Everyone is unique. We have different skin colors, hair colors, body
shapes and sizes — but we all look alike inside. If you could peek inside
your own body, what would you see? Hundreds of bones, miles of blood
vessels, and trillions of cells, all of which are constantly working
together, doing all kinds of different things.
Skin
Main job: To protect your internal (inside) organs from drying
up and to prevent harmful bacteria from getting inside.
How much: The average person has a total of six pounds of
skin.
Main layers:
Epidermis: Outer layer of skin cells, hair,
nails, and sweat glands.
Dermis: Inner layer of living tissue,
containing nerves and blood vessels.
Skin facts: Your skin …
… is flexible so that you can bend and stretch.
… feels heat, cold, pain, pressure, moisture, irritation, and
tickles because it has nerves.
… heals itself when wounded.
… keeps heat in on cold days and releases it as perspiration on
hot days.
… is a watertight container for your body.
Bones
The largest bone in the body is the femur, or thigh bone; it is 20
inches long in a 6-foot-tall person.
Main job: To give shape to your body.
How many: At birth you had more than 300 bones in your body. As
an adult you'll have 206, because some fuse together.
The smallest bone is the stirrup bone, in the ear; it is .1 inch
long.
Kinds of Bones
Long bones are thin; they are found in your legs, arms, and
fingers.
Short bones are wide and chunky; they are found in your feet
and wrists.
Flat bones are flat and smooth, like your ribs and shoulder
blades.
Irregular bones, like the three bones in your inner ear and
the vertebrae in your spine, come in many different shapes.
Joints
Bones don't bend. It is the joint that allows two bones next to each
other to move.
Main job: To allow bones to move in different directions.
Ligaments
Main job: These bands of tough tissue hold joints together. They
are strong and flexible.
Muscles
Every day, the average person's muscles work as hard as if they were
placing 2,400 pounds on a 4-foot-high shelf.
Main job: To make involuntary or voluntary body movement
possible.
How many: Your body has more than 650 muscles. Each muscle does
only two things: contract when being used and expand when
resting.
Kinds of Muscles
Skeletal muscles move your bones. They are
called voluntary muscles because you decide when to move them. You have
more than 400 voluntary muscles.
The job of the cardiac muscle, or heart, is
to pump blood through your body. The cardiac muscle is involuntary; it
never stops working during your lifetime.
Smooth muscles control your internal
movements, such as moving food around in your intestines. These muscles
are also found in the blood vessels, where they assist the flow of
blood. Smooth muscles are involuntary.
Tendons
Your fingers are mostly powered by muscles in your palm and
wrist.
Main job: To hold your muscles to your bones.
Tendon fact: Tendons look like rubber bands.
Viscera
This term refers to the organs, including the trachea or windpipe,
lungs, liver, gallbladder, spleen, stomach, large intestine, small
intestine, and bladder, that fill your body's chest and abdominal cavity.
They belong to many different systems: respiratory, digestive, and
urinary.
Main job: To provide your body with food and oxygen and to
remove waste.
How many: The viscera are made up of 10 organs:
Glands
Main job: To manufacture substances that help your body to
function in various ways.
Kinds of Glands
Endocrine glands make hormones, which tell
the different parts of your body when to work.
Oil glands keep your skin from drying out.
Salivary glands make saliva, which helps to
digest carbohydrates in your mouth and aids in swallowing.
Sweat glands make perspiration, which
regulates your body temperature.
Cells
There are 26 billion cells in a newborn baby and 50 trillion cells
in an adult.
Main job: To perform the many jobs necessary to stay alive, such
as moving oxygen around your body, taking care of the fuel supply,
communications, and waste removal.
Some Different Cells
The egg is the largest human cell. Once it
is fertilized, all other cells begin forming.
Bone cells help build your skeleton by
secreting the fibers and minerals from which bone is made.
Fat cells store fat. They can shrink or
grow. Once you have them you can't get rid of them.
Muscle cells are organized into muscles,
which move body parts.
