Carl Zimmer has an interesting article about attempts to estimate how many cells the average human body contains. It depends on how you do it and previous estimates over the past few centuries have ranged widely from two billion to 200 million trillion cells.
Now that we have more sophisticated tools of measurement, one way to do this is by using the average mass of a cell (which turns out to be about one nanogram) and this gives a figure of 70 trillion cells. Another is to use the average volume of a cell (about 4 billionth of a cubic centimeter) and this gives 15 trillion cells. But there are other things to consider as well.
So if you pick volume or weight, you get drastically different numbers. Making matters worse, our bodies are not packed with cells in a uniform way, like a jar full of jellybeans. Cells come in different sizes, and they grow in different densities. Look at a beaker of blood, for example, and you’ll find that the red blood cells are packed tight. If you used their density to estimate the cells in a human body, you’d come to a staggering 724 trillion cells. Skin cells, on the other hand, are so sparse that they’d give you a paltry estimate of 35 billion cells.
That’s pretty huge range. To narrow it down, they took into account the fact that cells in different parts of the body and different organs have different numbers, masses, volumes, and pack in different densities.
After trying to take all these things into account, they arrive at a figure of 37.2 trillion cells.
Does anything actually depend on knowing this number precisely?
Curiosity is justification enough to ponder how many cells the human body contains, but there can also be scientific benefits to pinning down the number too. Scientists are learning about the human body by building sophisticated computer models of lungs and hearts and other organs. If these models have ten times too many cells as real organs do, their results may veer wildly off the mark.
The number of cells in an organ also has bearing on some medical conditions. The authors of the new study find that a healthy liver has 240 billion cells in it, for example, but some studies on cirrhosis have found the disease organ have as few as 172 billion.
It is nice to be able to figure this number out even if it did not have any usefulness.