5.3 Boffin questions

How many cells are in your body?

  1. You may have seen various estimates of the total number of cells in the human body in textbooks and online. A common estimate is 30,000,000,000,000 (30 trillion or 3 × 1013) cells. Have you ever thought about how these numbers are estimated? See if you can come up with a similar estimate for your own body.
    Hints: First, estimate your volume. You can assume 1 kg takes up about 1 l. Now convert your volume to m3.
    Mammalian cells typically have a volume ranging from 103 to 104 µm3.
    How many cells is that?
    Your answer should be a range based on the volume range. Does the range encompass the common estimate stated above?
  2. How much of us is human? Based on Sender et al. (2016), what percentage of our body weight on average is composed of bacteria?
Side Bar – Take care when writing down units as it is easy to make a mistake especially when the difference in the symbols is slight. The goal should always be to avoid any ambiguity! For example, nm refers to nano metres or 10-9 metres while nM refers to nano molar or (10-9 moles per litre). Similarly, it is each to confuse nM with mM.

 

A more accurate estimate

Your estimate above is a great starting point to think about this problem but is likely to be very inaccurate due to the assumptions made regarding both the weight-to-volume ratio and range of volumes for mammalian cells.

Different cell types of different volumes will account for different proportions of the total cell population. Erythrocytes (red blood cells) are much smaller than most cells and there are typically about 5–6 × 1012 cells per litre of blood. With a blood volume of around 5 L on average that means there are around 2.5–3 × 1013 erythrocytes in an average human. This makes them the largest contributor to the overall human cell count, as shown in the next figure.

 

A figure showing the relative contribution of the major cell types tot the total number of cells in the human body. Erythrocytes are clearly seen as the most abundant with between 25 x 1012 to 30 x 1012 cells in total making up 84% of the total cell count. The next most abundant cell type are platelets which make up 4.9% of the total cell count.
Figure 5.1: The relative contribution the major cell types to the total cell number in the human body (‘The distribution of the number of human cells by cell type’ by Ron Sender, Shai Fuchs and Ron Milo from Revised Estimates for the Number of Human and Bacteria Cells in the Body used under CC BY 4.0. https://doi.org/10.1371/journal.pbio.1002533)

 

Sender et al. (2016) have attempted a more accurate estimate of the total cell number by estimating the relative contribution of each of the major cell types. It is worth looking at their paper to see how they made their calculations. They have also estimated the total number of bacteria in an average human, which has been the subject of a lot of controversy.

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