Bones are rigid organs that form
part of the endoskeleton of vertebrates. They function to move,
support, and protect the various organs of the body, produce red
and white blood cells and store minerals. Because bones come in
a variety of shapes and have a complex internal and external structure,
they are lightweight, yet strong and hard, in addition to fulfilling
their many other functions. One of the types of tissues that make
up bone is the mineralized osseous tissue, also called bone tissue
that gives it rigidity and honeycomb-like three-dimensional internal
structure. Other types of tissue found in bones include marrow,
endosteum and periosteum, nerves, blood vessels and cartilage. There
are two hundred six bones in the adult body.
Bones have eight main functions:
1. Protection — Bones can serve
to protect internal organs, such as the skull protecting the brain
or the ribs protecting the heart and lungs.
2. Shape — Bones provide a frame to keep the body supported.
3. Blood production — The marrow, located within the medullary
cavity of long bones and the interstices of cancellous bone, produces
blood cells in a process called haematopoiesis.
4. Mineral storage — Bones act as reserves of minerals important
for the body, most notably calcium and phosphorus.
5. Movement — Bones, skeletal muscles, tendons, ligaments
and joints function together to generate and transfer forces so
that individual body parts or the whole body can be manipulated
in three-dimensional space. The interaction between bone and muscle
is studied in biomechanics.
6. Acid-base balance — Bone buffers the blood against excessive
pH changes by absorbing or releasing alkaline salts.
7. Detoxification — Bone tissues can also store heavy metals
and other foreign elements, removing them from the blood and reducing
their effects on other tissues. These can later be gradually released
for excretion.
8. Sound transduction — Bones are important in the mechanical
aspect of hearing.
The primary tissue of bone, osseous
tissue, is a relatively hard and lightweight composite material,
formed mostly of calcium phosphate in the chemical arrangement termed
calcium hydroxylapatite (this is the osseous tissue that gives bones
their rigidity). It has relatively high compressive strength but
poor tensile strength, meaning it resists pushing forces well, but
not pulling forces. While bone is essentially brittle, it does have
a significant degree of elasticity contributed chiefly by collagen.
All bones consist of living cells embedded in the mineralised organic
matrix that makes up the osseous tissue.
TYPES OF BONES
There are five types of bones in the
human body: long, short, flat, irregular and sesamoid.
1. Long bones are longer than they
are wide, consisting of a long shaft (the diaphysis) plus two articular
(joint) surfaces, called epiphyses. They are comprised mostly of
compact bone, but are generally thick enough to contain considerable
spongy bone and marrow in the hollow centre (the medullary cavity).
Most bones of the limbs (including the three bones of the fingers)
are long bones, except for the kneecap (patella), and the carpal,
metacarpal, tarsal and metatarsal bones of the wrist and ankle.
The classification refers to shape rather than the size.
2. Short bones are roughly cube-shaped,
and have only a thin layer of compact bone surrounding a spongy
interior. The bones of the wrist and ankle are short bones, as are
the sesamoid bones.
3. Flat bones are thin and generally
curved, with two parallel layers of compact bones sandwiching a
layer of spongy bone. Most of the bones of the skull are flat bones,
as is the sternum.
4. Irregular bones do not fit into
the above categories. They consist of thin layers of compact bone
surrounding a spongy interior. As implied by the name, their shapes
are irregular and complicated. The bones of the spine and hips are
5. Sesamoid bones are bones embedded
in tendons. Since they act to hold the tendon further away from
the joint, the angle of the tendon is increased and thus the force
of the muscle is increased. Examples of sesamoid bones are the patella
and the pisiform.
QUESTIONS AND ANSWERS ON BONE
1. How many
bones are in the human body?
An average adult has 206 bones. Some people have an extra rib or
two and may have more bones in their hands and feet.
2. How many
bones are in the spine?
There are 33. The bones in the spine are called vertebrae (ver-tea-bray).
Beginning at the neck, there are 7 vertebrae in cervical (sir-vick-all)
spine - 12 thoracic (thor-ah-sick) - 5 or 6 lumbar (lum-bar) - 5
sacrum (say-crumb) - 3 coccyx (cock-six). The coccyx is the tailbone.
3. How much
do all the bones in the body weigh?
Total bone weight depends on how much the person's entire body weighs.
Bones make up about 15% of a person's total body weight. For example,
the bones of a person weighing 100 pounds would weigh about 15 pounds.
4. Are bones
Yes! In fact bones contain thousands of living cells that are replaced
as they die. Like other organs in the body, blood brings food and
oxygen to bones. The blood also removes waste - you might say that
blood takes out the garbage!
5. What is
Bone is dense and hard yet, slightly elastic. It is made up of different
layers of bone tissue, cartilage, blood vessels, and other tissue.
The outer layer is compact bone. It is very hard and strong. The
inside layer is called spongy bone and it does look something like
a kitchen sponge! In the center of the bone is marrow. Within the
bone marrow the body makes red blood cells.
6. What is
In the center of bone is a jelly like matter called bone marrow.
Bone marrow is the heart of the body's blood factory. Not only does
bone marrow make red blood cells, but it also makes different types
of white blood cells. How do bones get blood? The outer layer of
bone cells form rings around spaces called Haversian canals. The
canals are connected to each other. Each canal is tiny and long.
A blood vessel runs through each little canal.
7. Do bones
Bones have an outer layer that is like a thin skin or membrane (mem-brain).
This membrane is called the periosteum (per-eye-os-tee-um). The
periosteum can be thick and is filled with blood vessels that help
feed the bone. Many nerves also fill the periosteum.