When we think about our bodies (if we think about it at all), we usually think of organs like the brain, the heart, the lungs, our digestive system, or our skin. The skeleton is often an after-thought. yet, our skeleton, and the bones that it comprises, are as remarkable for keeping us functional as the other organs and tissues in our bodies. This post explains why.

As living beings, we are both floppy and rigid, both hard and soft. Our bones (along with our muscles) are the primary reason why we do so many things with such grace and fluidity. Example: when our knees lock into position, we can stand. When they unlock, we can bend our legs up to 140 degrees, allowing us to sit and kneel and move about, day after day, for decades. When you think about how ploddingly today’s robots walk, how tippy they are on stairs or uneven ground, how hopeless they are in trying to keep up with the agility of a three-year old at a playground, you begin to appreciate what an accomplished creation we are.

Each of us has approximately 206 bones. But they aren’t evenly distributed. Your hands and feet account for more than half of the bones in your body. The 52 bones in our feet alone make up a quarter of all your bones. We have evolved to be agile with both our hands and feet, attributes that explain the geographical dominance of our species.

Our bones are strong as they must be. “Bone is stronger than reinforced concrete,” says Dr. Ben Olivere, a researcher at the University of Nottingham Medical School. “Yet they are light enough to allow us to sprint.” All your bones together weigh no more than 9 kg, yet they are so arranged that we can withstand up to a ton of compression.

Remarkably, bone is the only tissue in the body that doesn’t scar. Dr. Olivere adds: “When you break your leg, after it heals, you cannot tell where the break was. Even more remarkably, bone will grow back and fill a void. “You can take up to 30 cm of bone out of a leg, and with an external frame and a kind of stretcher, you can have it grow back.” Dr. Olivere says. “Nothing else in the body will do that.” Bones, in short, are super resilient.

What explains the remarkable toughness of our bones? The answer lies in their composition. About 70 percent of a bone is inorganic matter, mainly calcium salts. The other 30 percent is mostly collagen, the most abundant protein in the body.

Calcium is an essential element that plays numerous biological functions in the body. In bones, they are present in the form of calcium salt known as HA (for hydroxyapatite). HA has low solubility, so it does not easily dissolve in water. Crucially, it provides the skeleton strength and structure. It also plays an important role in bone regeneration.

Collagen (the other 30% of bone composition) is a very adaptable protein. Collagen makes the white of the eye and the transparent cornea. In muscles it forms fibers that behave like ropes – strong when stretched but collapse when pushed together. In hard structures like bones and teeth, it twins with HA which is strong when compressed, whici is why your bones and teeth retain their rigidity all the time.

As if all this isn’t enough, our bones also manufacture blood cells (red blood cells are formed in the red bone marrow of bones), store chemicals, transmit sound (in the middle ear) and even help to bolster our memory! The last function was discovered recently when a geneticist at Columbia University, Gerard Karsenty, realized that osteocalcin, a hormone produced in bones, is responsible for a large variety of regulatory functions, including keeping our memory in working order, a discovery which may explain why regular exercise not only helps build stronger bones but may even stave off Alzheimer’s disease [1].

Make no bones about it – your skeleton is a marvellous piece of engineering. It does more than keep you upright. As well as providing support, your bones help you move and carry load, protect your interiors, manufacture red blood cells, transmit sound, and possibly bolster your memory and keep your cognitive ability in good working order.   

Notes:

This post is adapted from Bill Bryson, The Body, Penguin Random House UK, 2019.

Further study

[1] “Anne Lennon, “Low bone density, poor bone health linked to higher dementia risk”, Medical News Today. Link: https://www.medicalnewstoday.com/articles/low-bone-density-linked-to-higher-dementia-alzheimers-risk