Page 21 - ASOP ROT Study Guide

Illustration showing how a bone develops and grows

Even though bones stop growing in length in early adulthood, they can continue to increase in
thickness or diameter throughout life in response to stress from increased muscle activity or to
weight. The increase in diameter is called appositional growth. Osteoblasts in the periosteum
form compact bone around the external bone surface. At the same time, osteoclasts in the
endosteum break down bone on the internal bone surface, around the medullary cavity. These
two processes together increase the diameter of the bone and, at the same time, keep the bone
from becoming excessively heavy and bulky.

4.3.3 Bone ageing and disease

Although the processes involved in controlling bone growth and the maintenance of a balance
between bone formation and breakdown, are known in some detail, they are not fully understood.
Ageing is an important factor, as both men and women lose bone as they get older. But the rate of
bone loss after the menopause is much faster in women than in men in the same age range, so we
know that sex hormones are also important. The bone wasting disease known as osteoporosis
occurs more frequently in post-menopausal women than in men of a similar age. This disease
results in a general loss of bone, leading to weaker bone which is more prone to fracture. Exercise in
old age helps to reduce this effect, by stimulating bone growth, and so being elderly is not an excuse
for physical rest! When we are older our bones will be weaker because of the loss of bone and
trabecular structure, but otherwise older bone generally performs as well as it used to in most
respects. For instance, if broken, older bones heal as well as younger bones, although not of
course as well as growing bone.

Sometimes bones maintain their organic component but not their inorganic component, so that the
bone is not hard. This is known as rickets in children and osteomalacia in adults. It is generally
caused by a shortage of vitamin D. It is mentioned here because it is a separate disease and should
not be confused with the far commoner osteoporosis.