The human body is a very complex machine, consequently its study is not so simple: it requires time, passion and willpower. But in studying as much as possible, one can come across “curious”, interesting and, often, astounding numbers.
Many of you will know the number of bones in the human body, 206; the less they will know the number of muscles (650). Today I show you those surprising numbers that I discovered during my studies and how these can help us understand how complex is the study of the human body, its functioning and the difficulties in research.

How many cells do we have in our body?

Let’s start with cells, which are the smallest functional biological unit in an organism. There are many “curious” numbers when it comes to cells, starting with their total number.
Assuming that this value can only be estimated, and that, of course, a person’s size will influence the result, the human body is estimated to be composed of over 37 trillion cells.
Let’s analyze this number: in scientific notation it is written 3.7*10¹³, but normally it would be written 37,000,000,000,000. If they were euros and we spent 1,000,000 a day, it would take us over 101 years to spend it all.

How is it possible that there are so many?

The answer is “simple”: they are very very very small.
To give you an idea, when I worked in the viral pathogens and biosafety laboratory I used the so-called Vero cells, that are kidney epithelial cells; in a plate of the area of 9.5 cm² (about 3 cm in diameter) there could be up to one and a half million cells to form a single layer.

Are they all so small?

No, there are cells that are smaller, but also bigger. An example is the sciatic nerve, which can reach up to one and a half meters in length; on the contrary, red blood cells measure 5 microns in diameter (take one millimeter, divide it 1000 times and take 5 parts).

As seen, the size of the cells depends on cell type.

But how many cell types do we have?

There is no univocal definition of “cell type”, but it can be said that a cell belongs to a cell type based on various characteristics: morphology, gene expression pattern, function and location in the organism.
Having said that, there are more than 200 cell types known to date, but it is certainly a number that is gradually increasing from year to year.

The nervous system, one of the most complex systems

One of the most complex systems to study, if not the most complex, is the nervous system, particularly the central nervous system (CNS). The main reasons are 2: the inability of the nerves to regenerate after trauma and the impossibilty to access the brain except during a very risky surgery or in an autopsy.
In addition, there are many different types of neurons both during development and in adulthood. But what’s impressive is that inside the brain there are about 100 billion neurons that communicate with each other via over 1,000 trillion synapses.
100 billion: 100,000,000,000
1,000 trillion: 1,000,000,000,000,000

To give you an idea, I remind you that the number of cells in the human body is estimated to be 37 trillion; there are therefore 27 times more synapses than cells in our body. And if these numbers have impressed you, wait until you see the next ones.

A highway for blood?

Here we are talking about the number that first impressed me during my studies: the length of the blood vessels.
Whenever I ask a friend/parent “in your opinion, how long are the blood vessels in the human body?", the answer I always get is on the order of “a few meters”; and it’s always fun to see his face when I say “you were almost there, it’s about 100,000 km” . You read that right: 100,000 kilometers, equal to two and a half times the turn of the Earth. This is because there are not only the famous veins and arteries, but also those capillaries that you may have heard from time to time, but which are often much thinner than a hair and form a very dense network.
I wish I was there now to see your faces, but let the imagination run wild.
Do you understand why I find it so fascinating to discover these “curious” numbers?!

A person in a nucleus

This metaphor represents an important number for me. I have always been interesed in DNA, its meaning and how it is the “code of life”. For those who need a refresher on this concept, I mentioned it in this article.
The DNA contained within a single cell is 2 meters long! This is because it is so thin that even in a space as small as the nucleus of a cell, it fits very well and, indeed, when a cell duplicates itself, and consequently the DNA is duplicated, there are still 4 meters of it. Here is the metaphor explained.

So, having 37 trillion cells, do we have about 75 trillion meters of DNA inside our body? No, because red blood cells are do not have a nucleus; there are about 25,000,000,000,000 (25 trillions) red blood cells, equal to 70% of the total number of cells. As a result, the total DNA in the human body is “only” 25 trillion meters long, equal to going to and from the Earth to the Sun 1,304,400 times.

Let's see how long it would take to travel this distance:

What if I told you that about 2% of the body weight is given by microorganisms?

Yep, we have so many commensal microorganisms (ie stable and non-pathogenic in the district where they are located) that contribute about 2% of the body weight; it is estimated that they are, in number, 10 times than our cells and help us in many processes of everyday life: intestinal bacterial that helps us digest and assimilate nutrients, skin flora that protects us from external pathogens, flora of the mouth that helps us in digestion, etc.

Conclusions

Here ends, for now, our journey through the numbers of the human body. I hope it may have intrigued and amazed you, but above all, to have made you think about how complicated study and biomedical research can be, dealing with such complexity, amplified by the fact that each of us is “different” and, consequently, that a treatment/ rug may not have the same effect on all 7 billion people worldwide. But we will address the theme of diversity and personalized medicine in a future article.
Stay tuned!