You might be surprised to know that until very recently, no one really knew the answer to this question. Estimates have ranged from 5 billion to 200 trillion (and that’s just the realistic guesses), but that’s not much of a useful range to go off of. Short of disassembling a human one cell at a time, how do you get at it?
Part of the problem is that all tissues are different, which seems obvious, but seriously complicates the problem. From minuscule sperm and diminutive red blood cells to enormous eggs and elongated neurons, all of our hundreds of cell types have a different average size, in both volume and mass. A pound of brain has different census results than a pound of muscle.
Averages are better, but still not enough. While one cell weighs on average one nanogram, it’s hard to extrapolate that mean to the varied sizes, heights and shapes that make up the rainbow of human forms.
Via Carl Zimmer, I heard about a paper by a team of European biologists that gives us our best estimate yet. The scientists broke the human body down into dozens of discrete units, organized by size, type, and tissue. Just like counting a jar of jelly beans would be much easier after sorting them into their various colors.
The answer? You’re the sum of about 37 trillion intricately cooperating, elegantly evolved parts, from blood to bone, epithelium to astrocyte, hair follicle to hepatocyte. It’s a wonder that we get anything done at all. Way to go, team.
Of course, none of that takes into account your bacterial bunkmates, the 100 trillion or so microbes that call you home.