You are a colony of (at least) ten trillion cells, both your own cells and stupefying numbers of guests.1 That is what a human being is — a very large social gathering of cells. Ten trillion is a conservative estimate, but it is one heck of a lot of cells. That is about 200,000 times more cells in a single person than there are people on planet Earth. That’s the sort of number that you really can’t get your head around. Even if you have a very big head.
We are, each of us, a multitude. Within us is a little universe.
How do they walk, I mean? Or even stand up? It’s a bit of a trick for ten trillion cells to do that. Individually, they certainly couldn’t pull it off. A single cell would be have trouble walking six inches for a hot date with another single cell.2
It’s a co-operative effort, obviously. There are probably many committees, subcommittees and review panels involved. But, in spite of the biological bureaucracy, the end product — walking — is quite efficient. Walking is so efficient, in fact, that is constitutes one of the great mysteries of how people work. We3 still can’t make a bipedal robot that can walk, not like us anyway. We humans simply don’t understand the details of our own locomotion.
Rocket science isn’t all that difficult. It’s not brain surgery.
a rocket scientist
But we do have some idea how we at least stay upright. Even I understand it. This superficially simple thing of rising up to a height of six feet or more is an impressive feat for a bunch of cells who are, individually, shorter than a coffee stain. But together they pull it off, and that basic accomplishment is what I’ll focus on here. This is stuff that cells probably learn in cell kindergarten.
Contrary to popular opinion, bones are not really stacked on each other like bricks. We do not really rest on our joints as much as you might think, nor in the way you might think. There is compression and friction in joints due to gravity, but this is not the supportive principle by which we manage to get upright each morning and stay that way.
Vertebrae in particular are not really made for “support.” We are one of the few creatures on Earth with an upright spine, the odd animal out; all other vertebrates on Earth has a horizontal spine, which is much more obviously not built for bearing weight by stacking. In fact, the spine we have is really not particularly well-constructed for verticality. It’s as though we borrowed a tool used by other species for hammering nails, and decided, “Let’s use this for screwing in lightbulbs.” It’s a bit queer, really.4Bare skeletons, as a general rule, fall over very easily.
In fact, rather than being stacked, we are held together and upright by muscles. Bare skeletons, as a general rule, fall over very easily. In the living body, even when we think that we are completely relaxed, our muscles are actually sustaining a constant level of tension — called “resting tone” — that holds joints together.5 When we are anaesthetized, surgeons must be cautious not to dislocate joints,6 because they become quite loose. This constant tension is what we really “stand on” — not bone resting on bone.
This idea, in which the rigid elements of a system “float” in a continuous tension network, was called “tensegrity” (tension/integrity) by Buckminster Fuller. For a long time the ideas were more widely known among architects than biologists.7 Biotensegrity could seem like quite a flaky concept at first glance, and the idea has certainly been co-opted for dubious purposes over the years,8 but “tensegrity biomechanics” and “biotensegrity” are slowly coming into their own as an important way of modelling and explaining biomechanical function.
Bones float in muscle, functioning more like “spacers” than bricks. They provide rigidity for leverage and as foundations for complex arrangements of high-tension wires (muscles and tendons). We are pulled upright, and held upright, in much the same way a circus tent pole is erected and held upright — not because it is resting on itself, but because it is being pulled equally in all directions by ropes. Unlike a circus tent pole, we actually need to move around, so this arrangement is extremely dynamic and active, constantly at work even when we are sitting.
There is one other major principle that keeps us upright: hydrostatic pressure (“hydrostatic” meaning “latin for something”).
Once again, bones are of secondary importance to another more important substance: soft connective tissue. Some aliens on Star Trek: The Next Generation referred to humans as “ugly bags of mostly water.”9 And right they were, at least about the bags and the water. Ugly depends on which bag of water we’re talking about (but let’s not go there).
The point the aliens were trying to make was that humans are mostly water — and everyone knows that, right? More specifically, and less widely known, is that our water is contained in flexible membranes. A bag. The “sack” is made of our connective tissue, intricate layers of a substance somewhat like Saran Wrap that literally holds us together. We have more connective tissue than anything else.10
The water (hydro) inside of us is under constant (static) pressure — hydrostatic pressure. The bag is tight. This is just like putting a tight elastic band around a water balloon: it squishes it into a more elongated shape. If you were to put several rubber bands in a row around a water balloon, it would start to look more like a tube than a balloon. In fact, it might start to resemble, say, a leg. If only it could balance, this “balloon leg” could stand upright — thanks to the pressure of the water inside.
The balloon analogy is surprisingly apt, because our anatomy actually is subdivided into balloon like subdivisions defined by thin, tough layers of Saran-wrap like tissue called “fascia” — same stuff as the gristle in steak. The compartments can swell even like a balloon (which can be quite disastrous).11 An even better analogy is that these are like sausage wrappings, giving the loose contents shape and firmness.
This is entirely how plants stand up. Spinach has no spine, no bones at all, but it still manages to stand up. Unless you don’t water it, and then it wilts — no water, no pressure, no standing up. Speaking of sausages, of course there is one part of the human body, the male human body specifically, that illustrates this principle perfectly.
Our ten trillion cells manage to walk into a bar by applying two major physical principles: biotensegrity and hydrostatic pressure. Our cells build tough membranes to tightly surround compartments of pressurized water, they make rigid bones to act as spacers and points of leverage, and they arrange themselves in complex systems of muscle tissue in order to literally “pull” us into the vertical position and keep us there like a tent pole.
How ten trillion cells order a tall cold one and generate bad pick-up lines is a completely different mystery altogether.
Other interesting reading:
From the article: “… extreme care must be taken in the positioning of the anaesthetised and paralysed child where the normal protection from cervical musculature is lost: extremes of neck rotation in children are dangerous.”BACK TO TEXT