the body

through the pipes of the circulatory system.

In any plumbing system,

several things can increase the force on the walls of the pipes:

the properties of the fluid,

extra fluid,

or narrower pipes.

So if the blood thickens,

a higher pressure is needed to push it, so the heart will pump harder.

A high-salt diet will lead to a similar result.

The salt promotes water retention,

and the extra fluid increases the blood volume and blood pressure,

and stress, like the fight or flight response,

releases hormones, like epinephrine and norepinephrine

that constrict key vessels,

increasing the resistance to flow and raising the pressure upstream.

Blood vessels can usually handle these fluctuations easily.

Elastic fibers embedded in their walls make them resilient,

but if your blood pressure regularly rises above about 140 over 90,

what we call hypertension, and stays there,

it can cause serious problems.

That's because the extra strain on the arterial wall

can produce small tears.

When the injured tissue swells up,

substances that respond to the inflammation,

like white blood cells, collect around the tears.

Fat and cholesterol floating in the blood latch on, too,

eventually building up to form a plaque

that stiffens and thickens the inner arterial wall.

This condition is called atherosclerosis,

and it can have dangerous consequences.

If the plaque ruptures, a blood clot forms on top of the tear,

clogging the already narrowed pipe.

If the clot is big enough,

it can completely block the flow of oxygen and nutrients to cells downstream.

In vessels that feed the heart,

that will cause a heart attack,

when oxygen-deprived cardiac muscle cells start to die.

If the clot cuts off blood flow to the brain,

it causes a stroke.

Dangerously clogged blood vessels can be widened

by a procedure called an angioplasty.

There, doctors thread a wire through the vessel

to the obstructed site,

and then place a deflated balloon catheter over the wire.

When the balloon is inflated, it forces the passageway open again.

Sometimes a rigid tube called a stent

is placed in a vessel to held hold it open,

letting the blood flow freely

to replenish the oxygen-starved cells downstream.

Staying flexible under pressure is a tough job for arteries.

The fluid they pump is composed of substances

that can get sticky and clog them,

and your typical healthy heart beats about 70 times a minute,

and at least 2.5 billion times during an average lifetime.

That may sound like an insurmountable amount of pressure,

but don't worry, your arteries are well suited for the challenge.