ABOUT YOUR BONE STRUCTURE
An essential part of a human being is the skeleton. The word ’skeleton’ comes from the Greek meaning ‘dried up’, because by itself, the skeleton looks like a completely dried up human, or a shrivelled mummy without skin (or so the Greeks thought). The skeleton is about 18 per cent of your weight – about 25 pounds (11 kg), made up of 206 separate bones to support and protect your body:
long, like the thigh bones, for example, short, like the wrist bones, flat, like your ribs, and irregular, like your vertebrae.
Almost every bone is designed to fit a particular need, with a notable exception being the coccyx, Man’s vestigial tail.
The common name for the bones that run down your back is ‘backbone’, which may sound like a single bone, although twenty-six articulated bones are involved in an adult, held together and kept upright by muscles and ligaments. Another common name is ’spinal column’ or spine, which again suggests a single bone. But if the spine were a single bone, your back would be completely stiff and unable to bend – like your thigh bone, which is one long piece. The spinal column is composed of many bones, separated by discs of fibre and cartilage that act as shock absorbers, enabling your trunk to turn to either side, bend backwards or forwards, or even in small circular movements. It doesn’t bend at one point, like the elbow in your arm, but slightly at several points. Because the spinal column can bend in different directions, its formal name is vertebral column, from the Latin ‘vertere’, to turn. So each individual bone in the vertebral column is called a vertebra, which is how our division of the animal kingdom came to be called vertebrates.
At birth, there are two curves in the vertebral column as in typical land-vertebrates: a downward curve at the neck and an upward curve in the back. So babies crawl. But during the second year, a child rises on hind legs, finding it increasingly easy, comfortable and natural like that. The vertebral column gradually bends back in the hip region to form a new curve that is shaped towards the back. The human spine, though still straight when seen from behind, now has a kind of double-S shape when seen from the side. The curves of the spine make it easy to keep upright and give a springy balance. Other animals, such as bears and gorillas, do not have this spinal curve in the region of the hip, so they can’t maintain an upright position for long.
Bone tissue consists of tiny particles of calcium and phosphorus in a network of collagen fibres. The calcium gives strength and hardness to bones, without which your bones would be like jelly. The collagen gives a certain amount of flexibility. Your bones also naturally contain fluoride, sodium, potassium, magnesium, citrate, plus other trace elements – all helping to hold the calcium and phosphorus building-blocks together.
The body of a 154 pound (70 kg) man contains about 2,5 pounds (1.2kg) of calcium – between 1.5 and 2 per cent of his total body weight.
Bone tissue is a storehouse for your reserves of calcium, containing 99 per cent of the calcium in your body, to be added to or withdrawn from as needed to maintain a balance of calcium in the blood. The other 1 per cent of calcium is found in soft tissues and fluids throughout your body, with the level of calcium in the blood kept fairly constant, as required for muscle contraction, pulse rate and heart contractions, normal blood clotting and brain functioning. When blood levels of calcium become low, calcium is resorted from the bones to help keep up blood calcium. When blood levels of calcium return to normal, bone resorption slows down.
Bones manufacture blood cells; they are tissues with blood vessels, nerve fibres and fluid-filled channels.
like other body tissues, living bone is always being rebuilt, bone remodelling is constantly under way: new bone is formed on the outer surfaces while small quantities of old bone on the inner surfaces are lost through breakdown and resorbed by the body. A bone-making cell is called an osteoblast and in its mature form in the bone it is called an osteocyte, important in the nutrition and maintenance of normal bone. The bone-resorbing cell which is responsible for remodelling and reshaping bones dining growth and repair is called an osteoclast. The delicate balance maintained between these processes is your bone mass – the total amount in your skeleton – a balance that is always changing according to your body needs, affected by heredity, diet, drugs, physical activity, hormones, stress, injury and disease.
Basically, there are two different types of bone tissue: cortical and trabecular. Cortical bone is very solid and dense, and is mostly in the long hard bones of arms and legs. Trabecular bone looks rather like a honeycomb, though much more porous, and this type is mostly in the spinal vertebrae. Each bone has both types of material, with the solid cortical bone on the outside as a shell around the spongy trabecular kind in the interior. The proportions of cortical and trabecular bone vary, depending on which bone it is and which part of the bone. Normally, at the ‘neck’ of the femur it is about 50 per cent cortical, 50 per cent trabecular. The lower part of the radius is about 75 per cent cortical, 25 per cent trabecular. Vertebrae are about 10 per cent cortical, 90 per cent trabecular.
The spongy bone in the inside of long bones forms a system of great strength – sufficient to carry loads with an economy of material. If your bones were all completely solid, your skeleton would be impossibly heavy and yet not have much more structural strength than it actually does. But it is trabecular bone tissue which is most susceptible to changes that occur in bone remodelling.
Bone tissue is formed when the human embryo is only two months old, then built up through childhood and adolescence until you are about twenty-five to thirty years old. When you are young, the build-up of bone growth predominates over breakdown; your bones get larger and more dense up to maturity. In adulthood the two processes are in equilibrium. Peak bone mass for cortical bone is reached at about the age of thirty-five, earlier for trabecular bone. Finally, as you reach old age, the natural process of breakdown prevails more than regrowth. Adults have about 10 to 15 per cent of their bone replaced every year, according to estimates, in a cycle of three to four months, with the formation, maintenance and breakdown of cells. The average life span of bone tissue is about ten years in an adult.
If you live long enough, a certain amount of bone loss is normal – about 0.5 per cent annually. The decline of trabecular bone in the spine can start in the early twenties, particularly among women, with a slight loss in both sexes until the age of fifty. But the rate can be as much as 1 per cent or more a year, so that by the time a woman is fifty, she might have lost 30 per cent of her skeletal mass. Loss of cortical bone starts in the early thirties, mostly from arm- and leg-bones. Bone loss can be accelerated if muscles are inactive, as with paralyzed patients, by prolonged bedrest or being confined to a wheelchair. Certain diseases can also accelerate decline in bone mass.
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