by Mary Hrovat
Most of the atoms in your body are hydrogen atoms. Because hydrogen atoms are so light, however, by mass you are mostly oxygen (60% to 65%). The amount of oxygen in your body varies because most of it is in the form of water (98.3% of your molecules are water molecules), and in the human body, water is always coming or going. You are wet star stuff.
Carbon is the second most abundant element in your body by mass (about 18%). We refer to all life on Earth as carbon-based because all of our biologically important molecules (carbohydrates, proteins, lipids, and nucleotides) contain carbon.
Carbon, oxygen, and hydrogen are three of the four most abundant elements in the visible universe. In fact, six of the elements essential for human life are among the ten most cosmologically abundant elements. It’s important to remember, though, that hydrogen and helium, the most common elements cosmologically, account for more than 99% of the universe we see. All the rest is almost not there. The third most abundant element cosmologically, oxygen, accounts for 0.1%. Our bodies reflect the relative abundances of elements in the universe, to some extent, and on Earth. At the same time, planets and all the life on them are astonishingly rare in the context of all there is.
Hydrogen was made in the Big Bang and is by far the most abundant element. Most other elements are produced in stars. The life story of a star is all about maintaining (or failing to maintain) a balance between gravity’s inward pull and the outward push of energy produced by fusion. Stars begin by fusing hydrogen into helium; later in their lives, as the hydrogen is consumed, the density, and thus the pressure and temperature, inside the star increase, and other fusion processes begin that produce carbon, nitrogen, and oxygen. At later stages, increasingly heavy elements are created. Much of the material making up most stars is eventually released and goes on to form other stars and their planets. And that’s where we come from.
Twenty-three elements are thought to be essential to human life. Six of them make up almost 99% of the human body: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Another five—potassium, sulfur, sodium, chlorine, and magnesium—account for another 0.85%. Twelve trace elements make up the rest.
Oxygen makes up 21% of Earth’s atmosphere and roughly 47% of the crust, where it’s present in water in the soil and oxides of silica, aluminum, and other elements. In fact, oxygen is the most abundant element by mass on Earth. It reacts easily with other elements to form various compounds by oxidation. Fire is a very rapid form of oxidation, and the rusting of metals is a very slow form, in which iron turns to iron oxide. Oxidation in the body can be thought of as a slow fire, controlled by enzymes, that produces energy and heat. Oxidation could be described as the fire in which we burn (with apologies to Delmore Schwartz, who is not wrong when he says that time is the fire in which we burn). It’s a renewable fire, proceeding in countless cycles as long as we continue to eat and breathe, but there’s some evidence that decades of oxidation can take a toll on our health.
Nitrogen is an important component of amino acids and proteins, as well as the heme in hemoglobin, which transports oxygen in the bloodstream. The word amino is derived indirectly from the name of the Egyptian god Ammon. Ammonia (NH3) was given its name because it was obtained from sal ammoniac (salts of Ammon, or ammonium chloride), which in antiquity was obtained from a region of present-day Libya near a temple of Ammon. (Or so the story goes. Actually Pliny the Elder gave the name hammoniacum to a substance produced in that region, but it probably wasn’t actually ammonium sulfide. Nevertheless, the name stuck.) Although the atmosphere is 78% nitrogen, we can’t use the nitrogen we inhale for our biological processes. Instead, we rely on plants to “fix” nitrogen in a biologically available form.
Humans knew about sulfur long before they knew that it was biologically important. Its common name in English was brimstone, and it was associated with hot springs and volcanoes. Bacteria in some hot springs digest sulfides in the water and excrete hydrogen sulfide, which has a distinctive smell (we smell it in rotten eggs, and other sulfur compounds give skunk spray and garlic their distinct aromas). Like those bacteria, you may also emit hydrogen sulfide from time to time if your food ferments a bit in your gut.
Like carbon, sulfur has a structure that’s suitable for forming long chains and rings. It’s a component of the amino acid cysteine, where it forms bonds that link peptide chains, in particular those in keratin, the main protein making up your hair and nails (as well as horns, feathers, hooves, and claws, and the scales of reptiles). Sulfur is also present in the essential amino acid methionine, which is missing from many plant-based protein sources, and in two B vitamins—thiamine and biotin.
You’re also somewhat salty star stuff. As a percentage of your mass, your sodium and potassium contents are relatively small. However, these essential elements are responsible for some of your most interesting and essential chemistry. They’re electrolytes, which means that they become ions in solution. The electric charges they carry are crucial to the small differences in electrical potential between the inside and outside of cells that drive nerve impulses and muscular contractions.
There is more potassium than sodium inside cells, and more sodium than potassium outside. According to one hypothesis, the reason is that the first cells developed in the Precambrian oceans, which had a much higher potassium/sodium ratio than the oceans do today. These first cells were relatively leaky, so their internal environment would have been very similar to that in the ocean where they developed, and cell processes developed in the context of this environment. As cells became more sophisticated, they developed mechanisms for maintaining this primordial environment to keep themselves running.
One of these mechanisms is the sodium–potassium pump, a protein that transfers sodium to the outside of the cell and potassium to the inside. The resulting gradients in the concentration and electrical charge make it possible for nerve signals to be transmitted. The sodium–potassium pump is powered by adenosine triphosphate (ATP, the molecule pictured above). ATP is the coin of the realm for energy transfer between cells and provides a link between the processes by which the body builds molecules from raw materials—anabolism—and those by which molecules are broken down to obtain energy—catabolism. About one-third of the ATP that your cells make is used to run sodium pumps throughout your body.
Calcium is one of the most important minerals in your body. Your bones are made up mainly of the mineral apatite, or calcium phosphate. Your tooth enamel is the hardest material in your body and consists mainly of hydroxyapatite, a crystalline form of the apatite. Calcium is also an electrolyte, and calcium pumps, like sodium–potassium pumps, maintain an electrochemical gradient that enables calcium ions to perform many crucial tasks.
Your body can store some elements. Your bones represent a reservoir of calcium and phosphorus, although it’s not a static reservoir. Calcium is released from the bones as needed (to maintain plasma calcium levels, for example) and is later redeposited. In fact, your bones are continually being remodeled by resorption and redeposition, which collectively replace about 10% of your skeleton each year.
Other elements are continuously depleted and renewed: hydrogen and oxygen, for example. Some organic compounds are used, broken down, and renewed all the time. In the process of cellular respiration, energy from oxygen molecules and nutrients in your food is turned into ATP, which supplies the energy for everything you do. Your body produces, uses, and recycles roughly its own weight in ATP every day.
Similarly, calcium, carbon, nitrogen, oxygen, phosphorus, and sulfur all move through the environment in cycles. Oxygen, for example, is excreted into the atmosphere by plants and taken up by animals; in addition, it cycles into and out of the rocks in the crust, although at much slower rates. Calcium is weathered from rocks and carried to the oceans, where it is fairly abundant. There it is used by marine animals to form shells or exoskeletons. Because of its connections to the carbon cycle via the sea, the calcium cycle is crucial to Earth’s climate.
The phosphorus cycle is somewhat unusual in that there is very little phosphorus in the atmosphere. It circulates among plants, animals, and microbes and eventually makes its way to the oceans. It spends a very long time in sediments on the ocean floor, where it’s unavailable to living things. It cannot return to the biosphere until the sediments are brought to the surface by geological uplift, so this part of the cycle can last for hundreds of millions of years. Because phosphorus is essential for life, this cyclic sequestration may be a limiting factor controlling the populations of land and sea mammals.
As satisfying as it is to contemplate the grand sweep of cosmology and human evolution, the details are what make these stories so endlessly fascinating. For example, while I was researching this essay, I learned that although aluminum is abundant on Earth, it’s not essential for life, but cobalt, which is less abundant and potentially toxic, is essential for metabolism in not just humans but all animals. It’s an essential trace element found in cobalamin, also known as vitamin B12. I also learned that magnesium, which is essential in the human body for various processes, gives seawater its slightly bitter taste.
Far and away the most interesting thing I learned, though, has to do with potassium. Most potassium is in the form of potassium-39, a stable isotope. However, about 0.012% of the potassium in your body is the radioactive isotope potassium-40, which is the source of most of your internal background radiation. In a 70 kilogram (150 pound) human, 4000 to 5000 potassium-40 nuclei decay per second. Most of them decay to calcium-40, and an electron and antineutrino are emitted. Around 11% of the time, potassium-40 decays to argon-40, and a neutrino and gamma ray are emitted.
Although it may be a source of natural mutations, the decay of potassium-40 in your body is nothing compared to what’s going on in Earth’s core. There radioactive potassium, along with thorium and uranium, is an important source of radiogenic heat, which drives plate tectonics and arguably helps make life on Earth possible. Still, think of potassium-40 the next time you have an idle moment, and imagine your body randomly emitting antineutrinos and the odd gamma ray, which zips off into the universe at the speed of light, a small return for your brief sojourn as star stuff.
You can see more of my work at maryhrovat.com.