A not-so-pleasant event recently reminded me of this fact. While rushing around a few weeks ago, I tripped and fell hard in a parking lot (gracefully, I'm sure). Two things kept me from bashing my face into the asphalt: my kneecap and my right hand (my left arm was wrapped protectively around my laptop case). Though nothing was broken, there was damage: a bruised patella and sprained wrist (although, since I didn't get x-rays, there was the possibility of minor cracks as well).
Having a biology background and being generally interested in how the body works... whenever I'm sick or injured, I often contemplate the amazing ability that the body has to heal. Bones, for example, are not inert, lifeless scaffolds on which our muscles and organs hang. They are living tissues -- a fact that usually escapes our thoughts until one is broken and then heals (or doesn't). And here's the punch line: Bone health requires the proper balance of creation and destruction.
Though I won't go into great detail about the process of bone formation and resorption, I will mention two kinds of cells -- mostly because of the poetry of their names: osteoblasts and osteoclasts. The former are responsible for building bone; the latter are diametrically opposed, digesting bone so that, for example, calcium can be used elsewhere. "Osteo" comes from the Greek word for bone (think osteoporosis: "porous bone"). "Blast" refers to being embryonic -- ie, formative and creative, whereas "clast" means broken.
When bones aren't actively trying to heal, the functions of these two types of cells must be balanced. If the bone-absorbing cells outperform the bone-depositing cells, brittle bones will result. The other way around will result in excessive deposition of bone, which also is problematic. For bone to heal after an injury, the balance must temporarily shift to favor bone deposition. This also is true in children, whose bones are actively growing. But those years favoring the osteoblasts are ultimately balanced by those marking the other extreme of life, when the osteoclasts are slightly dominant.
I could wax poetic about calcium dynamics and how they're affected by diet and physical activity, but that may put most of you to sleep. Instead, I'll just leave you to think about your busy bones and the fine balance of creation and destruction.