Ask the three main branches of science what the human body is mostly made from and their answers sound like the punchline to a geeky joke.
“Water,” says the biologist, giving a reply that has surprised generations of schoolchildren who understandably may have expected something more solid.
“Oxygen,” insists the chemist, raising the insubstantiality of our existence a step further.
“Nothing,” retorts the physicist, clearly winning what seems like a competition to make humans disappear in a puff of logic.
But each answer, more implausible than the next, is correct in its own way. Apparently.
Professor Shirley Hodgson, a fellow of the Royal Society of Biology and an expert in the genetics of cancer at St George’s University of London, explained: “The human body is made up of trillions of cells.
“Importantly, all of these cells contain a lot of water, meaning that humans are in fact around 65 per cent water. The water in cells helps with chemical reactions, transports oxygen and waste, and acts as a shock absorber.”
But humans are also not entirely, well, human.
“Some of these cells are our own, and form our organs, muscles and bones, but a surprising number of them are bacteria,” Professor Hodgson said.
“We are host to many millions of bacteria, particularly in the gut, the microbiome.
“Bacteria are very helpful in improving our immunity, and are vital for the digestion of food. Sometimes bacteria can also be harmful, but there is a remarkable mutual benefit from our coexistence.”
Within our cells are “molecular machines” that perform a vast array of different functions.
“In the nucleus is the DNA, the blueprint for all our characteristics, and the cell reads this DNA message to make tens of thousands of different proteins, all of which have important jobs, from acting as hormones to helping form your skin and hair,” Professor Hodgson said.
Various different cells make up organs that work together to make a functioning animal.
In addition to the reproductive, digestive, cardiovascular, respiratory and nervous systems are the less well known lymphatic system, which helps protect the body against pathogens, and the endocrine system, including the thyroid and adrenal glands, which is involved in regulating hormones.
The renal system, including the kidneys and bladder, helps eliminate chemical waste, while our skin, hair, sweat glands and nails protect the body and control its temperature.
But a very different picture emerges if one takes a “purely chemical” point of view.
“The human body is made up of a long list of ‘ingredients’, with the most abundant being oxygen (65% by mass), carbon (18%), hydrogen (10%), nitrogen (3%), calcium (1.4%) and phosphorous (1.1%),” Elisabeth Ratcliffe, of the Royal Society of Chemistry (RSC) wrote in an email.
“There are over 60 elements in our bodies in total, mostly in minute quantities. Some people are surprised at how little of some elements is needed to support life.
“For example iron, which is so important for transporting oxygen around the body, only makes up 0.006 per cent of our chemical composition.”
The typical human body contains miniscule amounts of poisonous materials like mercury, arsenic and even selenium. At high doses, the latter would be fatal. But if about 0.000019 per cent of our body was not made of selenium, we would be dead as it is a key component of healthy thyroid function.
Radioactive uranium, absorbed from the environment around us, is present in the body at 0.00000013 per cent.
“At those levels it’s easily passed through the body, so it’s not going to do us any harm,” Ms Ratcliffe said.
We also contain tiny amounts of precious metals like gold, silver and ruthenium, which is used in electronics.
The RSC once calculated that all the elements contained within actor Benedict Cumberbatch would fetch more than £96,500 if extracted and sold on the open market.
Dr Jess Wade, a physicist at Imperial College London, admitted that while about 99.9 per cent of an atom was “empty space”, it was “cheating” a bit to describe this as the main constituent of humans – as this is also true of all other forms of matter.
Each atom has a central nucleus with orbiting charges called electrons.
“If there wasn’t all the space, we’d be tiny,” Dr Wade said. Physics.org once calculated if all the space was removed, the entire human population would take up the same space as a sugar cube.
“An example of something that has undergone this compression is a neutron star, which is so dense a teaspoon of neutron star would weigh a billion tonnes,” she added.
The reason we are not all squished together is electromagnetic force, which Dr Wade said was “really strong, much stronger than the force of gravity”, which is pretty strong given it is what keeps us stuck to the surface of planet Earth.
“The electrical charges in the atoms of your body repel each other, keeping your body the right shape and size,” Dr Wade added.
It is at this point that high-end physics starts to sound really quite poetic. We are indeed, as Joni Mitchell sang, made of stardust.
But we also have a little starlight too.
“The movement of electronic charges creates magnetism, so the movement of the atoms inside your body could actually generate a magnetic field,” Dr Wade said.
“Starlight is a form of electromagnetic radiation – packets of energy, called photons, travelling at the speed of light.
“Photons can be described as the movement of electric and magnetic fields moving through space. The force that stops us from collapsing is the same one that lets us see stars and galaxies.
“It’s also the same force that helps us move. Electronic signals move through our bodies along a series of biological wires called neurons, and make us scratch our nose, wink – they even give us our memories.”
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