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NARRATOR: The human body
is a complex network

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of cells, tissues,
and organs that

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together make life possible.

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

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skeletal, muscular,
cardiovascular, nervous,

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endocrine, lymphatic,
respiratory, digestive,

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urinary, reproductive.

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The skeletal, muscular,
cardiovascular,

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and nervous systems
in particular

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create an infrastructure that
facilitates the other systems.

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

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They hold the body
together, give it shape,

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and protect its
organs and tissues.

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The skeleton also
provides anchor points

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for the muscular
system, which includes

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three types of muscles--

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skeletal, smooth, and cardiac.

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They are found throughout the
body and facilitate movement.

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Nestled within these muscles
is the cardiovascular system,

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a pipeline that includes
the heart, blood vessels,

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and the blood itself.

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Also called the
circulatory system,

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the cardiovascular
system delivers

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oxygen, white blood
cells, hormones,

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and nutrients
throughout the body.

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Lastly, the nervous system
is a communication network

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of nerve cells
that the body uses

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to transmit information and
coordinate bodily functions.

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It's comprised of
the brain, the hub

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of sensory and intellectual
activity, the spinal cord,

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

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This infrastructure, created
by neurons, blood, muscles,

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and bones allows
three other systems

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

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lymphatic, and urinary systems.

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The endocrine system
is a series of glands

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that use information carried
by the nervous system

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to help regulate the
body's processes.

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Thanks to this
neural connection,

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endocrine glands,
such as the thyroid,

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are aware of the
amount of hormones

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and other chemicals
they need to produce.

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These chemicals are then
distributed throughout the body

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by way of the
cardiovascular system.

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The cardiovascular
and nervous systems

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are also utilized by
the lymphatic system--

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a collection of lymph
nodes and vessels

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that help regulate
the body's defenses.

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Also called the immune
system, the lymphatic system

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uses neural pathways
to transmit information

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about affected
areas of the body,

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

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via the bloodstream.

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Another key regulatory
system is the urinary system,

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

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The urinary, or renal
system, maintains

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the body's electrolyte
levels and filters waste

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from the blood.

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This waste is sent
through the blood vessels,

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into the kidneys, and
then expelled as urine.

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All of these systems
require energy to function,

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

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The respiratory system is a
group of passageways and organs

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

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Air enters the body
through nasal cavities,

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

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The lungs extract
oxygen for the body

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to use, and then expel a
carbon dioxide byproduct

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when we exhale.

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Energy can also come
in the form of food.

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The digestive system is an
approximately 30-foot series

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of organs that convert
food into fuel.

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Food enters the system
through the mouth,

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then moves into the
esophagus, the stomach,

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and the intestines.

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Nutrients are absorbed
into the body,

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while solid waste is expelled
through the anal canal,

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the end of the digestive tract.

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No matter the role,
size, or shape

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of any of the
body's systems, each

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began with the
reproductive system.

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This system is responsible
for creating life.

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The primary organs involved
differ between the sexes,

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with ovaries, fallopian
tubes, the uterus, and vagina

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

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Together, fertilization may
occur, organ systems form,

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and then a child is born.

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Humans are
complicated organisms.

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But when our 10 major
organ systems are healthy,

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they ensure our well-being.

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