Science Burrito is exploring what it takes to make a person, if we wanted to use technology instead of biology. Last time, we started out the project by looking at bones, the foundation on which the rest of the body is built. We learned that bones is where the blood is produced. But once it’s made, it needs to be moved around the body to where it is needed. That is done by the circulatory system.
The circulatory has two main parts, the heart, which is the pump which moves blood around the body, and the blood vessels, the veins and the arteries, which are the tubes which the blood moves through.
The heart is easy. It’s a pump. We can do pumps. We can make so many different kinds of pumps. There are gear pumps, centrifugal pumps, peristaltic pumps, diaphragm pumps, vanes pumps… the list goes on. They all sound kind of complicated, but they all work on the same basic principle: a pressure difference cause liquid to move from an area of high pressure, to one of low pressure.
The heart is most like a diaphragm pump, which work by squeezing fluid in tubes to push the fluid through.
It has a problem though. The heart pumps a lot of blood. It pumps about 100 ml every second, or 5.7 litres a minute, or over 8000 litres a day. There are diaphragm pumps that can perform at those rates, but they require something to run the diaphragm, usually compressed air. Then you need to keep the compressed air topped up, and stored somewhere. It’s all looking a bit much for our body.
But we don’t have to use this kind of pump. We can use a centrifugal pump instead. They work by spinning blades around to push fluid around. More importantly they can flow rates into the tens of litres a minute, are much smaller and use far less power.
We actually already have artificial hearts, but they are powered by pumps outside the body in backpacks and are only a temporary solution. The reason we don’t use centrifugal pumps to replace human hearts is that they don’t beat in the same way. They flow continuously. The rest of the circulatory system just isn’t set up for continuous flow. But that’s not a problem for us, we can design our system for our pump.
So we have our pump, in fact, we can buy it at any good hardware store. Now we need tubes to send the blood around the body.
There are a few things we need to consider here and the first is pressure. The pressure that your heart puts on your veins and arteries varies from around 16% of atmospheric pressure (that’s the pressure the air above you is pushing down on your head with) to about 2% atmospheric pressure. That’s not actually all that much, so it looks like it will pretty easy to find tubing that can cope with this pressure. A rubber balloon, for instance, 1 meter wide and 1 mm thick can withstand twice as much pressure as that.
So we could, perhaps, make our veins and arteries out of rubber.
Fine, but there are three types of blood vessel. Veins, which carry blood to the heart, arteries, which take blood from the heart out into the body, and capillaries.
And this third type is very important because it is the capillaries that transfer good stuff like oxygen and nutrients, out of the blood and into the body, and bad stuff like carbon dioxide and toxins out of the body and to the lungs or liver to be dealt with. It does this via a special little function it has called permeability.
Permeable is just a posh way of saying leaky. But not, you know, very leaky or all your blood would leak out. It’s just leaky enough to let little things, like oxygen molecules though, but not bigger things, like blood cells.
Fortunately, filtering (separating the big a little bits) blood is something we’ve been doing for a while now, in something called dialysis. We’ll get back to that in a later build-a-body, but for now it is enough to say dialysis tubing should work just fine for our capillaries. This tubing is made out of cellulose, which is the material that makes up the walls of plant cells. That makes sense. Plant cells need to take in nutrients and gases and excrete toxins, just like human cells, but they are also very robust. This makes cellulose perfect for our needs.
The difficulty comes in how tiny we need to make the tubes. Capillaries are around 8 microns wide. That is 125 times smaller than a millimetre. How can we get our cellulose capillaries down to that size? The best 3D printers around have a resolution (the smallest they can print) of around 25 microns. Why am I telling you that? Because it turns out we can do 3D printing with cellulose, in fact, it is getting to the point where we can 3D print with almost any material!
The problem is that the machines which build capillaries are cells, and they are far smaller than any machines we currently build ourselves of our bodies today. Ultimately, I don’t think the technology currently exist for us to make capillaries that small, although it may well soon as 3D printing and micro-machine technology improves.
Of course, there is one simple solution to this problem… We just make our person 3 times bigger than normal!