Why Do We Have Bones?
The bony skeleton is a remarkable organ that serves both a structural function—providing mobility, support, and protection for the body—and a reservoir function, as the storehouse for essential minerals. It is not a static organ, but is constantly changing to better carry out its functions. The development of the bony skeleton likely began many eons ago, when animals left the calcium-rich ocean, first to live in fresh water where calcium was in short supply, and then on dry land where weight bearing put much greater stress on the skeleton. The architecture of the skeleton is remarkably adapted to provide adequate strength and mobility so that bones do not break when subjected to substantial impact, even the loads placed on bone during vigorous physical activity. The shape or structure of bone is at least as important as its mass in providing this strength.
The skeleton is also a storehouse for two minerals, calcium and phosphorus, that are essential for the functioning of other body systems, and this storehouse must be called upon in times of need. The maintenance of a constant level of calcium in the blood as well as an adequate supply of calcium and phosphorus in cells is critical for the function of all body organs, but particularly for the nerves and muscle. Therefore, a complex system of regulatory hormones has developed that helps to maintain adequate supplies of these minerals in a variety of situations. These hormones act not only on bone but on other tissues, such as the intestine and the kidney, to regulate the supply of these elements. Thus one reason that bone health is difficult to maintain is that the skeleton is simultaneously serving two different functions that are in competition with each other. First, bone must be responsive to changes in mechanical loading or weight bearing, both of which require strong bones that have ample supplies of calcium and phosphorus. When these elements are in short supply the regulating hormones take them out of the bone to serve vital functions in other systems of the body. Thus the skeleton can be likened to a bank where we can deposit calcium or phosphorus and then withdraw them later in times of need. However, too many withdrawals weaken the bone and can lead to the most common bone disorder, fractures.
Both the amount of bone and its architecture or shape are determined by the mechanical forces that act on the skeleton. Much of this is determined genetically so that each species, including humans, has a skeleton that is adapted to its functions. However, there can be great variation within a species, so that some individuals will have strong bones and others will have weak bones, largely because of differences in their genes (Huang et al. 2003). Moreover, bone mass and architecture are further modified throughout life as these functions and the mechanical forces required to fulfill them change. In other words, bones will weaken if they are not subjected to adequate amounts of loading and weight bearing for sufficient periods of time. If they are not (such as in the weightless condition of space travel), rapid bone loss can occur. In other words, as with muscle, it is “use it or lose it” with bone as well. Conversely, the amount and architecture of the bones can be improved by mechanical loading. However, as described in Chapter 6, some types of exercise may be better than others in strengthening the skeleton.
To respond to its dual roles of support and regulation of calcium and phosphorus, as well as to repair any damage to the skeleton, bone is constantly changing. Old bone breaks down and new bone is formed on a continuous basis. In fact, the tissue of the skeleton is replaced many times during life. This requires an exquisitely controlled regulatory system that involves specialized cells that communicate with each other. These cells must respond to many different signals, both internal and external, mechanical and hormonal, and systemic (affecting the whole skeleton) and local (affecting only a small region of the skeleton). It is not surprising that with so many different tasks to perform and so many different factors regulating how the skeleton grows, adapts, and responds to changing demands, there are many ways that these processes can go astray.