Question: How many blood cells are in our boddy

Ciarán O'Brien answered on 7 Nov 2014:

It’s impossible to get an exact number because your body is constantly making new blood cells (at the rate of about 2.5 million PER SECOND!) to replace old blood cells that die off and get eaten by your body’s defences.

We can do a quick calculation to get a rough estimate, though. For, say, the average 9-10 year old, there are usually between 3.8 and 5 million red blood cells per microlitre of blood (That’s 1/1000 mL, or a millionth of a litre). We know this because medical researchers counted the cells in tiny drops of blood from hundreds of children over the years. Doctors also know that the average adult has 5L of blood, so a child would have about half that, so let’s say you have 2.5L of blood, or 2,500,000 microlitres.

So doing the maths:
3,800,000 x 2,500,000 = 9500,000,000,000

5,000,000 x 2,500,000 = 12,500,000,000,000

So at any given time you have somewhere between 9.5 and 12.5 TRILLION red blood cells. If you factor in white blood cells and macrophages, those numbers are probably closer to between 10 and 15 trillion blood cells in total. Adults would have about twice as many as that. Cells are really, REALLY small.

Since starting my PhD, I learned that there are actually more bacterial cells in your body than human cells (Bacterial cells are much, much smaller than human cells, so you can pack a lot more of them in the same space). One of the reasons we look at gut bacteria is because there are just so many; There are maybe 1,000 times more genes from bacteria in your body than human genes. They have to be having an effect on us.