The Skeletal System and Its Movements 3Collagen imbues bone with its great tensile strength
(the ability to resist a pulling force that would tend to
pull a bone apart; L. tensio, to stretch) and flexibility.
The composition of bone can be compared to that
of reinforced concrete, with the collagen playing the
role of the steel and the calcium crystals serving the
role of the sand and rock. The compression strength
of bone is actually greater than that of reinforced
concrete (Guyton, 1976), and the tensile strength of
very dense bone is estimated to be 230 times greater
than that of muscle of a similar cross section (Rasch
and Burke, 1978).
Functions of Bone
The composition of bone allows it to serve in the
following key functions.
- Support: Bones provide an internal framework for
the body that is essential for stability and form. - Protection: Some bones protect fragile structures
within. For example, the skull helps protect the
brain; the rib cage, the heart and lungs; and the
pelvic girdle, vital internal organs. - Movement: Many bones serve as levers to enhance
movement capabilities (see Muscles, Levers, and
Rotary Motion in chapter 2 [p. 44] for more infor-
mation). Having long levers in our body allows
our limbs to move through a large distance, at a
fast speed, or both. - Blood cell production: Some bones contain tissue
(red bone marrow) that is responsible for the
production of red blood cells. Red blood cells
are vital for the transport of oxygen and carbon
dioxide. - Mineral storage: Various important minerals
such as calcium, phosphorus, and magnesium
are stored within the bones. When necessary,
hormones can stimulate release of some of these
minerals into the blood for the body to use. These
minerals are vital for important processes such
as blood clotting, nerve transmission, muscle
contraction, and energy metabolism.
Types of Bone
Bones come in a large variety of shapes and sizes.
They can be classified according to their shape
into the five types described next and illustrated in
figure 1.1.
- Long bones are tubular in shape and much
longer than they are wide. They are found in the FIGURE 1.1 Types of bones in the skeleton (anterior view).
limbs, where they serve as levers to enhance move-
ment. For example, the “thigh” bone, or femur, is a
long bone (figure 1.1). Other examples include the
clavicles, humerus, radius, ulna, and metacarpals
and phalanges of the upper limb or extremity and
the tibia, fibula, and metatarsals and phalanges of
the lower limb or extremity (figure 1.4). The long
bones in the lower extremity are generally larger and
stronger to meet their weight-bearing needs, while
those in the upper extremity are generally smaller
and lighter to meet their role in reaching and in
manipulation of objects.- Short bones are cubical in shape and are found
in the upper portion of the hand (carpals; see figure
1.4) and feet (tarsals; see figures 1.1 and 1.4). These
bones aid with shock absorption, transmission of
forces, and small complex movements.