WEBVTT

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You breathe in about 17,000 times per day.

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It's a process you rarely think about,

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but behind the scenes, a huge coordinated
effort is playing out.

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Your vital organs,

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the gut,

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brain,

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bones,

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lungs,

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blood,

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and heart

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work together to sustain your life

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by delivering oxygen
to tissues throughout your body.

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Most of our cells need oxygen

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because it's one of the key ingredients
of aerobic respiration.

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That's the process that produces
a molecule called ATP,

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which our cells use to power their
many incredible functions.

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But getting oxygen throughout our
bodies is a surprisingly difficult task.

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Gas enters cells by diffusing in
from their surroundings.

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And that only happens efficiently
over tiny distances.

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So for oxygen to reach the cells
within our bodies,

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it needs a transportation network.

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This is where our 20 trillion 
red blood cells come in.

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Each one contains about 270 million
oxygen-binding molecules of hemoglobin,

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which is what gives blood its scarlet hue.

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To make these cells, the body uses
raw materials

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that become available from
the food we eat.

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So in some ways, you could say
that oxygen's journey through the body

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really begins in the gut.

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Here, in an amazing display of mechanical
and chemical digestion,

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food gets broken down into
its smallest elements,

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like iron, the building block
of hemoglobin.

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Iron is carried through 
the cardiovascular system

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to the body's hematopoietic tissue.

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This tissue is the birthplace 
of red blood cells,

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and it can be found enclosed within
our bone marrow cavities.

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The kidneys regulate 
our levels of red blood cells

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through the release of erythropoietin,

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a hormone which causes marrow
to increase production.

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Our bodies churn out roughly 2.5 million
red blood cells per second,

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a number equivalent to the entire
population of Paris,

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so that oxygen that makes it to the lungs
will have ample transportation.

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But before oxygen 
can even reach the lungs,

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the brain needs to get involved.

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The brainstem initiates breathing

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by sending a message
through your nervous system,

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all the way to muscles 
of the diaphragm and ribs.

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This causes them to contract,

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thus increasing the space
inside the rib cage,

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which allows the lungs to expand.

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That expansion drops your lungs
internal air pressure,

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making air rush in.

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It's tempting to think of our lungs
as two big balloons,

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but they're actually a lot more 
complicated than that.

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Here's why.

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The red blood cells in the vessels
within your lungs

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can only pick up oxygen molecules
that are very close to them.

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If our lungs were shaped like balloons,

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air that was not in direct contact
with the balloon's inner surface

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couldn't diffuse through.

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Luckily, our lungs' architecture ensures
that very little oxygen is wasted.

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Their interior is divided into
hundreds of millions

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of miniature balloon-like projections
called alveoli

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that dramatically increase 
the contact area

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to somewhere around 100 square meters.

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The alveolar walls are made of
extremely thin flat cells

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that are surrounded by capillaries.

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Together, the alveolar wall and
capillaries make a two-cell thick membrane

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that brings blood and oxygen close enough
for diffusion.

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These oxygen-enriched cells are then
carried from the lungs

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through the cardiovascular network,

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a massive collection of blood vessels
that reaches every cell in the body.

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If we laid this system out
end to end in a straight line,

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the vessels would wrap around the Earth
several times.

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Propelling red blood cells
through this extensive network

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requires a pretty powerful pump,

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and that's where your heart comes in.

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The human heart pumps an average
of about 100,000 times per day,

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and it's the powerhouse that ultimately
gets oxygen where it needs to go,

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completing the body's team effort.

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Just think - this entire complex system
is built around the delivery

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of tiny molecules of oxygen.

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If just one part malfunctioned,
so would we.

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Breathe in.

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Your gut, brain, bones, 
lungs, blood, and heart

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are continuing their incredible act
of coordination that keeps you alive.

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Breathe out.