The Structure of Bone Controls
Function and Growth
LEARNING OBJECTIVES
1.
Outline
the six functions of bone and the skel-
etal system.
2.
Describe
the parts of a long bone and the mi-
croscopic structure of bone.
3.
Explain
the formation of bone during different
phases of a person's lifetime.
4.
Discuss
how bones heal after fractures.
he skeletal system consists of all the bones
attached at joints and the cartilage between
the joints. The functions of the skeletal sys-
tem include support, protection, movement,
mineral homeostasis, blood cell protection, and triglycer-
ide storage. Bones are classified according to shape, with
a bone’s structure determining its function. Throughout
life, bone is constantly made and destroyed in an ongoing
process of bone remodeling.
The Skeleton Is More Than Just a
Supportive Framework for the Body
The adult skeletal system is made of 206 different bones.
During development, some bones (such as those that form
the skull) fuse to create a solid
unit, while others connect to the
adjacent
bones
at
articulations
that offer varying degrees of move-
ment. Bones and associated tissues
perform several basic functions: •
Support
—The skeleton provides a scaffold or framework
to support soft tissues and points of attachment for
skeletal muscles.
Protection
—The skeleton protects many internal organs
from injury. For example, the skull protects the brain,
and the rib cage protects the heart and lungs.
Movement
—Muscles provide
the
force, while bones
serve
as
the
levers. Together, bones and muscles
produce movement of the body in its environment.
Mineral homeostasis
—Bone tissue stores calcium and
phosphorus. When needed, endocrine glands secrete
hormones, which act on bone to either release calcium
and phosphorus into the blood or store excess minerals
in the bone extracellular matrix.
Blood cell production
—Red bone marrow, a connective
tissue within bone, produces
red blood cells, white blood
cells,
and
platelets
in
a
process called hemopoiesis.
Triglyceride storage
—Yellow bone marrow within bone
is
composed mostly of adipose
cells, which
store
triglycerides (fats). In a newborn, all bone marrow is
red, but it eventually changes into yellow marrow with
increasing age.
A Bone's Structure Determines
Its Function
The skeletal system contains four types of bones, based on
shape:
Long bones are longer than they are wide and have
knobby ends where
the
articulations
form.
Their
slightly curved structure gives them strength. Long
bones include those of the arms, legs, fingers, and toes.
Short bones are equal in length and width, making
them nearly cube-shaped.
Examples
include
most
bones of the ankles and the wrists.
Flat bones are thin and provide both protection and
surfaces for muscle attachments. The bones of the
skull, sternum, and ribs are all flat bones.
Irregular bones have complex shapes, such as those of
the face and vertebral column.
Let’s take a look at the structure of bones, from gross
(macroscopic) to microscopic.
Long bones are typically used as examples when study-
ing gross bone structure (Figure 5.1). Long bones are hol-
low in the middle and more solid on the ends. Typically,
the ends of an adult’s bones contain the red marrow, while
the hollow cavity contains the yellow marrow. The outside
of the bone has a connective tissue covering known as the
periosteum. The ends of a bone that form movable joints
are covered with a layer of articular cartilage. As we will
discuss later, long bones lengthen by adding new bone to an
area known as the epiphyseal plate that lies between the
epiphysis and diaphysis at each end of the bone.
articulation
(ar-tik'-
a-LA-shun) A location
at which two or more
bones make contact.
Also known as a
joint
.
hemopoiesis
(he-mo-poy-E-sis)
Blood cell production.
114 CHAPTER 5
The Skeletal System
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