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Osteoporosis   p 4
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2. What is osteoporosis? Link to the Medical Research Council web site
 
2.2. Age related changes in bone
The growth of the long bones (arms and legs) during childhood and adolescence can easily be recorded on a height chart. During this time, the density and size of bone are increasing. The amount of bone increases because, in the millions of BMUs throughout the skeleton, the osteoblasts are building bone faster than the osteoclasts are breaking it down. This process is known as modelling. The bone mass continues to increase until, in early adulthood, it achieves a maximum value, known as the peak bone mass. It remains at this level from about 25 to 35 years; the rate of bone formation is now exactly balanced by the rate of resorption, a process known as remodelling. Then, from 35 to 40 years, the balance shifts to favour resorption and the skeleton begins to lose bone mass (Figure 10).
BMD graph
Figure 10. The graph shows the change in bone mineral density (BMD) with age in women. After the age of 35 – 40 bone mineral density declines in both sexes, but in women the loss accelerates after the menopause.
About Figure 10.
You will see that the vertical scale is labelled ‘Bone Mineral Density (BMD)’ and that the units are g Ca/cm2. They are based on the method, X-ray absorbsiometry, used to study the density of bone in vivo. In this technique, which is explained later, the amount of bone mineral in the path of the beam at a particular site in the body is measured. Then, if the mass is m and cross-sectional area of the beam is a, the BMD is m/a.
After 35 – 40 years bone loss occurs in both sexes, but for women it accelerates during the first menopausal decade. This acceleration is due to the reduced secretion of oestrogen after the menopause. Oestrogen acts to inhibit the action of the osteoclasts. When there is a lack of oestrogen there is an increase in osteoclast activation, so bone is resorbed faster than it is rebuilt leading to a net bone resorption. This effect explains why osteoporosis is more common in women than in men but, although there is no accelerated phase of bone loss, men still have age-related bone loss.
Age related bone loss
Vitamin D deficiency is one of the causes of bone loss in elderly people. To explain this we need to see how the calcium level in the plasma is regulated (Box 2). The concentration of calcium is determined by the amount absorbed by the intestine, the amount excreted by the kidney, and the exchange of calcium with the skeleton.

Vitamin D and PTH are the main regulators of the calcium balance. The production of vitamin D by the skin using sunlight falls as we age and elderly people, particularly if they spend most of their time indoors, tend to become relatively deficient in vitamin D. Since vitamin D promotes the absorption of calcium from the intestine, a deficiency results in a fall in the concentration of calcium in the plasma. The feedback system responds by increasing the secretion of parathyroid hormone (PTH), which in turn causes more calcium to be resorbed from the bone.

Box 1. The calcium balance in the plasma

Calcium is essential for innumerable functions including neurotransmission, hormone action, cellular growth and enzyme action.

The concentration of calcium in the plasma is closely maintained at between 2.15 – 2.55 mmol/dm3, of which nearly half is ionised and the rest bound to proteins or complexed. The skeleton contains approximately 1 kg of calcium. The level of calcium in the plasma is determined by:

Calcium cycle Calcium cycle
Calcium cycle Calcium cycle Calcium cycle Calcium cycle
Calcium cycle Calcium cycle
Calcium cycle Calcium cycle
Calcium cycle Calcium cycle
Calcium cycle
Figure 11. Calcium movement. Roll over the highlighted words to build up the diagram. The numbers on the arrows represent the daily movements of calcium in an adult.

If the level of calcium in the blood falls, a negative feedback system stimulates the parathyroid gland to release parathyroid hormone (PTH) which stimulates the raising of blood calcium levels. It does this by:

  • increasing calcium resorption through the kidney;
  • promoting the absorption of calcium from the gut (through the action of vitamin D);
  • encouraging osteoclastic bone resorption.
 
Question 3
The arrows in Figure 11 show the direction in which calcium is moving.

a) Which hormones favour the movement of calcium from the plasma to the skeleton?

b) How does this explain the acceleration in bone loss which occurs after the menopause?

c) A simple negative feedback system causes PTH to be secreted when the calcium level in the plasma falls. PTH favours the movement of calcium in the direction of three arrows in Figure 11. Which are these?

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