According to Plato, how does blood move around the body?

It's a tricky problem in hydraulics, especially for ancient thinkers who don't know that the heart pumps blood.

There are plenty of things that ancient people don’t know about the human body. For instance, they don’t know that it was possible for the organs of our bodies to do things automatically and without our conscious awareness. Of course, they know that our bodies do things involuntarily, such as sneezing. But they don’t know that our intestines move food through them by means of wave-like contractions or that our heart pumps blood through our arteries and veins. In fact, for most of the history of ancient Greece, they don’t even know that there is a difference between the arteries and veins.

It took a very long time for people to discover the fact that the heart functions as a pump. Even after we discovered the way that many involuntary, unconscious activities of our organs are controlled by the autonomic nervous system, it still wasn’t obvious how the heart works. In 1644, William Harvey (1578- 1657) made this important scientific discovery.

So, it’s no surprise that Plato (428 - 348 BC) had no idea about any of this. But he, like many of his peers, was curious about how blood did move around the body.

This is a tricky problem when you don’t know about the heart’s function as a pump. It’s not hard to explain how blood can get from, say, the stomach to the feet. Whatever mechanism you think explains why things fall will explain that. But it’s very hard to explain how blood can travel upwards.

The natural thing for a modern person to do is to reach for some anatomical resource to explain this. But because Plato doesn’t think that any organ of the body can shuttle blood around, this isn’t something that occurs to him.

So, what does the heart do?

Well, for Plato, the heart does nothing to help with this problem. Here’s what the heart does, for Plato: the heart transmits messages from our brain (where the part of us that thinks is located) to the rest of the body. The thinking part of us is in charge, and it can rein in our limbs when that is needed and appropriate. Plato is one of the first thinkers to recognize that the heart is located at the center of all the veins in our body, and so even though he doesn’t think it is a pump, he does think that it helps to facilitate the transmission of orders or commands to the rest of the body from the thinking part of us.

The solution to the problem of blood moving around the body, Plato thinks, is the laws of nature that the gods created.

To see what I mean, we should start with the beginning of this process.

Plato, like all ancient Greeks, thinks that blood is made of food. There was an important contrast between the immortal gods, who ate ambrosia and had ichor instead of blood, and humans, who ate food, were mortal, and had blood.

In Plato’s mind, when we breathe in air, some fire gets into our bodies. That makes sense because the air we breathe in is hot, at least to some extent. So, it’s got to have some fire in it. Fire is composed of sharp little tetrahedra, or pyramids, that Plato thinks are in constant motion back and forth in our body as they get tossed around by the inhalations and exhalations of breath.

We eat food, and it comes into contact with tetrahedra that are jostling around, and the tetrahedra chop the food into blood. When fire “burns” something, it’s actually being cut up by the very sharp, fine edges of the small tetrahedra that compose fire.

So, now we have blood sitting in our bellies.

How does it get around the body?

To understand this point, we have to dig a bit deeper. Inside blood are all the four elements: water, air, fire, and earth. They exist in blood because they are in the food that we eat.

This is incredibly crucial because it helps explain how blood moves around the body and gets to where it needs to go.

One of the major laws of nature that Plato believes in is this: there is no such thing as empty space, and so, when things move, they have to displace something else.

The universe is entirely crowded. No spot is vacant. Every spot is occupied.

You might wonder how things can move around in a situation like this, but that’s not such a mysterious thing. Think about how fish move in the ocean. They don’t move into empty space. They move by displacing the water in the spot that they are moving into. The water in the spot that the fish comes to occupy rushes behind to fill the spot that the fish had just occupied.

This explains how blood can move up and down the body.

As fire and air jostle around the body because we are breathing in and out, the blood that was made from our food is caught up in the jostling. It is lifted up and deposited into the veins.

But this is a continuous process. As blood is put into the veins, the blood that is already in the veins is pushed forward. As more blood is put into the veins, even more is pushed forward.

It’s like this: imagine that you have a pipe that starts in your basement and then goes up the wall to your kitchen. You want to know how to make the water flow up. One way to do that is to put a lot of water continuously in the pipe. The water you put in will push the water already in the pipe up more.

Now this helps explain how blood gets around, but there’s one thing missing.

We haven’t explained anything about why we need blood to begin with.

We need blood because the things it contains (earth, air, water, and fire) are also the constituents of tissues (skin, sinew, etc.). Our tissues are constantly wasting away, and so the movement of blood delivers to our tissues the constituents that they need in order to be replenished.

Let’s say that our ankle’s tissues are depleted. Blood will replenish them with what they need. Let’s say that our shoulder’s tissues are depleted. Blood similarly will replenish them.

We push blood around the body by depositing a continuous flow of blood into the veins. As the blood goes around the body, elements in the blood are deposited where they need to go.

For the most part, anyway.

Another law of nature that the gods designed: each of the elements has geometrical features that allow them to naturally hook onto each other.

Fire is made of tetrahedra, and the other elements are made of other polygons.

It is easy to imagine, then, that our tissues are large compounds of shapes. Our ankles are some mixture of water, earth, etc. That means that they are large compounds of the polygons that compose these elements. Sometimes the elements in them fall away due to wear and tear, and so they need the missing elements to be replenished by the blood.

As the elements in the blood move past the tissues, elements get deposited in the spots left open by the depleted elements. This process happens instantly, because there can never be an empty space. The blood rushes in immediately and fills the gap.

There is no guarantee that the element that fills the spot will be the right one.

Is that a problem? Absolutely. But that’s the explanation of how we can get diseases and be unhealthy.

Some people have excessively dry skin, for instance. Their skin needs water, but earth has ended up in the spots where water should go. It would be preferable for there to be water.

But if Plato thought that there was a mechanism to prevent that earth from ending up there, and ensuring water would go there, then he would be denying the reality that diseased tissues can result from the absence of what’s needed or the excessive accrual of what is not needed.

So, in this way, Plato is able to explain the movement of blood in our bodies and the replenishment of tissues. He does this without ever ascribing responsibility to organs of the body or the beating of the heart.