What Are the Components of Blood?

Take a guess as to how much blood is in your body right now. Just like a ballpark guess. Think about all the blood in your heart, all the blood in your arteries, veins, and all the smaller blood vessels. Add up all those sources of blood and you get five whole liters. That's about this much. This glorious red stuff is so valuable to our physiology as humans that we're going to spend an entire episode introducing you to the cast of characters that make blood what it is. For instance, you've probably heard about the big players like red blood cells and white blood cells, but blood is a diverse and multifaceted substance that also holds clues to diseases that affect more than just your blood. I want you to think back to the last time you got a cut. The blood that oozed out of that cut was probably bright red, shiny, and dripped quite a bit more slowly than water. That may give you the impression that blood is a uniform fluid, but it's actually multiple substances, all with different purposes. You can tell because if you look at a vial of blood and let it sit around for a few hours, or if you spun it in a centrifuge, you'd notice three distinct layers emerge as the denser components of blood sank to the bottom. The creamy colored and lightest layer on top is the plasma, which is mostly water. But just like a mug of hot cocoa, it's what you dissolve in the water that makes it interesting. Yes, I made my cocoa with water growing up. All you kids who made your hot cocoa with milk must be nice. Plasma is where you'll find the dissolved solids in your blood. Substances like proteins, sugars, hormones, and a ton of other things. The red dense layer at the bottom is made of erythrocytes, also known as red blood cells, the most common type of cell found in your blood. We're talking around 250 million cells per drop of blood. That takes up about 40% of your blood by volume, which leaves this little layer in the middle here making up less than 1% of your blood volume. This is where you'll find your leukocytes, or white blood cells alongside platelet cells. And even though they only make up a tiny volume of blood, these cells still have a very important function that all comes back to our immune system. Now, you'll notice that a lot of cell names we've been talking about so far end in cyte, which is no coincidence. Whether it's an erythrocyte or a leukocyte, cyte denotes a mature cell. And the prefixes here literally mean red and white, so basically we have these fancy words that translate back to red cell and white cell. Now, we do still have a handful of different leukocytes. There isn't just one single type of white blood cell. Like B and T cells, what are collectively called lymphocytes, are just two types of white blood cell. If you want to learn more about those cells, by the way, we'll dive into them and the immune response later in the playlist. Other white blood cells include a diverse group of immune cells called granulocytes, as well as monocytes, which become big chunky boy cells called macrophages. Collectively, white blood cells float around in circulation waiting for some kind of signal that recruits them to the scene of injury or infection. That signal might be a chemical like interleukin-1 that plays a part in fever, or histamine, which you're familiar with if you've ever sought allergy relief. Now, it might seem surprising that there are so many immune cells in your blood. After all, there's a whole separate immune system for this exact purpose, right? Well, if you want to constantly patrol the body to find something, traveling through the blood is actually a good strategy. Take the other component in that little sliver of blood, for example, platelets. These little cell fragments travel through the blood until they bump into an injured blood vessel. At that time, they start the process of making a blood clot. If this injury took place on your skin, this blood clot might be better known as a scab. Just like that one Blink-182 song, platelets wander around with no purpose or direction, and they don't owe us a single explanation. So, white blood cells help protect us, platelets clot our blood after an injury, but what do red blood cells do? These donut-shaped cells are squishy, bendable cells with one main purpose. Ferry oxygen through the blood to the rest of the body. These things are tiny, only six microns wide. That's a good thing, too, because they need to fit through the tiniest blood vessels in the body without clogging them shut. Red blood cells can carry oxygen because they contain a protein called hemoglobin, made of iron, which is the heme part, and four globin proteins, hence globin. Oxygen molecules bind onto those hemoglobin proteins so they can hitch a ride through the bloodstream. All those iron molecules give red blood cells their color, too. Just like how Mars has a bunch of iron on its surface and looks red, it's the same iron atoms coloring our blood. And these cells are totally optimized to carry oxygen or to be oxygenated. In mammals, red blood cells don't have a nucleus, which gives them more space for hemoglobin, and their donut shape gives them more surface area for oxygen to seep through. Plus, oxygen binds to this protein in a way that when one oxygen molecule binds to hemoglobin, it becomes easier for more oxygen molecules to hop on that same hemoglobin afterwards. And as those oxygenated red blood cells are pumping through your arteries, they'll drop off some oxygen at active tissues or anywhere in the body that oxygen is needed. Now, red and white blood cells are the major cells in our blood and can tell us a lot about our health. But that other stuff, all the substances dissolved in the plasma, can also give us valuable health data. Like if you go in for a checkup and your doctor orders a blood test, they'll probably measure things like cholesterol, triglycerides, midichlorians, the usual. There are definitely times when they'd want to measure red and white blood cell counts, but a lot of the substances you're used to hearing about are dissolved in plasma. And one of the ways to use that information is to predict your risk of disease. For example, a fasted blood sugar test is one of the things doctors might use to diagnose diabetes. This test is built on the idea that people with diabetes may have elevated levels of glucose in their blood compared to non-diabetics. But all kinds of blood tests can tell us different things about your health. Like measuring blood electrolytes can give us information about kidney disease and counting the number of white blood cells can tell us the status of the immune system. But doctors' tests are getting sensitive and specific enough to find all kinds of really small compounds in the blood that can give them hints about diseases elsewhere in the body. And not just body parts directly in the circulatory system, but all over. These compounds are called biomarkers, literally biological markers. They can be used as clues for doctors to make better diagnoses or to predict outcomes more accurately. Now, technically metrics like pulse and blood pressure count as biomarkers, but scientists are finding all kinds of new biomarkers in the lab that can help them make decisions about healthcare. For example, scientists can measure extremely tiny fragments of genetic material in someone's blood called microRNA and make predictions about that person's risk of specific cancers. Here's what's so cool and exciting about that, though. For just about every form of cancer, the outcomes are much better if the disease is recognized and treated early or prevented altogether. And microRNA are very specific to cell type and disease states so they can give us very precise information about someone's cancer diagnosis and where to look. So this biomarker in your blood might give doctors the ability to detect certain cancers earlier and target therapies more precisely. Take one of the deadlier forms of cancer, colorectal cancer. Scientists noticed that if they found a type of microRNA called microRNA21 in someone's blood, there was a very good chance that that person had colorectal cancer. Or if they had more of microRNA141, participants tended to have a worse chance of survival, which means this biomarker could give doctors a more accurate prognosis. So not only does blood carry out so many important functions for our survival, it can give us clues about diseases that may help us survive longer. But we left out a pretty important detail in this video. You know that whole ABO blood typing thing? Well, our next video will get you up to speed on how that system works and how we're getting close to turning everyone's blood into the universal donor type. Speaking of biomarkers, I've had a few concussions throughout my youth and every time, doctors ordered a CT scan for me. These days, a few biomarkers exist for diagnosing concussions, which not only improves diagnosis accuracy, but lets doctors objectively measure how severe the concussion is. I'm Patrick Kelley. Thanks for watching this episode of Seeker.