NARRATOR: The human body
is a complex network

of cells, tissues,
and organs that

together make life possible.

10 major systems are responsible
for the body's functions--

skeletal, muscular,
cardiovascular, nervous,

endocrine, lymphatic,
respiratory, digestive,

urinary, reproductive.

The skeletal, muscular,
cardiovascular,

and nervous systems
in particular

create an infrastructure that
facilitates the other systems.

The adult skeletal system is
a framework of over 200 bones.

They hold the body
together, give it shape,

and protect its
organs and tissues.

The skeleton also
provides anchor points

for the muscular
system, which includes

three types of muscles--

skeletal, smooth, and cardiac.

They are found throughout the
body and facilitate movement.

Nestled within these muscles
is the cardiovascular system,

a pipeline that includes
the heart, blood vessels,

and the blood itself.

Also called the
circulatory system,

the cardiovascular
system delivers

oxygen, white blood
cells, hormones,

and nutrients
throughout the body.

Lastly, the nervous system
is a communication network

of nerve cells
that the body uses

to transmit information and
coordinate bodily functions.

It's comprised of
the brain, the hub

of sensory and intellectual
activity, the spinal cord,

and the many cranial and spinal
nerves that emanate from them.

This infrastructure, created
by neurons, blood, muscles,

and bones allows
three other systems

to regulate the body's
environment-- the endocrine,

lymphatic, and urinary systems.

The endocrine system
is a series of glands

that use information carried
by the nervous system

to help regulate the
body's processes.

Thanks to this
neural connection,

endocrine glands,
such as the thyroid,

are aware of the
amount of hormones

and other chemicals
they need to produce.

These chemicals are then
distributed throughout the body

by way of the
cardiovascular system.

The cardiovascular
and nervous systems

are also utilized by
the lymphatic system--

a collection of lymph
nodes and vessels

that help regulate
the body's defenses.

Also called the immune
system, the lymphatic system

uses neural pathways
to transmit information

about affected
areas of the body,

and then sends out healing
agents, like white blood cells,

via the bloodstream.

Another key regulatory
system is the urinary system,

which includes the kidneys,
ureters, bladder, and urethra.

The urinary, or renal
system, maintains

the body's electrolyte
levels and filters waste

from the blood.

This waste is sent
through the blood vessels,

into the kidneys, and
then expelled as urine.

All of these systems
require energy to function,

and that's where the respiratory
and digestive systems come in.

The respiratory system is a
group of passageways and organs

that extracts life-giving
oxygen from the air we breathe.

Air enters the body
through nasal cavities,

travels down the throat, and is
then transported to the lungs.

The lungs extract
oxygen for the body

to use, and then expel a
carbon dioxide byproduct

when we exhale.

Energy can also come
in the form of food.

The digestive system is an
approximately 30-foot series

of organs that convert
food into fuel.

Food enters the system
through the mouth,

then moves into the
esophagus, the stomach,

and the intestines.

Nutrients are absorbed
into the body,

while solid waste is expelled
through the anal canal,

the end of the digestive tract.

No matter the role,
size, or shape

of any of the
body's systems, each

began with the
reproductive system.

This system is responsible
for creating life.

The primary organs involved
differ between the sexes,

with ovaries, fallopian
tubes, the uterus, and vagina

found in women, and testes and
a sperm channel found in men.

Together, fertilization may
occur, organ systems form,

and then a child is born.

Humans are
complicated organisms.

But when our 10 major
organ systems are healthy,

they ensure our well-being.

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