It's common to think of your skeleton as existing in a sort of stasis, as though once you've finished growing, your skeleton merely supports your body and doesn't ever change. However, the skeleton is very dynamic; it exchanges calcium with the bloodstream and is constantly remodeling itself in response to changing demands.
Skeleton
Your bones consist of an organic matrix made up mainly of the protein collagen -- the same one that gives your skin its elasticity -- and an inorganic calcium-based salt. The calcium salt is called hydroxyapatite. Without the inorganic calcium-based matrix, your bones would be very elastic and malleable, rather than rigid and supportive. Without the collagen matrix, however, your bones would be excessively brittle.
Calcium
It's common to think of calcium as nothing more than the mineral you need to maintain your skeleton -- and in fact that's one of its important roles -- but it has other roles as well. Your muscles depend upon calcium both in the muscles and in the blood for contraction and proper function. The heart, for instance, can't beat regularly if you don't have enough calcium in your bloodstream. Calcium also has roles in cell-to-cell communication.
Homeostasis
The word "homeostasis" means "steady state." Your body attempts to maintain all parameters -- calcium in the blood, for instance -- at steady, healthy levels. Your skeleton serves as a source and sink for calcium, and helps maintain blood calcium homeostasis. If your blood calcium is too high, the bones take up calcium and increase in density. If your blood calcium is too low, the bones give up calcium and lose density.
Vitamins and Minerals
Vitamin D and calcium are critical to bone and blood homeostasis. Vitamin D helps you take up calcium from the gut, and without it, you wouldn't be able to absorb or use the calcium in the food you eat. By taking up calcium from the gut, you ensure that there's plenty in the blood, meaning that the bones can take it up from there -- helping them to maintain critical strength and density -- as opposed to having to give up calcium to the bloodstream.
References
- "Human Physiology;" Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology;" Gary Thibodeau, Ph.D.; 2007
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