Skeletal
System
Vertebrates have a strong supporting
structure. This is their internal skeleton, or endoskeleton.
Invertebrates such as arthropods have external skeletons, or exoskeletons.
The bones that make up our framework, or skeleton,
are very efficient. They give the greatest support with the least weight, and
allow us to move freely. However, there is one disadvantage to having an
internal skeleton. It does not offer protection against injury as does an
exoskeleton. Many soft parts of the body are exposed. An organism with an
endoskeleton must rely on its nervous system and sense organs for protection.
The Functions of the
Skeleton
The
bones in your body function in several ways.
1.
They serve as sites of
attachment for skeletal muscles, and they serve as levers that produce movement
of body parts when these muscles contract.
2.
They give the body its general
shape and support body structures.
3.
They protect delicate
structures, such as the brain, spinal cord, heart, and lungs,
4.
They serve as a storage site
for minerals, such as calcium and phosphorus.
5. Marrow
is found within the skeleton bones. It has an important function in producing
red blood cells.
Bone Structure
Bone
is made up of living bone cells, connective
tissue fibers, and inorganic compounds. It is a very active tissue, and there
is a constant absorption of old tissue and laying down of new tissue. A basic
part of the structure is collagen, a type of connective tissue. In bone
formation, living bone cells called osteoblasts secrete collagen
molecules and certain polysaccharides. The collagen molecules form fibers that
are then bound together by the polysaccharides, which act as cement. Bone is
formed when calcium and phosphate ions from the body fluids combine, forming
calcium phosphate, and precipitate as crystals within the mass of collagen
fibers and cement. The hardness and heaviness of bone is due to the presence of
the calcium phosphate. The osteoblasts are entrapped in small cavities within
the bone substance to form cells called osteocytes.
In
the bone, the osteocytes are arranged in concentric circles. In the center of
each series of circles is a cavity called the Haversian canal,
which contains blood vessels and nerves. Tiny canals connect the osteocytes
to each other and to the Haversian canal. The blood vessels within the
Haversian canals carry oxygen and nutrients to the bone cells and remove
wastes. If a bone is broken, the osteocytes become active, producing new bone
tissue to heal the wound.
The
outside of a bone, except at its ends where it connects to other bones, is
covered by a tough membrane called the periosteum. The chief function of
the periosteum is the production of new bone for growth and repair. The
periosteum also serves as the point of attachment for muscles to bones. This
membrane contains blood vessels and nerves that enter the bone.
There
are two types of bony tissue—compact bone and spongy bone.
Their composition is the same, but compact bone is very dense and strong, while
spongy bone is comparatively porous. Most bones contain both compact and spongy
hone tissue.
Some
of the bones of the body are hollow, and there is also much space in spongy
bone. These spaces are filled with a soft tissue called marrow. There
are two types of narrow—red marrow and yellow marrow. Red
marrow produces red blood cells, platelets, and some types of white blood
cells. In adults, red marrow is found in the spongy bone of the vertebrae,
ribs, breastbone, cranium, and long bones. Yellow marrow consists of fat cells.
In adults it is found in the hollow central region of long bones.
Cartilage,
like bone, is a type of connective tissue. While bone is rigid, cartilage is
flexible. In the embryo, most of the skeleton is cartilage. As the
embryo develops, minerals are deposited, and the cartilage is gradually changed
into bone. This process, called ossification, continues into adulthood.
The bones of small children contain more cartilage than the bones of adults and
are therefore more elastic and not as easily broken. In adults cartilage is
found at the ends of ribs, at joints, and in the nose and outer ear. Cartilage
provides support, while still permitting some bending or motion. It provides
flexibility at joints and cushions against impact or pressure.
Regulation of Bone formation
Bone is formed when
calcium and phosphate ions from the body fluids combine, forming calcium
phosphate, and precipitate as crystals within the mass of collagen fibers and
cement. The factors influence the concentration of materials in the bones and
in the blood
-
The quantity of
vitamins and minerals in the diet,
-
Hormones
-
Genetic factors
Calcium and phosphate is
continuously lost via the alimentary tract and the kidneys. Insufficient
calcium and phosphate cause the bones become softened and fragile. During
pregnancy, the mineral requirement of the fetal skeleton is supplied by the
mother. Pregnant women should therefore supplement their diet with either
calcium tablets or calcium-rich foods.
The
Roles of Vitamins in the formation of bone; vitamins A, C and D play
important roles.
The lack of vitamin D, calcium and phosphate
absorption decreases, consequently formation of teeth and bone takes
considerable time and fail to harden completely.
As a result, symptoms of the disease rickets appear.
In a deficiency of
vitamin A, the growth rate of the bones decreases.
In a deficiency of vitamin C, weak and fragile bones result.
The formation of bone is
regulated antagonistically by parathormone secreted by the parathyroid gland,
and calcitonin secreted by the thyroid gland
Parathormone secreted when the level
of calcium in the blood decreases
. Calcitonin secreted when the level of calcium in the blood
increases
Milk
is the natural food of all young mammals. It is an excellent source of
calcium compounds, but the minerals alone are not enough. Certain vitamins,
especially vitamin D, are also needed for the normal growth of bone. Supplied
with the minerals and vitamins, bone cells deposit calcium phosphate and calcium
carbonate
Types of bones
There are three types of bone
in the human skeleton; short bone, long bone and flat bone.
-
Long
Bones: Arm and Legs
Flat
Bones: Sternum, rib cage, hip bones,
Patella of the knee joint and skull
Short
Bones: Hand, foot,
wrist joint, ankle joint and finger
Parts of the
skeleton.
The human skeleton contains 206 bones. The
skeleton has two main divisions;
n Axial
skeleton
n Appendicular
skeleton.
The
axial skeleton includes the skull, vertebrae, ribs, and breastbone. The
upper part of the skull, the cranium, houses
and protects the brain. The rest of the skull includes the facial and jaw
bones. The spinal column, or backbone, consists of 33 bones called vertebrae. The vertebrae are separated
from each other by disks of cartilage. The disks act as shock absorbers and
give the spine flexibility. The ribs are attached at the back to the upper
vertebrae and at the front to the breastbone, or sternum. The area enclosed by the sternum, ribs, and backbone is
the chest cavity. Within the chest
cavity, the heart and lungs are supported and protected by the ribs and
sternum.
The appendicular skeleton
includes the arms and legs and two ring like sets of bones called the pectoral
girdle and the pelvic
girdle.
The
pectoral girdle consists of the shoulder blades and collar bones. It connects
the arms to the spine.
The
pelvic girdle is made up of the hip bones, or pelvic bones. It connects the
legs to the spine.
The Bones
Form Joints
The area
where two bones meet is called a joint. The
bones in the human body are connected by several different kinds of joints.
These linkages are designed in a certain way to fulfill a particular function.
Some joints, such as those between the bony plates of the skull, do not move at
all- these joints are called as immovable
joints. Other joints have a large range of power and movement-movable
joints. For example, at the elbow and knee, hinge joints combine strength
and mobility. This joint is designed for only bend-stretch movements.
When
you throw a ball, another type of joint is used. The ball-and-socket joint of
your shoulder allows you to "wind up" for a good throw. If you have
watched a pitcher during a baseball game, you have observed that his whole body
is used to produce the force to throw the ball. His leg is able to swing up and
over because of another ball-and-socket joint in the hip. This joins
the thighbone, or femur, to the socket
in the hipbone, the pelvis.
The
wrist bones are connected by angular joints. These can supply a twisting and a flicking motion to give a baseball
a fast thrusting motion. The ankle bones also have angular joints. The
vertebrae are connected by gliding joints. A slight motion is possible with the vertebrae also. You can
twist your head to look around you because of the pivot joint connecting
your head to your spine.
Some
joints are only partially movable - partially movable joints. The joints attaching your ribs to the
vertebrae in your backbone are examples of partially movable joints. Some of
the ribs are attached to the breastbone, or sternum, by long strands of cartilage.
Partially movable joints of the ribs allow your chest to expand and contract
when you breathe.
Movable joints are held in
position by tough strands of connective tissue, called ligaments. The ligaments
can be stretched by exercise. This loosens the joints and allows you to move
more easily
Structure of joints
Structure of joints
The
inside surfaces of your joints are covered with layers of cartilage. A secretion
lubricates the joints called synovial
fluid, is secreted into movable
joints by surrounding membranes. This fluid acts as a lubricant and reduces
friction at the joint.. The fluid makes the cartilage surfaces slippery so they
can slide freely. In some joints, such as your knee and shoulder, a sac acts as
a cushion between the bones. Such a sac is called a bursa. Sometimes stress on a joint causes the bursa to produce more
fluid, which results in inflammation, pain, and stiffness. This condition is known as bursitis.Muscle System
