♪♪ ♪♪ -Think about it... when you're sitting still and not breathing... ...what is the only thing in your body that's palpably moving?

-Every second, your body gives you a little sign to prove that it's still there.

♪♪ A pulse.

-It is billions of cells beating for billions of times in a lifetime, never pausing to rest.

-Behind that beat is an unbelievably vast system whose one and only job is to make sure every cell in your body stays alive.

♪♪ -The speed, the strength, the frequency, the rhythm -- it tells us so much.

-A heart that thumps... blood that flows through thousands of miles of arteries and veins... All so we can attempt the crazy feats that make humans... unique.

♪♪ ♪♪ ♪♪ Human life depends on one substance above all others -- oxygen.

We can go about three weeks without food, three days without water.

But without oxygen, we wouldn't last three minutes.

But why?

Well, if you think about it, each of us is just a huge pile of cells, somewhere around 37 trillion.

Blood cells, skin cells, brain cells.

And every single one of those cells contains an engine that runs on oxygen.

Think of it like a fire that's always burning.

Oxygen is the wood or coal.

And to keep the fire stoked, all we have to do is breathe in.

But getting a supply of oxygen is the easy part.

Once it's inside the body, all that fuel has to somehow find its way into those 37 trillion engines.

And that requires a system that never rests.

-The cardiovascular and circulatory systems represent our major engine in our bodies.

Now, this is to make sure that all of our cells have oxygen going in, carbon dioxide leaving, plenty of nutrients, and essentially keeps us going.

-The body itself is pretty small, but the network of vessels is enormous.

The vessels themselves laid end-to-end would be about 100,000 miles long, which would encircle the globe probably four times.

-This vast network of pumps, valves, and tunnels delivers oxygen to cells in every corner of your body, without you ever having to think about it.

And what's more insane than that is we can train this system to work even more efficiently.

Especially when oxygen is harder to come by.

♪♪ -It would be difficult to sum me up in one word because I'm a bit of a contradiction.

On one hand, I'm a Southern Belle, a debutante, and afraid of heights.

But I'm also an extreme alpinist, an ice climber, and a rock climber.

♪♪ My name is Kitty Calhoun.

Ice climbing is who I am.

-High-altitude ice climbing is the ultimate test for the cardiovascular system.

Imagine you're scaling a vertical slab of ice.

Your body is screaming for oxygen to feed your muscles to keep moving upwards.

It's true that any elite athlete has to have an extremely efficient cardiovascular system.

But climbing is different.

The air at high altitudes is depleted of oxygen and freezing temperatures tax a system that's already pushed to the brink.

♪♪ Then add in one last factor -- the heart-pounding knowledge that at any moment, you could slip.

Training to climb means teaching the body to adapt to an atmosphere that can barely sustain human life.

And that training begins in one place -- the heart.

-The heart is the first organ to start functioning and is typically the last one to stop functioning in the human lifetime.

-It's only the size of a fist, but it beats more than 100,000 times a day.

The heart itself is just two pumps side-by-side.

On the left, the tricuspid valve collects blood from all over the body.

On the right, the pulmonary valve takes that blood and shoots it into the organ that surrounds the heart.

The lungs.

There, the blood picks up oxygen from tiny air-filled sacs called alveoli that expand like balloons every time we inhale.

Then it goes back to the heart so that all that oxygen-rich blood can get pumped to the rest of the body.

After a minute, it returns, ready for another trip.

That cycle happens every second of every day.

But throw a mountain into the mix, and things get interesting.

♪♪ -Ice climbing is an activity where you have to be in good cardio shape.

When I'm not ice climbing, I'm getting a good cardio work in a short amount of time.

-In order for Kitty's heart to perform well on the ice, she needs to train it to pump blood as dynamically as possible.

-During exercise, there are a lot of changes that occur in our cardiovascular system.

The heart obviously speeds up.

The more beats per minute, the more blood flow out of the heart per minute, therefore the more oxygen delivery to the tissues.

The efficiency of oxygen transfer also increases so that muscles can use the oxygen more efficiently.

-Consistently exercising the heart muscle trains it to push more blood out with each beat, which means it doesn't need to beat as often.

-The human heart is like any other part of our biology -- highly adaptive.

The more demands that are placed on our heart, the more the heart responds in terms of its capacity.

Over time, you'll find the resting heart rate of a marathon runner would be much lower than that of a person who is much more sedentary.

-Kitty has trained her heart to withstand the physical stresses of a climb up a solid sheet of ice.

But in this sport, there's more than just exhaustion at stake.

If you're hanging by an axe from a cliff, your heart is also being challenged by something else -- fear.

-You know, there's risk in life, right?

If you don't step out the door, then you won't get hit by a car.

If you don't go ice climbing, then you won't have an accident ice climbing.

Risk is something that people try to shut out in their lives, and I think to live life fully, to experience all that life has to give, that what you have to do is minimize the risk by getting experience and making good decisions.

♪♪ ♪♪ I do feel fear, standing down at the bottom and looking up at the climb.

I still have a healthy fear of falling.

So one of the biggest tricks that I use to keep it together mentally is breathing.

♪♪ -When the system runs amuck like in a panic attack, the heart speeds up, the heart pumps less efficiently, and is less able to meet the metabolic demands of the body.

So an ice climber, for example, would have to get control of that response, to slow down their heartbeat and enable the heart to fill properly and pump blood more efficiently.

-Climbing is as much mental as it is physical.

As Kitty's heart has transformed over time to be able to handle the extreme conditions, so has her brain.

-I start climbing, and I'm focused on breathing and making sure that I work my feet up high and then push up off my legs like you're doing a squat.

And when you do pull with your arms, you really try to use your back as much as you can.

Everything is a balance.

-It's all a highly coordinated dance.

The mind keeps everything in check while muscles all over the body reach their limit, in constant need of more oxygen.

Kitty's well-trained heart handles all these tasks easily.

The hard part is getting the supply of oxygen in the first place.

At high altitudes, the atmosphere contains a lot less oxygen.

That means there's less fuel for red blood cells to extract from the air entering the lungs.

If you've ever been up that high, you know how heavy your breathing gets.

To try and compensate, the heart goes into overdrive to pump the blood to starving muscles.

The low levels of oxygen in the blood can cause headaches and fatigue as the body tries to function with less fuel.

It can also affect the mind.

-Every organ depends on the heart for its function.

The brain cannot function without a moving heart.

-Without oxygen, the brain gets foggy.

That's another reason spending too much time up in the clouds is dangerous.

Your judgment goes right out the window.

♪♪ But once again, Kitty's body is different.

Her red blood cells are better at adjusting to the changing atmosphere, which gives her the edge she needs to get to the top.

♪♪ ♪♪ -Yay!

Oh, man.

It feels good.

When I'm at the top, I feel a sense of relief for sure, a sense of accomplishment.

Climbing is a form of therapy for me because I can just escape, and I enjoy breathing hard, I enjoy the strength aspect of it, I enjoy just moving in a different way.

♪♪ ♪♪ -This frozen ice face melts when the seasons change... ...leading streams into rivers... that snake through the landscape and bring life.

Humans use these waterways for transport and trade.

We construct cities around them.

They hold spiritual meaning for us.

The constant flow of water through rivers has supported life for millennia, much like the labyrinth intertwined right underneath our skin.

♪♪ -Because our blood has to be conveyed to every cell in the body, the enormity of the blood vessels in our body is really mind-boggling.

-A lot of simpler life forms don't need a circulatory system because they're just one-cell beings and they can rely on the diffusion of molecules within their one cell to carry everything around that they need to.

But we as humans are made up of many cells, and we needed to evolve a system that would allow us to very efficiently move oxygen throughout our body.

-Over the course of a day, blood will travel about 12,000 miles through arteries and veins.

It's a system defined by constant motion, something that's also reflected in our evolution as a species.

♪♪ -Humans are absolutely built to move.

I mean, if you look at our ancestry and the way we used to move around, our musculoskeletal system is designed for us to be agile, to go long periods of time by moving, and our hearts can perfectly accommodate this by making sure we get the right amount of blood when we need it.

-But here's the tricky thing -- not only does our circulatory system allow us to move, it only works properly if we are moving.

For most of us, modern life doesn't involve hunting or crossing continents on foot.

And what we're finding out is that if we stay still for too long, the whole system breaks down.

♪♪ ♪♪ ♪♪ -Driving a bus, you're just sitting there driving.

Day in, day out, eight-hour shift.

Sometimes if you're really busy, you may spend your entire shift in the seat because you're running behind schedule or something.

♪♪ You're sitting in traffic, sometimes you might sit in the seat for three hours in traffic.

Again, you're not moving.

The advice I give people is just you gotta be moving.

If you're going to sit down, you're going to die.

I'm a prime example.

I'm Steve Dean, and I drive a city bus.

♪♪ -I think one of the life lessons that the cells I've studied has taught me is this concept of if you don't use it, you lose it.

Cells know what they're doing, but they need to be challenged.

-One of the biggest obstacles our circulation is up against is something that we have absolutely no control over -- gravity.

Humans are an upright species.

We walk on two legs.

So once our blood has traveled down the arteries and unloaded oxygen to all the cells below the heart, that same blood then has to move straight upwards again through our veins to get back to where it started, working against Earth's gravitational pull the entire time.

In most cases, arteries rely on the heart to pump blood, but veins have to get their power from another source -- muscles.

When you tense your leg muscles they propel blood upwards.

But this ingenious adaptation only works if the muscles are active.

Otherwise, the pumps have no energy to work with.

Think about sitting for long periods of time.

Your leg muscles aren't doing anything, so they can't pump that blood back to the heart with any real force.

And that can lead to major problems.

-A bus driver's heart, cruising through traffic with no issue, is beating at a resting pace.

There is no sense of urgency to increase a heart rate and throw more blood to the body.

-You're very sedentary when you're driving the coach.

You sit there a lot.

And that's kind of one of the downfalls of driving the coach.

Back in 2010, when I was working a lot, just sitting there driving, I was up to about 250, 260 pounds.

I felt sluggish.

-Our jobs have never been more stationary.

They've moved from farms to cities, and now onto the Internet.

And we're constantly told to exercise, because the damage can creep up on you, silently.

-On general inspection, the heart of a sedentary person might look normal.

However, if you probe more deeply and look, for example, at the arteries that supply blood to the heart muscle, it's more likely that those arteries would have more signs of fatty deposits, in part because of the sedentary lifestyle.

-Not moving actually makes the body worse at processing fat.

And probably everyone on earth will tell you that fat is the enemy.

And here's why -- it's really just a matter of space.

When we exercise, the arteries surrounding the heart expand.

When we don't, they get thinner.

And blood needs room to move.

-A river can get blocked by rocks, for example.

Similarly blood vessels can develop blockages, called atherosclerosis, which are essentially fatty deposits that start at the wall and start to protrude inward and eventually impede blood flow.

♪♪ -These blockages that build over time can get us into real trouble.

-I felt this burning in my chest.

I drove myself to St. Peter's emergency room.

They did an EKG.

The doctor, she said, "You've had a heart attack."

♪♪ -The heart not only pumps blood to the entire body, but it also pumps blood to itself.

It's self-sustaining.

No other organ does that.

The heart is so hard-working that it needs a constant supply of oxygen and nutrients to remain functional.

When those arteries get clogged up with fatty deposits, that can result in stoppage of blood flow to the heart muscle.

And when that heart muscle dies, that's called a heart attack.

-And unlike other muscles, heart cells barely regenerate.

When they die, scar tissue grows in their place, and scar tissue can't pump.

-It was the very tip of an artery that was blocked.

They said, "If you'd have went to sleep, you might not have woke up."

You know, it's an eye-opening experience.

I just realized my time on this earth now has a limit and made me realize that I need to do things in my life different.

Gosh, we're just about here.

Well, I drive Dial-A-Lift now.

Good morning.

-Good morning.

-How are you today?

Dial-A-Lift is a door-to-door service for passengers with disabilities or elderly, something that prevents them from riding the normal bus.

-Marianne is my first rider today.

I take her to the Sacred Heart Church.

-Is this an express?

-Yup, you're going straight there today.

-Oh, good.

That works for me.

-I probably added another 3,000 to 4,000 steps a day by driving Dial-A-Lift.

Good morning.

It's Dial-A-Lift.

You're out of the seat, and then you escort them out to the van.

After you.

I'll have you go on board there.

You get them on the lift on the van.

Okay, up we go.

And this step's a little higher.

Sometimes you have your arm out for them to hold on so they're steady.

It's things like that that keep you healthy.

-Well, here we are.

-Yeah, get to go work out.

-Yay!

Hey, that's good for you.

Well, you have a wonderful day today.

-Do you feel like a celebrity?

[ Laughs ] -I feel like Steve, as always.

-If you don't need a strong heart, your body won't give you one.

If you don't need strong muscles, why would your body make one?

So it's in a way like if you don't use it, you lose it.

But if you use it, you get it.

[ Laughs ] It's been really exciting for me to see this in practice, you know, with people who are experiencing the increase in strength that can come about so quickly just by exercising and giving ourselves a little bit of a challenge.

To me, it's just very life affirming to think that at any age, we always have the opportunity to adapt and to do better.

♪♪ [ Heart beating ] -The heart and vessels are a mind-bendingly vast system.

But to operate, they need blood.

-I mean it's not for nothing that we refer to the metaphorical life blood because the blood is literally the life giving fluid.

What is blood?

Blood is essentially composed of two substances.

One is the plasma, which is basically salts, water, and proteins.

And the other part is the cells.

-Red blood cells account for more than 80% of all of our cells, and the other 20% rely on them to survive.

1 million times every minute, red blood cells absorb the oxygen we need to live.

-It's an extraordinary piece of our physiology.

Our heart pumps out five to six liters of blood a minute, and that blood is constantly being circulated around the body with new oxygen that's breathed in every few seconds with waste products that are being cleaned out on that same time scale.

It powers our every moment of our day-to-day lives.

-Their never-ending job has given red blood cells a unique structure.

Unlike other cells, they don't have a nucleus.

Instead, they're filled with massive quantities of a protein called hemoglobin.

It's made of iron, which is why blood is red.

Hemoglobin grabs on to the oxygen we breathe in, then, at the right moment, it lets go, and oxygen molecules pass into the surrounding cells.

What's crazy is that hemoglobin is toxic to the human body.

If it escaped from even a tiny fraction of our red blood cells, it could kill us, but as long as it stays within the cells walls, it can't harm us.

Red blood cells have a margin error of exactly zero and they do their job perfectly.

That is, as long as there's enough blood in the body to go around.

[ Siren wailing ] Because when we lose blood, everything is then in jeopardy, and just a few minutes can mean the difference between life and death.

♪♪ ♪♪ [ Horns honking ] The city of Accra is home to 2 million people.

It's Ghana's capital, its beating heart.

♪♪ 150 miles away over rugged terrain is the tiny town of Asissinwa.

A place so remote that the word "emergency" takes on a whole new meaning.

[ Man speaking native language ] -One plus two is three.

Hello!

-The bond between mother and her child is profound, and it starts in the womb.

The first thing they share is blood.

-The fetus doesn't breathe.

It's floating in amniotic fluid, and it only receives oxygen through the mother's blood supply through the umbilical cord.

The mother's blood supply also ferries waste out of the fetus, which then gets cleared by the mother's kidneys.

-It's an amazing exchange, but it also means that when something goes wrong with one, both are in danger.

[ Woman speaking native language ] -[ Speaking native language ] It will not be safe.

-I'm Dr. Phillip Agyeman-Prempeh.

I'm an obstetrician/gynecologist at Saint Michael Hospital.

We make over 2,000 deliveries every year in this facility.

-Akua Nyarko came into our maternity emergency.

She was at 36 weeks plus 4 weeks in her pregnancy.

The abdomen felt very tense, and we could not hear the fetal heart rate, both with the Doppler and ultrasound.

-During the birthing process, the mom and the baby are going through huge amounts of changes that affect the physical aspects of both infant and mother.

Immediately after the baby is born, the uterus will contract to allow all of those blood vessels that have been opened during the pregnancy so that the woman actually does not hemorrhage, meaning she does not continue to bleed too heavily.

-For Akua, something had gone very wrong.

-The uterus was not contracting.

She was bleeding profusely, and we realized that her blood was not clotting.

-We lose blood all the time -- in small doses, and our bodies can handle it.

But if there's a major bleed that isn't stopping, the body can't recover.

The response has to be lightning fast, and that means a transfusion.

-We needed blood to stabilize her and save her life.

But we realized we didn't have enough blood.

-And getting blood into a remote area can take hours.

-In cases like this, we would ask for an ambulance to bring us the products, but that takes time.

It was a dilemma and a tough one.

So we called our biomedical scientist, and he made an order for us to get blood.

-Meanwhile the body starts its own rescue effort fast by sending messages.

-We have little brains all over our body, and the heart has a pretty big one.

It's been recently discovered that there are about 40,000 neurons on the heart.

Nerves that go to the brain as well as nerves that come from the brain.

It's very responsive.

It has the capacity to beat faster or slower to be able to best serve the needs of the body at any given time.

-This group of neurons reacts immediately when blood leaves the body.

Pressure sensors on the heart perceive the loss, and they tell the nervous system.

It sends out signals that start a chain reaction to keep cells alive.

It's a highly synchronized effort that depends on instant communication.

Much like the technology that Akua's doctors used to contact the only people who could send them blood in time to save her life.

♪♪ -Emergency.

Okay.

We'll get it to you right away.

♪♪ [ Bell dings ] ♪♪ ♪♪ ♪♪ -We had to perform an emergency cesarean section to stabilize her, and then to prevent her from losing more blood.

This is a patient who was very pale.

If we didn't get the blood, it was highly likely that she would bleed to death.

-Very little in the body can happen without blood.

If you empty the heart of blood and stopped it, then the brain and the kidneys and the vital organs wouldn't get oxygen and you'd develop massive organ damage within a period of three minutes.

The blood is the conduit for all these other organs to function.

-If cells go without oxygen for too long, the body goes into irreversible shock.

But the heart-brain knows when it's in trouble, and its response is smart.

The heart starts to beat faster.

Breathing becomes shallow and quick.

There's also a chemical reaction.

Hormones flood blood vessels in the skin causing them to constrict.

Blood moves inward to keep vital organs alive.

That's why people turn pale when they lose blood.

Skin is not so important, but the brain is vital.

It's the body's clever way of redistributing resources.

If the blood loss is really extreme, it might be impossible to reverse the damage.

Tragically for Akua's unborn child, it was already too late.

But Akua was still holding on.

♪♪ -Within 20 minutes, Zipline brought us the blood.

We started transfusing her.

And then she was able to pull through.

-With some support, the body begins to stabilize.

-Our bodies evolved the cardiovascular system to ramp up very, very quickly and ramp down because we can't maintain a degree of stress for long periods of time without damaging the system.

-The connection between the brain and the heart is complex and not one-sided.

The structures in the brain regulate the heart through nerve impulses and hormones, and it's in the brain that emotions and thoughts are processed, even though sometimes it doesn't seem that way.

-Anyone who's ever experienced a sense of panic knows that we feel it immediately in our heart.

So why would it be a surprise that the heart would be really smart at being connected to our emotional state when that is our internalized experience of the outside world.

-So if we feel things in our heart, does that mean it has a mind of its own?

-The idea that the heart might be telling the brain how to think or how to feel is an open question for me personally and one that I think is very hotly debated among scientists.

-If there are neurons on the heart, then maybe its power over our emotions isn't just poetic, but also biological.

♪♪ ♪♪ ♪♪ ♪♪ -My name is Bertha Martinez.

I am a volunteer at the Lincoln Heights Senior Center.

After we retired, my husband and I started helping at the center.

We love to dance, and we have been attending there for the past 18 years.

-Hombres!

[ Cheering ] -We always say that it's our second family.

-Buenos dias.

[ Speaking Spanish ] -Humans are social animals.

-[ Speaking Spanish ] -That's something that we share with primates in general.

We live in groups, and a lot of the way that we navigate the world around us, has to do with social relationships.

This is something that is reflected in our biology.

-The brain and the heart are connected by the most powerful neural network in the body, the vagus nerve.

It passes millions of messages back and forth every day.

The vast majority come not from the brain but from the heart.

And this network is responsible for communicating emotions Positive social interactions light up the vagus nerve and the heart.

In other words, our relationships are constantly shaping our internal worlds.

-This is my husband Ricardo.

This picture was taken on our 25th wedding anniversary.

I lost my husband last year.

This chair, my husband used it all the time to watch TV.

Someone told me why don't I take everything down so that way I cannot feel so bad?

But to me, it's just the opposite.

-We know that people who experience intense emotional disturbance like grief after the death of a loved one or the breakup of a romantic relationship can develop severe effects on their hearts.

The heart in these people tends to acutely weaken and change shape into the shape of a takotsubo, which is a Japanese pot with a narrow neck and a wide base.

It's been variably called the broken heart syndrome or takotsubo cardiomyopathy, and we know that emotional stress can accelerate the development of heart disease and increase the risk of cardiac mortality.

-The exact cause is unknown.

Our best guess is that a super dose of adrenaline floods the heart, shocking cardiac cells and weakening them.

Hormones like adrenaline are one of the main ways the brain communicates with the heart, and the risk of developing takotsubo might have something to do with the strength of neural connections and how they respond to stress.

The same regions in the brain that regulate your heartbeat have been linked to emotional conditions like depression.

But unlike a heart attack, the cardiac cells are just shocked, not lost.

-Fortunately for us, a lot of the injury to the heart is potentially reversible.

The heart does tend to self-correct, so when our emotional states return to normal, the heart itself can also return to normal.

-Okay.

-I think it's very important to come out, even when you're sad.

-There is a kind of takotsubo cardiomyopathy, or broken heart syndrome, that occurs after happy events, too, but the heart shape changes differently.

-In the same way that adrenaline can damage cardiac cells, there are other hormones that can have the opposite effect.

And there's one molecule that's most associated with the complex social behavior of humans -- oxytocin, often called the love hormone.

Think about the feeling you get when you hug a friend or dance with a flirty stranger.

That rush comes as oxytocin spikes and makes a beeline for the heart.

Just like fear can make your heart pound, the oxytocin released by touch can put your heart at rest.

Receptors on cardiac cells detect its presence and your heart rate slows.

It also helps reduce inflammation in blood vessels.

So it can actually have a therapeutic effect on stressed arteries and veins.

Even short interactions can stimulate a rush.

So for the heart, love looks like thousands of brief encounters over a lifetime.

Which is really well-suited to a function like a dance.

♪♪ -I've been working here 12 years, and there hasn't been one Thursday that that place hasn't filled up with at least 150 people.

Latins like to dance.

-Dance is a good exercise.

If you move, it helps a lot, especially at my age.

-I think we know that a lack of love can be very damaging.

The corollary to that is can the presence of love be beneficial for the heart?

We know that people in longstanding relationships tend to live longer.

-♪ Every little movement ♪ ♪ Every little thing you do ♪ -The brain-heart connection is definitely present and is something that we as a field need to investigate more deeply to really understand its roots.

-♪ Said a black magic spell you put me under ♪ -After my husband passed away, it was hard, but my second family from the senior center, it gave me so much strength.

It been easier as time passes by.

-♪ Is beyond improvement ♪ -Now I'm doing what I love, and I'll just keep doing it, whenever I can.

-♪ Every little movement ♪ [ Cheers and applause ] [ Man speaking Spanish ] -How does the body understand the passage of time?

We as humans have a rhythm and a cadence to our life that I believe is in many ways, tied to the heartbeat.

-The same pulse inside every one of us, setting the pace for our lives.

♪♪ With a tiny thump that we often forget about that's, in a way, much bigger than we are.

All life has a rhythm.

You just have to pause and take a step back to feel it.

-The physical world is made up of pulses of energy.

So it's fun to play with that shift of perspective, to think about our bodies, more like an ecosystem composed of trillions of cells conspiring every moment to create our thoughts and keep our heart beating and keep our breathing going.

♪♪ ♪♪ ♪♪ -To order "Human: The World Within" on DVD, visit shopPBS.org or call 1-800-PLAY-PBS.

♪♪