MadSci Network: Anatomy

Re: How many cells does a human lose every second?

Date: Wed Feb 7 19:07:19 2001
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Children's Hospital Oakland Research Institute
Area of science: Anatomy
ID: 978577566.An

Hi Yan,

I'm sorry that I have taken so long to answer your question, but it VERY specific, and I wanted to try to figure out the best answer that I could.

First of all, lets take a look at the scope of your question. To start, we have to think about how many cells we have to start with. Our bodies are made of cells of course, but no one has been able to sit down and count each and every cell to see exactly how many go into making a person. We have to make estimates based on the size and distribution of the different types of cells, and the mass of an adult human body (in this case, the adult male body). These estimates lead us to conclude that, each of us has on the order of seventy-five to one hundred trillion cells that make up their body.

However, not all of those cells are what you would call, 'human' cells (i.e. cells that contain your own genetic material). About 40 trillion of your cells (comprising almost 50% of your body's cell count) are bacterial cells that live in your digestive system, primarily in your large intestine. However, these bacterial cells are much smaller than the cells that make up your body, so more of them can fit in a small space.

Of the remaining ~50% of your body's cell count, only about 10% (or ~4 trillion cells) make up the solid tissues that we think of when we think of the human body (muscles, spleen, kidneys, bones, brain, stomach, skin, etc.). The remaining 45% percent of the cells in your body are blood or lymph cells of some sort that are not associated with any solid tissue. These cells actually comprise most of the cells in your body. There are approximately 30 trillion Red Blood Cells, 2 trillion Platelets, and 500 million White Blood Cells in your Circulatory System. In your Lymph system there are about another trillion lymphocytes and immune related cells. That makes a total of about 38-40 trillion cells. Naturally, most of the MASS in your body is comprised by the 4 trillion cells that make up your solid tissues, especially your muscle and skeletal cells.

Now, the process of staying alive is a constant balance between losing cells and making more cells to take their place. When we are young and growing, we are making many more new cells than we lose existing cells, and when we reach really advanced ages, or when we get sick and start to die, we are losing more cells than we are making new cells to replace them. Because every person is different, and because we are all at different ages, it seems like it would be difficult to determine how many cells we lose every second, because there is no such thing as a model or average person. However, I am willing to try and make an estimate, as long as you realize that it is a very rough approximation.

First of all, I think I will focus only on those cells in your body which are your own. That means that I won't count the 40 trillion bacterial cells that live in body. However, as you note above, most of the cells in your body are Red Blood Cells (aka Erythrocytes, or just RBCs). There are about 30 trillion of these, and we know that they have a lifespan of only 120 days. That means that every 120 days, your body has gone through 30 trillion RBCs. That doesn't mean that all 30 trillion of them are made at the same time, or that they all die at the same time, but at any given time you can be certain that all of the RBCs you had 4 months ago are gone.

So, in 120 days, there are about 10.4 million seconds (10,368,000 to be exact). That means that in any given second, 30 trillion / 10.4 million or 2.89 million RBCs die. So, our estimate for RBCs is about 3 million cells lost per second, or less than the number of RBCs found in one cubic millimeter (aka one microliter) or blood.

Since most of your other cells (the cells in your solid organs) don't turn over that rapidly, the actual rate of loss will probably not be much higher than that when you consider it over the entire healthy portion of your life.

We can calculate a low estimate by thinking about the issue in an entirely different manner. The atoms in your body go through a complete turnover about once every seven years. As with the loss of RBCs, this does not mean that your body is rebuilt miraculously on your 7th, 14th, 21s, 28th, etc. birthdays. Rather at any given time, most if not all the atoms that were in your body seven years ago will have been lost through metabolic turnover. So, if you consider that replacement of all of the atoms in a cell to be the 'loss' of a cell (a loose definition at best), then in any given second you are lose ~40 trillion / (7 * 32 million) or 181,000 cells on average.

So, to answer your question, the number of cells that an adult male human loses per second is somewhere between 200,000 and 3,000,000 cells per second, and is probably closer to 3,000,000. Still since these are very rough average estimates, and since the difference between 200,000 and 3,000,000 is only one order of magnitude (one factor of 10 times), so you would be safe saying anywhere around 1,000,000 cells per second. When you consider that you have 40-50 trillion of your own cells in your body, that means that on average one cell out of every 40-50 million or more dies each second. Considering it that way, it doesn't seem like that much at all.

I hope this helps you.

If you want to know more about where the estimated numbers for the number of cells in a body comes from, check out Chapter Eight of the Nanomedicine web site. This is an online version of Nanomedicine by Robert Freitas.

For information about Red Blood Cell turnover, take a look at this Red and White Blood Cell page on the Virtual Hospital web site.

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