Our bones are pretty important to us. Without them we’d be unable to do much other than dance ‘the worm’. But what is it inside bones that allow us to do all these things and how can we keep them healthy?
To understand what makes bone what it is, we should first ask ourselves what it does. Along with being an amusing synonym for an erect penis, bone has three major roles. The first is support, providing the scaffolding which holds up the body. The second is protecting all the important and easily damaged stuff in the body. The third is to allow movement by giving the muscles something to attach to and work against.
In essence, all of these jobs involve withstanding forces, either tensile (pulling forces like those from muscles) or compressive (crushing forces like supporting the weight of the body above). To resist these forces the bone material needs to be hard enough to maintain its shape but tough enough to absorb the forces without breaking.
Structures that resist forces rely on four basic classes of material; metals, ceramics, polymers and composites. Unfortunately our bodies are unable to internally produce steel girders so we can strike metals off that list. While ceramics include non-biological materials like cement and glass, it also includes mineral crystals which the body produces. I’ve previously mentioned one such crystal, bioapatite (often referred to as hydroxlapatite), which forms a material that is very hard like glass. However, as you can see if you drop a glass from a height, its hardness means it cannot absorb much energy and instead it breaks easily. So a ceramic type material won’t do the job.
A polymer is made from long chains of the similar molecules like sugar (forming cellulose) or the amino acids in protein (silk and collagen). The human body mostly uses collagen with the amino acid chains arranged in fibres lined up parallel to each other. They don’t stretch very much but they are held together strongly and are difficult to break when under tension. However, collagen is kind of floppy and so it is unable to deal with compressive forces.
That leaves us with composite materials, which as the name suggests, are made up of combination of ceramics and polymers and provide resistance to both tension and compression. Bone is a combination of the mineral crystals of bioapatite suspended amongst the long collagen fibres. Together they create a substance both hard and tough like another composite material, fibreglass (made up of a glass ceramic and a resin polymer). It is this composite that perfectly suits the roles that bone plays in the body.
Next time I’ll tell you what happens when bone lacks bioapatite or collagen and a cool way to see the effects for yourself at home. For now, I’ll quit before my immaturity takes over and I start mentioning the penis references I think of when writing words like bone, hard and floppy.
I used my old university biophysics notes and physiology textbook, but Wikipedia offers a decent explanation provided no-one has been messing with the entry.