All your little bitty bits

kw: book reviews, nonfiction, science, atoms, popular treatments

I weigh a bit over 200 pounds, about 91 kg. Dry me out, and the residue would weigh about 85 lbs or 38+ kg. One could then (someone with a sufficiently strong stomach) divide up the dry mass into bone and muscle and so forth. But what is my atomic composition? If you also have that question, you'll find out in Your Atomic Self: The Invisible Elements That Connect You to Everything Else in the Universe by Curt Stager.

You can also get an answer of sorts from this table, but what fun is a table? In Your Atomic Self we find out, not just the amounts of the major chemical elements in us, but something about where they came from, how long they spend as a part of us, (not as long as you think!), and where they go when they leave us, or, ultimately, when each of "us" leaves our body behind.

For example, you and I are 2/3 oxygen, by weight. Most of that is in the water that makes up roughly 50-60% of our total weight (depending on bone/muscle/fat ratios). Where did all that oxygen come from? Surprisingly, the water we drink doesn't all become body water. Much of it is dissociated by various processes and some exits the body, rather soon, in our breath as carbon dioxide. The foods we eat all contain lots of oxygen, so some of that winds up in our body water, some in our tissues (fats and bone contain lots of oxygen), and some also gets breathed out as CO₂.

Though our lungs may have a capacity of half a gallon to a gallon (2-4 liters), we seldom breathe this deeply; less than one liter (1 quart) per breath is typical when we are at rest, which is nearly all the time for sedentary Westerners. About 20% of the air we breathe is oxygen, and all but 1% of the rest is nitrogen, which contributes to air pressure, but is not chemically active in this form—for which we ought to be very grateful! We use about 1/3 of the oxygen we breathe in and exhale the rest (which is why mouth-to-mouth resuscitation is effective). But at 20 breaths per minute, we allow nearly 30,000 liters of air in and out of our lungs daily, including about 5,800 liters of oxygen, of which about 2,000 liters enters our blood stream, and an equivalent amount, attached to carbon, exits as some 2,000 liters of CO₂.

Did you ever realize that a liter of CO₂ weighs 37.5% more than a liter of O₂? You'd lose a lot of weight if you did nothing but breathe all day! (Not really a lot; about a kilogram.) But as the author writes, there is more to the story than that, and it is not only the amount of water you take in and excrete.

Not all molecules of oxygen, and not all molecules of CO₂, are the same. Most oxygen is the isotope O-16, but a small amount (0.2%) is a heavier isotope, O-18 (there's a tiny amount of O-17 also). Then, most carbon is C-12, but ~1% is C-13 and about one atom of carbon in a trillion is C-14, a radioactive isotope produced mainly by cosmic rays. So while most O₂ molecules weigh 32 AMU (atomic mass units) and most CO₂ molecules weigh 44 AMU, the weight of stable O₂ can range up to 36, and that of stable CO₂ can range up to 49, while rare C-14·O₂ molecules can weigh between 46 and 50 AMU.

Why should that matter? The proportions of different molecular masses of these two substances can reveal the source of your diet and the air you've been breathing. Similar mass differences in water exist not only because of oxygen isotopes, but also hydrogen isotopes H-2 (deuterium) and H-3 (tritium). Physical processes such as evaporation tend to leave behind heavier molecules, and chemical processes, including photosynthesis in plants, prefer one isotope over another. This preference is not absolute, but it is enough that some kinds of foods have less O-18·O-16 in them compared to others, and so forth.

We also learn that each element connects us to the stars and to all life, each in its own way. Most hydrogen is primeval, created in the Big Bang, but some very small amount arises by spalling from processes such as cosmic ray collisions with atmospheric atoms. No elements heavier than lithium are primeval, but were created in extra-large stars that later exploded as supernovae, scattering them into the universe. So the hydrogen in you is billions of years older than your other elements…although a very few H atoms might be just a few days old! And all that oxygen and CO₂ that you've breathed out? Something or someone else (a great many "else's") are breathing it in, at least some of it, right now.

Each chapter of the book discusses primarily one element, or sometimes two. So, while we are sometimes told most life is composed of CHON (carbon, hydrogen, oxygen and nitrogen), the chapter on sodium and potassium reveals why your nerves wouldn't work without them, nor without calcium (the chapter after). Calcium isn't just about bones, and sodium isn't just about food tasting good. They are essential to second-by-second life processes. As it happens, one of the most essential is phosphorus. So much so, that this element may determine just how many humans Earth can support. It is really rather rare for an element that must make up 1% of your body's weight! That is more than ten times its abundance in the Earth's crust in general, but thousands of times as abundant as the 'available P' in the biosphere. Hmmm. I've predicted that coming wars will be over water. Perhaps later wars will be waged for access to phosphorus bearing minerals…if indeed those come later.

Though the book discusses 9 elements (I didn't mention iron above), that leaves a couple dozen "useful" elements in our makeup, so another book is not out of the question. I'd like that. I really enjoyed Your Atomic Self.