The 206 bones that make up your skeleton come in all shapes and sizes,
ranging from the flat, plate-like bones of your skull to the long,
cylindrical bones in your limbs. Their internal structure makes bones
light but very strong - human bones are four times stronger than
concrete; a 5cm cube of solid bone could take the weight of an elephant.
What do bones do?
The most obvious functions of your bones are to give your body its
basic shape, support your weight and work with your muscles to produce
controlled, purposeful movement. Your skull and rib cage also help to
protect your brain, heart and lungs from external damage, while the
bones of your spine safeguard your spinal cord.
Your bones are also your primary store of calcium, the most abundant
mineral in your body. They contain around 99 per cent of all your
body's calcium in the form of salts (mostly calcium carbonate and
calcium phosphate). As well as being a vital constituent of your bones
and teeth, calcium plays an essential part in many chemical processes,
including muscle contraction, nerve function and blood clotting.
During clotting, calcium reacts with proteins and other substances to
thicken and coagulate the blood, and tiny particles called platelets
stick together to plug the damaged blood vessels. Like much of the
calcium in your blood, these platelets originate in bones.
Bone Marrow
When you are born, your bones contain a jelly-like substance called red
marrow. This makes red blood cells to carry oxygen around your body,
white blood cells to fight infections and platelets to help your blood
to coagulate. By adulthood, yellow marrow (largely fat) will have
replaced most of the red marrow in the long bones of your limbs, but
your other bones will contain red marrow.
Bone Structure
Each of your bones is covered with a fibrous outer membrane, the
periosteum. This contains blood vessels to transport nutrients and
hormone signals to the inner bone, and nerves to carry information
about things such as pain and position.
Beneath the periosteum, the bone itself consists of a dense outer layer
of smooth, hard bone called compact bone and inner layers of softer,
more porous cancellous bone.
Compact bone consists of cylindrical
structures called
osteons, each measuring about 0.2mm across and about 10mm long, which
are tightly packed together to form the strong, dense bone. Blood
vessels and nerves, connected to the periosteum, occupy the hollow
central canals of the osteons.
Cancellous bone lies beneath the compact
bone. It consists
of a honeycomb of rigid struts (trabeculae) and contain the bone marrow.
What is bone made of?
The basic material of both compact and cancellous bone is a matrix of
collagen (fibers of strong protein) impregnated with calcium salts. The
calcium salts give bone its hardness while the collagen fibers give it
a degree of resilience. Collagen, a very flexible material, is also
found in cartilage, ligaments, tendons and skin, and is the main
component of your body's connective tissue, which separates, supports
and protects your vital organs.
Bone Cells
Although bone is mostly calcium-hardened collagen, it is not simply an
inert material. It is an amazing, ever-changing structure created,
maintained and constantly renewed by the specialized living cells
embedded within it. Each type of bone cell plays its own individual
part in the laying down, maintenance and repair of the bone tissue.
Osteoblasts - located near the surfaces
of the bones, the
bone-forming osteoblast cells produce the collagen matrix that hardens
into thin layers of bone when calcium salts crystallize within it.
Osteocytes - the osteoblasts eventually
trap themselves
within the bone they create and are then known as osteocytes. The main
function of osteocytes, the most numerous type of bone cell, is to
maintain the bone tissue.
Osteoclasts - the osteocytes work in
conjunction with cells
known as osteoclasts. These large cells use acidity and enzyme action
to break down bone tissue. They do this either to release calcium from
the bone so that it can be used elsewhere in the body or to remove bone
damaged by everyday usage or fractures. Osteocytes and osteoblasts then
repair the damage.
4 Ways to Keep Your Bones Strong
Eat well - make sure you get plenty of
vitamins and
minerals and lots of calcium. Good sources of calcium include dairy
products and green leafy vegetables.
Stay strong - take plenty of exercise,
particularly
bone-loading, weight-bearing exercise to build strong bones.
Stop smoking - chemicals in cigarettes
can interfere with
calcium absorption and may speed up the aging process.
Drink sensibly - excessive alcohol
intake interferes with
nutrient absorption and increases bone breakdown.
