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Ugly Bags of Mostly Water

The chemical composition of human biology

Paul Ingraham • 10m read
Underwater photo of beautiful deep blue water penetrated by sunbeams, with a column of shiny bubbles.

What’s our chemistry? Everybody knows that we are mostly made of H2O: about 65% of our substance, actually. In 1988 in real life, stardate 1463.9, the crew of the starship Enterprise, under the command of Captain Jean Luc Picard, was once creatively insulted by a dry, crystalline life form: the humans were slandered as “ugly bags of mostly water.”1 The wee crystal beasties certainly pegged us chemically! As for the aesthetics, I suppose it all depends on who’s looking.

Let’s go a little deeper than that, though.

Off to the chemist

Suppose you wanted to make a person from raw elements. What would be on your shopping list?

If you had a ray gun that could reduce a human being to elements, you’d have a whole lot of hydrogen and oxygen from all the water — approximately ten gallons worth — plus a bunch of carbon and nitrogen. You could just barely make a brick out of the remaining couple dozen elements. Here’s the whole order:

Chemical ingredients of the human body
Oxygen 65% Sulfur 0.3
Carbon 18.5 Sodium 0.2
Hydrogen 9.5 Chlorine 0.2
Nitrogen 3.2 Magnesium 0.1
Calcium 1.5 Iodine 0.1
Phosphorous 1.0 Iron 0.1
Potassium 0.4 Everything else 0.1

“Everything else” is aluminum, boron, chromium, cobalt, copper, fluorine, manganese, molydenum, selenium, silicon, tin, vanadium, and zinc. I had a teacher who liked to humble his students by reminding us that we were really just a couple bucks worth of chemicals (which is actually quite untrue2). Funny guy. I’m a very organized couple of bucks worth of chemicals, thanks very much.

But organized how?

The plumbing

All that water needs a bunch of pipes.

This fascinating trivia about chemistry helps in understanding the tissue structures of our bodies. A very great deal of how we are built is related to all that water. Like a civilization made great by aqueducts, wells, canals and sewers, the shape of our tissues is all about water.

Just like nerves infest all tissue, so too is tissue shot through with plumbing. Capillaries are as fine and as numerous as nerve endings, packed into muscle at the staggering density of about 300 per square millimetre, which means that all ten trillion (ish) of our cells are never further than about a fortieth of a millimetre from the nearest blood supply.3 Even the walls of the larger vessels have their own blood supply — pipes within pipes!

Extracted undamaged from our bodies, a standing circulatory system would be a person-shaped pink fog of microscopic capillaries — individually nearly invisible, literally about a third the diameter of the finest human hair45 — surrounding around an impressively complex network of ropier veins and arteries ranging in size from silk threads to the massive aorta, as thick as your wrist near the heart. Meat has even more pipe than it has nerve.

But wait, there’s more (pipes)

In fact, the circulatory system accounts for only about two thirds of the fluid-carrying tubes in the human body. The lymphatic system gets almost no publicity at all — yet there are just as many lymphatic vessels, large and small, as there are veins.

Lymphatic vessels transport a clear fluid which is very similar to blood, but without the red blood cells. Without the cells, lymph is as clear as water, and lymphatic vessels are not pressurized, so it’s not obvious when they bleed — but they do bleed, right along with blood vessels, every time you cut yourself. The lymph just blends into the blood.

The job of the lymphatic vessels, incidentally, soak up excess fluid from between cells and return it to the blood. When they don’t work, you slowly but steadily swell up like a balloon. Swelling, of course, is a feature of many injuries, and therefore knowledge of the existence of lymphatic vessels is significant when healing. Without functioning lymphatic vessels, swelling would be irreversible, resulting in the amazing phenomenon of elephantiasis.

Up your nose with a rubber hose

I used to wonder, before I was biologically literate: what are blood vessels themselves made of, exactly? Were they like tendons, but hollow? Were they like neurons — long tubular cellular pseudopods? Were they, perhaps, built from a rubber-like cellular secretion, somewhat like a garden hose? Knowing nothing, anything seemed possible.

None of the above. The walls of blood and lymphatic vessels are made of layered cells, muscle and connective tissues. A section of a really small vessel, a capillary, is literally made of a single layer of cells, just one or two of them flattened and bent in a circle, blood cells barely squeezing between them, like children crawling between each other’s legs.

It’s also interesting to note that the circulatory system, while impressively branching, is not quite as bewildering as the nervous system. The circulatory system is only ultra-complex at the edges, whereas the centre is relatively simple: a four-chambered pump whose function can be duplicated by a surgically implanted machine. The nervous system, by contrast, is complex both at the edge and at the centre, and the idea of replacing the brain with a machine is quite science fictional!

The necessity of leaking

It’s important to leak. I mean that in more ways than one, of course. This is going to take some explanation, so hopefully I’ve piqued your interest.

Normally, only about 25% of the water that isn’t actually inside our cells is being pumped through these vast networks of pipes. So where’s the rest?

Most of it is simply between cells. I once imagined that it was wet only inside blood vessels, that organs and cells were packed in close together and lubricated but were otherwise “dry as bone” — but of course it turns out that even bone isn’t dry in the human body.

The inside of people is wet everywhere, wet like a rain forest, wet like a Vancouver winter. All the extra water is filling up the microscopic spaces like the water in a sponge. Cells float in a bath of remarkably oceanic fluid, salty and rich in nutrients, dissolved gases, and the waste products of life.

So although the bulk of our bodily fluids are kept moving in pipes, it’s important to understand that they also steadily leak, seep and soak hither, thither and yon. In fact, the small vessels are definitely not water-tight. In your house, such leaky plumbing would require an emergency telephone call to a man with a lot of tools and a steep hourly rate. In your body, you’d have an emergency if you weren’t constantly leaking.

Bleeding, obviously, is a sort of leaking that you don’t want. But otherwise, water constantly moves across many of the membranes and barriers of the body, including the skin (sweat), and this is essential to life. Wherever it goes, water carries molecules and ions and the handful of other elements that we’re made of. On the way to cells, fluids carry nutrient molecules. Going the other direction, fluids wash away the junk molecules — pollutants — produced by the chemistry of life.

And, of course, many of those waste products are constantly being pumped out of our bodies in urine — another leak you wouldn’t want to do without.

So the structure of the circulatory system is intricate and elaborate, but it also has fuzzy, indistinct edges. The distinction between the inside and the outside of a small blood vessel is a loose wall of cells less than a thousandth of a millimetre thick that is literally full of holes. Blood cells stay inside the vessel, but virtually everything else you find in blood moves freely across the boundary.

The spaces between cells are simply filled with fluid, and our cells float in that soup. Anatomy is a swamp.

About Paul Ingraham

Headshot of Paul Ingraham, short hair, neat beard, suit jacket.

I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter., or subscribe:

What’s new in this article?

2020 — Added a substantive footnote about the cost of the chemical ingredients of a human.

2017 — Added a small paragraph about the therapeutic significance of our watery nature.

2004 — Publication.

Related Reading

This article is part of the Biological Literacy series — fun explorations of how the human body works, what I think of as “owner’s manual stuff.” Here are ten of the most popular articles on this theme:


  1. Sabaroff, Robert et al. and ibid and lorem ipsum. “Home Soil,” Star Trek: The Next Generation, Episode #18, Season 1, Original air date: February 22, 1988.
  2. This is something many chemistry teachers have been repeating for decades without fact-checking it. In Bill Bryson’s charming 2019 tour of human physiology, The Body: A Guide for Occupants, he points out that serious efforts to estimate the cost of the ingredients for a human have produced vastly higher price tags. Pure carbon is rather precious, as are several other ingredients like calcium, phosophorus, and potassium:

    Altogether, according to the RSC, the full cost of building a new human being, using the obliging Benedict Cumberbatch as a template, would be a very precise $151,578.46. Labor and sales tax would, of course, boost costs further. You would probably be lucky to get a take-home Benedict Cumberbatch for much under $300,000—not a massive fortune, all things considered, but clearly not the meager few dollars that my junior high school teacher suggested.

  3. Chilibeck PD, Paterson DH, Cunningham DA, Taylor AW, Noble EG. Muscle capillarization O2 diffusion distance, and VO2 kinetics in old and young individuals. J Appl Physiol. 1997 Jan;82(1):63–9. PainSci Bibliography 57058 ❐
  4. Potter RF, Groom AC. Capillary diameter and geometry in cardiac and skeletal muscle studied by means of corrosion casts. Microvasc Res. 1983 Jan;25(1):68–84. PubMed 6835100 ❐
  5. It was surprisingly difficult to compare capillary diameters to spider silk and human hairs. Both hair and silk come in a wide variety of thicknesses, but this fact is routinely ignored. For instance, spider silk will be described as being a tenth the thickness of a human hair. Which spider? Whose hair? I haven’t documented my sources because it’s a trivial, gee-whiz point, but the process of clearing this up was interesting. Turns out that human hair diameter ranges from about 15 micrometres at its finest, all the way up to 200 micrometres at the thickest: a full order of magnitude difference! Capillaries, on the other hand, are more consistent at around 4-6 micrometres: something like a third to a fortieth the thickness of hair, depending on the hair. Definitely smaller! Now spider silk turns out to have a really wide range of sizes. The very thinnest is measured in nanometres, just 10 of them, which is really impressive (nanometres are used to measure things on the molecular scale). At the other end of the range, spiders sometimes pump out silk as thick as 150 micrometres, a relatively gargantuan tenth of a millimetre and about the same size as the heaviest hairs. So, capillaries can be up to 500 times larger than really fine spider silk, or about thirty times smaller than the thickest.
  6. Schipke JD, Pelzer M. Effect of immersion, submersion, and scuba diving on heart rate variability. Br J Sports Med. 2001 Jun;35(3):174–80. PubMed 11375876 ❐ PainSci Bibliography 53699 ❐Immersion under pool conditions is a powerful stimulus for both the sympathetic and parasympathetic nervous system.


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