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[light mystical music]

Let us start today's episode by getting to know our master of microscopes, James, a little more up close and personal. Your screen is currently awash in James's blood. Fortunately, this was a planned affair. James didn't accidentally cut himself on a slide and bleed all over it. He simply placed a drop on the slide and let the cells do the rest. And so, in observing his blood under the microscope, James is joining a long tradition of scientists who have examined blood belonging to themselves, to other animals, and to other organisms so that we can decipher the role that this fluid plays in keeping us alive.

It's been hard for us as humans to miss the fact that blood is important to our bodies, but understanding the nature and mechanisms of it has required untangling the macroscopic structures that send blood coursing through our veins and the microscopic details that make blood what it is. One of our early methods of describing and categorizing the body were the four humors, a system developed by ancient Greek physicians who believed our body could be described as the interplay between yellow bile, black bile, phlegm, and blood.

But blood is more than just the substance itself. The fluid is nothing without a way to use it, and so our understanding of blood is tied up in our understanding of the whole system, the circulatory system. To get to what that means, let's start by looking at the blood of something a bit different from us -- daphnia.

One thing that makes daphnia handy for our purposes is that you can you can, you know, literally see right through them, and, with a microscope, you can observe the rapid beating of its heart and the flow of blood cells through its body. Daphnia are also an invertebrate, and, like many other invertebrates, it passes cells and ions and other important molecules throughout its body using a watery fluid called hemolymph. 

Their hemolymph isn't constrained to veins or arteries; instead, it's pumped into an open cavity called the hemocoel, where it then comes in contact with the organs it needs to deliver nutrients to. This is an open circulatory system. Vertebrates like us, on the other hand, use a closed circulatory system, where blood is directed throughout our body in a contained network of blood vessels. That system allows for more efficiency in delivering oxygen throughout our body, but it's also more complex and requires more energy compared to the open circulatory system.

Now, it took us quite a while to piece together what our circulatory system would look like. The second century physician Galen theorized that blood is made in our livers using the remnants of digested food, and, when blood arrived at whatever spot it was needed, it was then consumed. Galen's hypothesized path mistook the nature of blood and the structure of the heart, and yet it was a theory that would prevail in medicine for centuries.

In the 13th century, the Arab physician Ibn Al Nafis drew on his study of animals to correctly describe how our circulatory system works, and yet, his revolutionary work was not widely known until 1924, when an Egyptian physician found a copy of it in the Berlin State Library while working on his dissertation. In the 17th century, English physician William Harvey came to draw the same picture of the circulatory system, turning to demonstrations on dissected animals to gruesomely illustrate his point. And his work would, in turn, go on to change our understanding of blood, emphasizing that it is a thing made and circulated by our bodies, and not, as Galen predicted, a product of our digestion.

So what are some of the pieces that make up our blood? It's complex and variable, of course, but let's first focus on the color. For many invertebrates, the hemolymph is either clear or tinted yellow with pigment molecules, and you might be thinking right now, "I've squashed a fly before, and there was a lot of red." And yes, perhaps there was red, but that was actually due to the pigments in the insect's eyes, not their blood. 

But biology does love an exception, which takes us back to the daphnia, as well as to another invertebrate called artemia. Both of these invertebrates' blood contains a protein that is familiar to us -- hemoglobin. Hemoglobin is the four-part protein inside our red blood cells that makes them red, and, more importantly, hemoglobin attaches to oxygen. When a red blood cell gets to our lungs, the hemoglobin binds to oxygen, turning the red blood cell into our own personal oxygen transport machine. 

Now, there are other oxygen-binding molecules used by other animals, but daphnia and artemia both turn to hemoglobin when times get tough. When the waters around them get low in oxygen, they adapt by producing more and more hemoglobin so they can capture more of that precious resource. As they produce more hemoglobin, the populations of daphnia and artemia become visibly redder, like they're blushing, which might explain why they don't just have hemoglobin lying around in large quantities all the time. Not only does it take energy to make hemoglobin, it makes the animals more visible to predators.

When you look at James's blood cells, you can see the red of the hemoglobin, but as we said at the start of this episode, James is one of a long line of scientists who have examined their blood under the microscope. Some of the early microscopic studies of human blood relied on samples taken from clots or from a recently-fed louse. James has actually made a note on the script here; it says, "Glad I didn't have to do this!"

But of course, one of the early adopters of the self-study approach was none other than the early master of microscopes, Antonie van Leeuwenhoek, who described the "sanguinous globules" of his own blood in a letter to the Royal Society of London. Centuries later, Dr. Karl Landsteiner would collect samples from six of his coworkers and himself to study how these different blood samples and the antibodies within them reacted to each other. These observations would lead him to describe the human ABO blood grouping system.

We have always known that our blood was vital, in both senses of that word, and though we have now teased apart much of its true nature, its significance reaches beyond scientific understanding. We talk of blood relations and being hot-blooded or red-blooded. Blood is always available to us, both for science and symbolism, for medicine and metaphor. 

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. We'd also like to say thanks again to Skillshare for supporting this video. You can learn many, many things on Skillshare, even how to make your own blood. Well, digital blood, at least. With classes like Animating Blood Flow in a Vessel with Cinema 4D, you'll learn, unsurprisingly, how to animate blood flow in a vessel in Cinema 4D, but you'll also learn skills like modeling, texturing, lighting, animating, rendering, and compositing that will help you cross any other 3D animation project that you hope to tackle in the future.

Skillshare is an online learning community that offers membership with meaning. With so much to explore, real world projects to create, and the support of fellow creatives, Skillshare empowers you to accomplish real growth. It's curated specifically for learning, meaning there are no ads to distract you, and they are always launching new premium classes, so you can stay focused and follow wherever your creativity takes you. And an annual subscription to Skillshare is less than $10 a month. If you're one of the first 1000 people to click the link in the description, you can get a free trial of Skillshare's Premium Membership.

Now, onto the people on the screen right now. These are our Patreon patrons. Without them, our show literally could not exist, and we are so thankful to them. If you like what we do and you want to help us keep making it, you can join those people at If you want to see more from our master of microscopes James Weiss, check out JamAndGerms on Instagram, and if you want to see more from us, there's always a subscribe button somewhere nearby. 

[light mystical music]