There are some 100 trillion (100,000,000,000,000) of them and they are with us all the time, night and day, waking and sleeping, 24-hours around the clock, i.e. whatever we are doing and whenever we are doing it. Being 100 trillion of them firmly fixes them among the most ordinary things we know. Yet we almost never think about them. And when we do, we almost always have a confused and inaccurate picture of them. What are they? The 100 trillion individual cells that make up every human body.
If you are like I used to be, you are probably saying to yourself, “I’m more attuned to mathematics and the physical sciences. I never really understood biology. It just seems to be too incredibly complicated without any simplifying fundamentals.”
Indeed, throughout secondary school and university, I even thought that biology didn’t really classify as a science because of its lack of simplifying fundamentals. Then one day it all changed. I discovered there are simplifying fundaments, and even better one simplifying fundamental that underlies all the others. Suddenly, the multitudinous hormones, ions, proteins, lipids, antibodies, etc. running around the body all seemed to make sense. It still takes a superhuman effort to understand all of them in detail, which is why the biological sciences are divided into so many arcane specialties. However, to educated laymen, this one key idea is sufficient to help us understand why these specialties exist, and in broad outline to understand what these multitudinous specialists are saying about them.
What is this amazing item capable of making the arcane understandable? And why should it be considered an apt candidate for the title of an extraordinary ordinary thing? First, let me set the scene.
How the Human Body Resembles a City
We are probably all familiar with the mantra that the human body is “like a well-oiled machine.” The problem is, this idea is misleading and thus virtually useless. Unless we are mechanics or mechanically inclined, most machines would seem to be bafflingly complex. Thus, when described in terms of its subsystems (circulatory, digestive, nervous, respiratory, urinary, etc.), the human body may still seem to be bafflingly complex.
By contrast, a city is made up of all kinds of people (doctors, lawyers, merchants, teachers, architects, bus drivers, refuse collectors, etc.) all engaged in activities that in one way or another contribute to keeping the city viable. However, much of what happens in a city is less about keeping it viable than about keeping it running.
The same is true of the human body. Rather than being made up of cogs and wheels, it is made up of trillions of individual cells, each one trying to stay alive long enough to carry out the role for which it was designed.
It is the task of keeping each one of these individual cells alive and active that puts most of the structures and functions of the body sharply into focus.
Once this idea has been fully accepted, much about the human body—or the body of virtually any other living organism—becomes almost self-evident. So the idea is well worth repeating.
In a city, most of what happens is dedicated to keeping its individual inhabitants alive and active. In the human body, most of what happens is dedicated to keeping each one of its 100 trillion individual cells alive and active.
What Is a Cell?
If the individual cell, this extraordinary ordinary thing, is the center of attention, just exactly what is it?
By simplest definition, a cell is the smallest possible living unit. It cannot be divided into two or more pieces that we would still classify as living. Why? Because anything smaller would not be able to fulfill the basic criteria of being alive.
We all believe we know what is alive and what isn’t. However, even biologists, whose job it is to study and understand life, can’t fully decide on how to define it so that everyone would agree. Nevertheless, over the centuries they have come to accept that for anything to be alive, it must fulfill at least four criteria. It must be able to:
- Convert nutrients (food) into energy
- Use energy to carry out vital functions
- Expel waste materials
- Reproduce itself (undergo mitosis) in order to ensure future generations
Let’s look at these four criteria in a bit of detail.
While the human body is made up of trillions of cells, some living organisms are made up of only one cell. Because cells are so small, they are invisible to the naked eye. Until they were discovered under the first primitive microscope in the 17th century, no one even suspected that they actually existed.
There is a wide variety of unicellular (single-cell) organisms, both animals and plants. But since they all fulfill the four criteria of life defined above, we do not need to look at any one type in particular in order to understand the human body.
Building the Body Cell by Cell
We are going to examine the structure and functions of the body from the bottom up, i.e. see what happens when unicellular (single-cell) organisms get together to form more and more complex multi-cellular organisms.
Imagine a unicellular organism living all by itself. Such an organism would generally live in an aqueous (watery) environment. Why? Because the nutrients (food) it needs to survive is floating in the water all around it. All the cell needs to do is take in the pieces it wants. It never has to go looking for food, because the food comes to it.
Once the food is inside, the cell has mechanisms for converting it into the energy it needs to maintain itself and allow it to function. It also has mechanisms for expelling any waste materials it may have taken in by mistake or created by the process of converting food into energy.
Now let’s image that the single cell decides to link up with another cell to form a bi-cellular organism. With this association, very little changes. Both cells still independently take in food from the aqueous environment, convert it into energy, and expel waste materials. But what would happen if these two cells decided to associate with a third cell to form a tri-cellular organism? Again, very little, because each cell would continue acting almost as if it were still completely independent.
But notice, a single cell has total access to the aqueous environment to take in food and expel waste. As two cells, access to the aqueous environment would still be very high but no longer total because of where the cells are joined. Each time a new cell associates, access to the aqueous environment decreases. The cells on the outside of the group maintain nearly total access; however, for those near the center of the group, access becomes considerably reduced.
What happens when the organism becomes so big that some cells have no access to the aqueous environment at all?
Being unable to take in food and expel waste, these interior cells would rapidly die. We might therefore reasonably conclude that there must be a limit to the number of cells that can form an organism. Once the number becomes so great that certain cells no longer have access to the aqueous environment, growth must cease.
However, we know this isn’t true; otherwise we wouldn’t exist. So another structure must be possible.
Instead of completely blocking inside cells from the aqueous environment, the organism arranges itself to create a tiny tunnel. This allows the aqueous environment to flow to and from the cells near the center. As the organism takes on more and more cells, it creates more and more tunnels for the aqueous environment to flow to and from the cells inside.
This is a fundamental concept. No matter how deep they are inside the organism, all cells must have access to the aqueous environment that contains food and takes away waste; otherwise they will rapidly die.
This is true not just for microscopic organisms visible only under a microscope; it is true for all living organisms. Thus, the more cells an organism contains, the more “tunnels” it needs to ensure that each and every one of its cells can take in food and expel waste.
The fact is, the vast majority of the 100 trillion cells in the human body are located deep inside. This means that the body needs an almost countless number of tunnels. Moreover, it must organizes them into complex systems to ensure that even cells in the very center of the body are still surrounded by an aqueous environment containing food and capable of carrying away waste. Otherwise, they rapidly would die — and you along with them.
If this idea seems somewhat abstract, let’s return to our comparison with a city.
How Cells Build a City
Suppose we start with only one person living in the middle of a field. The field supplies all of his needs. When he is hungry, he simply goes into the field and picks some crops to eat. When the food is converted into energy, the waste materials can also be expelled into the field.
Now let’s suppose that a second person arrives on the scene. No problem. Both of them can fully satisfy their needs from the crops in the field. If more and more people arrive, eventually it will become inefficient for each individual to go into the field to find food. So the group decides that some people should gather the food while others should stay in the house to cook it.
The people in the house are now cut off from the source of food. In order to keep the cooks alive and functioning, the people who go into the field now have to bring back enough food for everyone, not just for themselves. This is like creating a “tunnel” in our multi-cellular organism.
Without putting too fine a point on it, the people who go into the field to gather the food can also expel the waste materials their bodies create (urination and defecation). But what about the people in the house? When the group is still rather small, they too can occasionally go into the field specifically to expel waste materials from their bodies. However, as the community becomes bigger and bigger, more and more houses are built. The houses in the center become increasingly distant from the field, so going there to expel waste materials becomes increasingly difficult. Something has to be done.
At this point, the community decides to reorganize itself. Now instead of having only two specialized jobs (gathering food and cooking it), they add two more for a total of four.
- Some people (those living closest to the field) gather the food.
- Some people (those living nearer the center) receive the food and divide it among all the members of the community.
- Some people cook the food.
- Some people collect waste (human and food) from the houses and expel it back into the field.
In short, a smaller and smaller proportion of the community is now actually gathering food, while a larger and larger proportion is becoming involved in distributing it, cooking it, and disposing of waste.
We are of course still talking about a very small community. But if you think about it, this is exactly what happens even in a major city, only more so.
A very small number of people are engaged in gathering or cultivating food, while the majority are engaged in warehousing it, processing it, and distributing it. So the city has to create a food distribution system. Again, without putting too fine a point on it, the people who eat the food but never go outside the city into the field still have to eliminate waste. They can’t do this just anywhere, so the city also has to create a sanitation system.
As the city gets even bigger, people must be able to move around in order to carry out their assigned functions, so the city has to create a road and transportation system. And so on.
In short, most of the organization of a city is devoted to providing solutions to the needs of each individual inhabitant, most of whom are too far removed from the source of those solutions to take care of them themselves.
The same thing is true of the human body. However, the body would seem to have a considerably harder job on its hands than a city. After all, Tokyo-Yokohama, the world’s largest city, has a population of only 33 million persons. The population of cells in the human body is about 100 trillion; this means that the body has 3 million times more “inhabitants” than does Tokyo.
To put this comparison on a global scale, the total population of the world recently passed 7 billion (7,000,000,000). Although an enormous figure, you would still have to multiply it by 14,000 to achieve the population of cells in just one human body!
No wonder the structures and functions of the body at first glance may seem to be frustratingly complicated. However, let’s keep in mind the fundamental principle: Most of what goes on in the body is to serve the needs of each individual cell rather than the body itself.
Based on this fundamental principle, the fundamental systems that the body needs to survive and function would include:
- digestive system
- circulatory system
- respiratory system
- urinary system
- nervous system
- defense system
These six systems clearly reflect the fundamental principle. Others, such as the skeletal system, the muscular system, the endocrine system, the reproductive system, etc. have an impact on ensuring that each individual cell survives and prospers, but their contribution is indirect. However, first understanding how the six “direct” systems contribute to the survival and prosperity of each individual cell makes it easier to understand and appreciate the body’s important indirect systems. And how they all work together to make us who we are. Like a city.
Could the Human Body Better Be Compared to a Computer?
The whole thrust of this blog has been to demonstrate how understanding the human body is aided by comparing it to a city. However, traditionally it has been compared to a machine. Is there any merit in this approach?
Well yes and no. For me, comparing the human body to an automobile, airplane, dishwasher, or any other mechanical device only confuses matters. This is why for years I have thought that the human body in particular, and all biology in general, were bereft of an evident logic. However, we might gain something by comparing biological structures to a machine that transcends being just metal and wires, namely the computer.
The fundamental purpose of most machines is to facilitate or speed up things that already needed to be done. For example, the dishwasher was invented, and then constantly improved, to take the drudgery out of the unavoidable task of cleaning eating utensils.
What distinguishes the computer from other machines is that
its software is alterable. It can “become” any other computing
machine simply by changing its software. Its basic function — to
manipulate electronic impulses to produce information—is not changed by
altering its software.
If a dishwasher could also mow the lawn, it would no longer be a dishwasher. However, a computer will always be a computer.
Today when so many people use computers and have a general (if not specific) idea of how they work, it is not unusual to find educators trying to explain the human body by analogy to a computer.
For instance, they say things like: “In order for a computer to operate, each of the necessary pieces must be present. This is also true of the human body.” And so it is self-evident; no system, biological or electronic, can operate properly if some of its parts are missing. Next, they say, “All the parts of a computer, and all the parts of the human body, must work harmoniously together to complete desired tasks.” This is also self-evident; no system, biological or electronic, can produce desired results if the components function in conflict. Both statements, therefore, are essentially useless; they give us nothing.
They then go on to make comparisons between the components of the human body and the computers, such as:
- The human brain is like the computer’s CPU.
- The mind is the software running on the CPU.
- The cache is comparable to human short-term memory.
- The nervous system is the body’s communication system, just like computer buses.
- The eye provides human vision just like the computer’s video card generates images for display on a monitor.
- The skeletal system can be comparted to the computer’s motherboard. The skeleton holds all the body’s systems and organs together, with the bones giving structure and providing a location for each bodily part. Similarly, the computer’s motherboard provides a location to mount much of the computer’s functional hardware.
And on . . . and on . . . and on.
I grew up in the 1950s, long before computers were an integral part of daily life, so perhaps I am not a fit judge of the educational value of the body-computer analogy. However, what I find lacking in this analogy, and any other so-called explanatory analogy, is a sine qua non, i.e. the heart and soul of an idea whose presence illuminates everything else.
When I left university, one of my first assignments was to prepare materials to train medical representatives of a major pharmaceutical company. Medical reps are the people who visit doctors to remind them of the specific characteristics and benefits or a product that is already on the market, counter misconceptions about such products, or to prepare them for introduction of a new product about to be launched.
I was prettified. Surprisingly, most such reps had little or no background in either biology or medicine. They had been recruited from other industries where they had demonstrated the ability to talk with and persuade people of what they were saying. If they had no more appropriate background that I did, how was I going to explain things to them that I didn’t really understand myself?
Somehow while going through the background material I had to work from, I began to see that the human body (and probably all other biological forms) depends on the functioning and maintenance of each one of the 100 trillion individual cells.
From then on, everything fell into place. It wasn’t easy, but I produced materials and conducted training sessions based on the analogy between the human body and a city rather than the human body and a machine. It worked like a charm. It was a high point of my life when one of these trainee medical reps came up to me and said, “I have absolutely no background in biology or medicine, yet I understood every word of this training program. How did you do that?”
I did it by drilling down to find an absolutely key concept on which everything else would depend, and built up from there.
I have no doubt that making an analogy between the human body and the computer can be useful, particularly in this day and age when so many people are at least minimally computer literate. However for the life of me, I simply don’t see how the educational value of the body-computer analogy even comes close to the body-city analogy. But of course I am prejudiced.
What do you think?