HOW MANY CELLS DOES THE HUMAN BODY HAVE? - BIOLOGY - 2025

Scientists have tried to find out how many cells the human body has, proposing very different figures. These figures range from 5 trillion to 200 trillion, and it is that counting cells is not an easy task.

To begin, you should know that all living things are made up of one or more cells. Cells are the fundamental units of organisms, they form their structure and perform different tasks. All of them come from existing cells, and contain information that we have inherited from our parents.

Instead of being a chaotic war of microorganisms, cells cooperate in an amazing way forming an organized whole. There are living beings that have a single cell, and they are called unicellular organisms (like bacteria); while many others are made up of a large number of cells, the so-called multicellular (such as animals and plants).

Obviously, it is easier to know the number of cells of small and simple organisms than of other complex ones such as humans.

In simple organisms, the number of cells appears to be under strict genetic control. On the contrary, the factors that determine the number of cells in higher organisms are more varied. Homeostatic mechanisms (which maintain a balance) are involved, such as proliferation (or cell birth), differentiation, and cell death.

Unicellular organisms (bacteria)

For example, a hormone that our body secretes, growth hormone, can modify the number of cells by regulating cell proliferation, reproduction, and regeneration.

On the other hand, there are genes that prevent the excessive reproduction of cells. If these have any mutation, they can lead to cancer because the cells would grow out of control.

In complex beings there are individual differences in the number of cells (according to size, weight, age…). In addition, there may be moments in life when you have more or fewer cells than others; if you have increased your muscle mass, or, on the contrary, you begin to develop a degenerative disease. So calculating the number of cells in the body can be quite a difficult task.

How can we calculate the number of cells in the human body?

Most plant and animal cells (including humans) are only visible through a microscope, as they measure between 1 and 100 microns. Remember that a micron or micrometer is one millionth of a meter.

Pretending to count all the cells in the human body through the microscope is not very practical. First, there are about 200 different types of cells in humans, and within each of them there are about 20 subtypes of structures or organelles. Some types of cells cannot be seen so easily, but rather stick together in tangles that are difficult to define, such as brain neurons.

Second, even if you didn't have this problem and could identify 10 cells per second, it would take thousands of years to count them all.

However, a team of scientists from Italy, Greece and Spain found a better solution; they reviewed everything that had been written in the past about the number of cells in our body, finding that there were very mixed estimates. They all had one thing in common: they did not explain how they had done the calculations.

The researchers took into account that each structure of our body has a different weight, without ignoring that the cells that make up each area have different sizes and densities.

To solve this, the authors established the measurements of an average man, 70 kilograms in weight and 1.72 meters in height. They then went through a large body of bibliographic material to find the volume and density of the cells that make up the bones, intestines, cartilage, muscles, blood, veins, etc. As well as the various organs separately.

How many cells were obtained?

Animal cell

Finally, they added up the amounts found in each structure of the body and estimated that the human body has about 37.2 trillion cells.

Of these, the vast majority are erythrocytes, cells found in our blood, also known as red blood cells. Its function is to transport oxygen throughout the body.

The second common type is glial cells, which are those found in our nervous system, followed by endothelial cells (inside the blood vessels), dermal fibroblasts (in the skin) and platelets (in the blood).

Regarding weight, the cells of the muscles and fat tissue constitute 75% of the cell mass, being the heaviest.

Number of cells and bacteria in the human body

Bacteria

Something that the study authors did not count was the number of bacteria. For a long time it was thought that we had more bacteria than cells, but this seems to be wrong.

A study published in 2016 has shown that the body contains the same number of bacteria as human cells (Sender, Fuchs & Milo, 2016). And that, in addition, the largest number of bacteria are concentrated in our digestive system, mainly in the colon.

Even these authors indicate that we tend to have more body cells than bacteria, depending on the frequency of our bowel movements. In fact, we get rid of a few trillion bacteria through the intestine.

It appears that this varies by gender, as women have 30% more bacteria than body cells. This proportion is due to the fact that they tend to have less blood volume than men, but the same amount of bacteria as men.

On the other hand, researchers have not yet calculated the viruses, fungi, and other microbes that are also part of the human body. In fact, it is believed that the number of viruses could greatly exceed that of bacteria.

Furthermore, it is not known whether a greater amount of these agents in our body could be a real risk to our health. Science News commentators claim that a reduced proportion of bacteria does not mean less effect of bacteria on our health.

Ultimately, a 1: 1 bacteria-to-human cell ratio is still an impressive number of bacteria. It is incredible to think that half of our body is made up of external agents that slip into our body and modify it.

Microchimerism, when our cells come from another being

Not all the cells that are in our body come from us. In addition to external agents such as bacteria and viruses, cells of other beings appear to exist. This mechanism is called microchimerism, and it involves the presence of some cells that are genetically different from those of our body.

This phenomenon has been observed mainly in pregnant women. It seems that cells from the fetus can pass into the mother's bloodstream and settle in some of her organs. It can also happen the other way around, that is, the maternal cells travel to the fetus and are deposited in the body.

These cells have been found mainly in organs such as the brain, heart, lungs and kidneys, through autopsies of women who died during their pregnancy (Rijnink et al., 2015).

The number of these cells increases as the pregnancy progresses, decreasing dramatically when the mother gives birth to the baby. However, the amount of cells of the fetus that are accumulated in the mother is different in each one.

Furthermore, these cells seem to remain for many years in the maternal organs. In fact, fetal cells were found in the brain of a 94-year-old woman (Chan et al., 2012).

It seems that microchimerism also occurs in species such as primates, dogs, mice and cows.

Number of cells and diseases

It is also necessary to indicate that the number of cells in an organ can vary according to certain medical conditions. For example, a liver with cirrhosis, a degenerative disease, can have millions of fewer cells than a healthy liver.

The same can happen with conditions such as Alzheimer's, where there is a progressive degradation of neurons (cells of our brain).

On the other hand, there are diseases that are associated with a greater number of cells. Thus, an individual who has developed cancer will have a greater number of cells than he should.

Ultimately, trying to define how many cells the human body has is a daunting task. There are quality studies that have been able to approximate the number of cells that we have, however, those that come from outside such as bacteria, viruses, or those transmitted by our mother (or twin brothers) are not included.

On the other hand, the number of cells is different in each person according to weight, age, height, size… and even the qualities of our organs, blood, veins, bones, etc.

Not counting the presence of diseases that affect the normal number of cells that we have.

Therefore, new research is needed to explore these aspects, or check if there are new ones that influence, to get closer and closer to a more exact number.

References

1. Cell (biology). (sf). Retrieved on October 31, 2016, from Wikipedia.
2. Chan, WF, Gurnot, C., Montine, TJ, Sonnen, JA, Guthrie, KA, Nelson, L. (2012). Male microchimerism in the human female brain. Plos One, 7 (9); e45592.
3. How Many Cells Are in the Human Body - And How Many Microbes? (2016, January 13). Obtained from National Geographic.
4. How many cells are in your body? (October 23, 2013). Obtained from Phenomena.
5. How many human cells are there in our body, on average? (sf). Retrieved on October 31, 2016, from Biology.
6. Padilla, AS (March 2, 2016). Fetal microchimerism: The undeniable mother-child bond. Obtained from Neuromexico.
7. Sender R., Fuchs S., Milo R. (2016). Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biol 14 (8): e1002533.
8. The Cells in Your Body. (sf). Retrieved on October 31, 2016, from ScienceNetLinks.