Science High School Reviewer | Grade 4 Science | Life Science | Systems of the Human Body | Lesson 13: The Skeletal and Muscular Systems

Complex organisms are made of many organ systems.
The skeletal and muscular systems work together to support
and move your body.

A group of organs that work together makes up an organ
system. The heart and blood vessels are organs that make
up the circulatory system. Each part of an organ system is
important. If one organ is damaged, the other organs in
the system will be affected.

Different organ systems work together and depend on
one another. For example when you in-line skate, your
skeletal and muscle systems help you stand. Your nervous
system sends information to control the movements of your
arms and legs. Your respiratory and circulatory systems
work together to give your muscles the energy they need.

The Skeletal System
Can you touch your backbone, your ankle
bone, and your knee? These are bones that are
all part of your skeleton. Your skeleton is made
mostly of a tissue called bone.
Without the support of your skeleton, you
would fall into a blob-like heap. Just as a house
could not stand without its wooden frame, your
body cannot stand without its skeletal system.
Your skeleton also protects organs inside
your body, such as your brain, heart, and
lungs. Your skeletal system also helps you
move. Muscles that move your body are
attached to your skeleton.

Types of Joints
Joints hold your bones
together and allow them
to move. The photos
show examples of two
types of joints.

Ball-and-socket
joints allow
the most
movement
of any type
of joint. Your
shoulder and
your hip have
ball-and-socket
joints. The end of one
bone (ball) fits into a
bowl-shaped area (socket) in
another bone. Ball-and-socket
joints allow your bones to have
circular movement, such as
swinging your leg.

A hinge joint
lets your
bones move
backward and
forward, the
way a door
moves. You can
bend or straighten
your leg because of
the hinge joint at your knee.

The Human
Skeleton
The adult human
skeleton has 206 bones.
Some of these bones are
labeled in the picture.
Bones have different
shapes. The shape of a
bone is related to the
job it does.

Building Strong Bones
Your skeletal system needs certain minerals to
help it stay healthy. Minerals, such as calcium,
help build bone tissue and keep bones strong.
Calcium also helps muscle and nerve tissues work
properly. The bones of your skeleton store calcium
and other minerals your body needs.
In addition to supporting the body and storing
calcium, many bones produce different kinds of
blood cells. Some of these blood cells carry oxygen
to cells in other systems. Others fight bacteria
that cause disease. Some cells help stop the
bleeding from a cut.
One bone attaches to another bone at a
joint. Joints can be classified by the type of
movement they allow. Tissues around joints
protect them. The tissues also help hold the
bones together at the joint.

The Muscular System
Muscles that move your body are
attached to your skeleton. Without
your muscular system, bones could
not move at their joints. Because your
muscles shorten, or contract, you can
walk, run, smile, or sing. Without
muscles, you could not breathe or
swallow food.

Voluntary Muscles

Muscles that you control are
voluntary muscles. You choose
when you want voluntary muscles to
work. Smiling, running, and chewing
are actions controlled by voluntary
muscles.
Most of the muscles in your body
are skeletal muscles. Skeletal muscles
are voluntary. They are different
shapes and sizes. They help your
skeleton move. Skeletal muscles
work in pairs. For example, a pair of
muscles works together in your arm.
When you first bend your arm, one of
the muscles contracts and the other
relaxes. Then when you straighten
your arm, the first muscle relaxes and
its partner contracts.
Involuntary Muscles
Muscles that you cannot control
are involuntary muscles. You use
involuntary muscles to breathe or
digest food. They keep blood flowing
through your blood vessels. They help
your eyes adjust to light that gets
brighter or dimmer.

Muscle Tissue

The photos show each type of
muscle under a microscope.

Skeletal muscles are
attached to your
bones. They seem
to have stripes.

Cardiac muscle
is involuntary. It
is found only in
the heart. Like
skeletal muscle,
cardiac muscle
looks striped.

Smooth muscles
are involuntary.
They work
automatically in
different organ
systems.