section
33.1
Skeletal System
Key Concept The skeletal system includes bones and tissues that
are important for supporting, protecting, and moving your body.
When you jog, your body absorbs a force of more than twice your
body weight. When you jump, you land with about 12 times your body
weight. Your bones and muscles must be strong enough to handle these
forces along with the everyday stresses your body experiences.
Your skeletal system is made up of the
appendicular and axial skeletons.
The skeletal system is the organ system that supports your body.
It protects your internal organs and allows the body to keep its shape.
The skeletal system includes the bones and the connective tissues that
hold the bones together. The human skeleton has 206 bones.
Appendicular Skeleton
The appendicular skeleton is the part of your skeleton that can move
easily. It includes all the bones that extend* out from the trunk of
your body—your arms, legs, feet, and hands.
The appendicular skeleton also includes two sets of bones, called
girdles, that connect your arms and legs to your body. The girdles
allow your arms and legs to have a wide range of motion,* such as
throwing or kicking a ball, and moving sideways.
Axial Skeleton
The bones in the body’s trunk and head make up the axial skeleton.
These bones move less but support the body’s weight and protect
internal tissues. The bones of the skull protect the brain. The ribs and
breastbone protect the heart and lungs. The bony vertebrae surround
and protect the spinal cord.
The axial skeleton does allow some movement. The ribs are connected by flexible tissue that enables the chest to expand when you
breathe in. Flexible tissue in the spine gives people the ability to bend,
turn, and look behind them.
* Academic Vocabulary
extend reach out
range of motion how much you can move a part of your body
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The skeletal system is made
up of an appendicular
skeleton (light blue) and an
axial skeleton (dark blue).
Cartilage
The bones in your body are connected to each other, but they cannot be
connected bone-to-bone. Such hard materials would not allow you to
move. Instead, bones are connected to each other with cartilage, a
flexible connective tissue. Cartilage is found between your bones. It
allows for smooth movement and prevents bones from rubbing against
each other, which would wear down or damage their surfaces.
Name one part of your body that is part of the appendicular
skeleton. Name another part that is part of the axial skeleton.
Bones connect to form joints.
The place where two bones meet is called a joint. Some joints give bones
a full range of movement while others allow little or no movement.
No movement. Joints that allow no movement are called fibrous joints.
They are made of the same dense material as your bones. For example,
fibrous joints hold your teeth in place and hold the plates of your skull
together to protect the brain.
Some movement Joints that allow some movement are called cartilaginous joints. These joints are held together by cartilage. The vertebrae in
your spine are separated by discs of cartilage, giving the spine some flexibility. For example, you can bend to the side, forward, and backward.
Full range of movement Other joints, called synovial joints, allow a full
range of movement. These joints are cushioned with cartilage but held
together with ligaments. A ligament is a long, flexible band of connective tissue. It holds bones together but is loose enough to allow bones to
move. There are several different types of synovial joints:
Gliding joints allow flat bone surfaces to slide across
each other. The ankle and wrist contain gliding joints.
Pivot joints allow two bones to rotate against each
other. The skull’s top two vertebrae form a pivot joint,
allowing the head to turn from left to right.
Ball and socket joints include a ball-shaped end of a
bone, and a cup. The ball can move and rotate inside
the cup. The hips and shoulders contain ball and
socket joints.
VISUAL VOCAB
A ligament is a long band of
connective tissue that connects
two bones across a joint.
ligament
Saddle joints allow a bone to move front to back and left to right.
One bone rides inside the other, as if the second bone were a saddle.
Your thumbs are connected to your hands with saddle joints.
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Hinge joints allow bones to move in one direction, like a swinging
door. The knees, fingers, and toes contain hinge joints.
Some bones are connected with more than one type of synovial joint.
These are called compound joints. In your elbow, for example, a hinge
joint connects your forearm to your upper arm. Your elbow also has a
pivot joint that allows you to turn your hand upward or downward.
Find one of each kind of joint on your own body. Explore the
range of motion that you have for each joint.
Bones are living tissue.
Bones may seem hard and unchanging, but they are actually living
tissue. They produce blood cells and store minerals. Each bone is coated
with a layer of connective tissue called periosteum. This tissue holds and
protects blood vessels. Like the body’s other tissues, bones rely on blood
vessels to bring nutrients and carry away wastes.
Bone Structure
compact bone
There are two types of bone tissue: compact and spongy. Compact
bone is the outer layer of a bone and protects the body against jolts
and bumps. It is made up of rings of calcium-rich tissue.
Spongy bone is porous, like a sponge. It is located at the center
of the bone. Spongy bone is surrounded by hard, dense, compact
bone. A young person’s spongy bone is mostly red bone
marrow. Red bone marrow, which is part of the circulatory
system, produces red blood cells. As a person grows, some
red bone marrow is replaced by yellow bone marrow,
which is mostly fat. It can change back into red bone marrow to produce blood cells if the body suddenly loses blood.
Bone Growth
At first, human embryos have a skeleton made only of flexible cartilage.
As they develop, the cartilage becomes hard bone.
Bones cannot form without the help of cells called osteoblasts. These
cells release collagen, a strong connective tissue that forms fibers. They
also release calcium phosphate, a mineral that hardens the collagen. The
collagen combines with calcium phosphate to form bone. This process is
called calcification. Once calcification occurs, the osteoblast is trapped
in the bone. It is then called an osteocyte.
Bones grow from their ends, where the cartilage is located. Children’s
bones grow longer, wider, and thicker until puberty. During the teenage
years, sex hormones stimulate bones to become more dense. Bones are
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periosteum
yellow bone
marrow
spongy bone,
contains red
bone marrow
Bones have many layers for
protection and support.
strongest when a person is between 18 and 30 years old. After age 30,
calcium is taken from the bones and used in the rest of the body. As a
result, the bones become less dense. But even after you stop growing, the
osteoblasts in your bone can still create new tissue to heal broken bones.
Bones store calcium for the body to use. The process of moving
calcium into and out of the bones is controlled by hormones and helps
to maintain chemical homeostasis in your body. The thyroid gland
releases a hormone that stimulates osteoblasts to take calcium out of the
blood and put it into bone tissue. The parathyroid gland releases a
hormone that stimulates other bone cells to take calcium out of bone
tissue so the body can use it.
What materials are needed for calcification?
33.1 Vocabulary Check
skeletal system
appendicular skeleton
axial skeleton
vertebrae
cartilage
joint
ligament
calcification
Mark It Up
Go back and highlight
each sentence that
has a vocabulary
word in bold.
Choose the correct term from the list for each description.
1. A flexible band of tissue that connects bones across a
joint
2. The bones that surround the spinal cord
3. The place where two bones meet
4. The organ system that includes the bones
5. Making hard bone by combining collagen and calcium
6. The bones found in the trunk and head
7. Bones found in the arms, legs, hands, and feet
8. Flexible connective tissue found between bones
33.1 The Big Picture
9. Why is cartilage such an important part of the skeletal system?
10. Name one way spongy bone differs from compact bone.
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