High school biology
Introduction to the circulatory system and the heart. Created by Sal Khan.
Where I left off in the last video, we talked about how the hemoglobin in red blood cells is what sops up all of the oxygen so that it increases the diffusion gradient-- or it increases the incentive, we could say, for the oxygen to go across the membrane. We know that the oxygen molecules don't know that there's less oxygen here, but if you watch the video on diffusion you know how that process happens. If there's less concentration here than there, the oxygen will diffuse across the membrane and there's less inside the plasma because the hemoglobin is sucking it all up like a sponge. Now, one interesting question is, why does the hemoglobin even have to reside within the red blood cells? Why aren't hemoglobin proteins just freely floating in the blood plasma? That seems more efficient. You don't have to have things crossing through, in and out of, these red blood cell membranes. You wouldn't have to make red blood cells. What's the use of having these containers of hemoglobin? It's actually a very interesting idea. If you had all of the hemoglobin sitting in your blood plasma, it would actually hurt the flow of the blood. The blood would become more viscous or more thick. I don't want to say like syrup, but it would become thicker than blood is right now-- and by packaging the hemoglobin inside these containers, inside the red blood cells, what it allows the blood to do is flow a lot better. Imagine if you wanted to put syrup in water. If you just put syrup straight into water, what's going to happen? The water's going to become a little syrupy, a little bit more viscous and not flow as well. So what's the solution if you wanted to transport syrup in water? Well, you could put the syrup inside little containers or inside little beads and then let the beads flow in the water and then the water wouldn't be all gooey-- and that's exactly what's happening inside of our blood. Instead of having the hemoglobin sit in the plasma and make it gooey, it sits inside these beads that we call red blood cells that allows the flow to still be non-viscous. So I've been all zoomed in here on the alveolus and these capillaries, these pulmonary capillaries-- let's zoom out a little bit-- or zoom out a lot-- just to understand, how is the blood flowing? And get a better understanding of pulmonary arteries and veins relative to the other arteries and veins that are in the body. So here-- I copied this from Wikipedia, this diagram of the human circulatory system-- and here in the back you can see the lungs. Let me do it in a nice dark color. So we have our lungs here. You can see the heart is sitting right in the middle. And what we learned in the last few videos is that we have our little alveoli and our lungs. Remember, we get to them from our bronchioles, which are branching off of the bronchi, which branch off of the trachea, which connects to our larynx, which connects to our pharynx, which connects to our mouth and nose. But anyway, we have our little alveoli right there and then we have the capillaries. So when we go away from the heart-- and we're going to delve a little bit into the heart in this video as well-- so when blood travels away from the heart, it's de-oxygenated. It's this blue color. So this right here is blood. This right here is blood traveling away from the heart. It's going behind these two tubes right there. So this is the blood going away from the heart. So this blue that I've been highlighting just now, these are the pulmonary arteries and then they keep splitting into arterials and all of that and eventually we're in capillaries-- super, super small tubes. They run right past the alveoli and then they become oxygenated and now we're going back to the heart. So we're talking about pulmonary veins. So we go back to the heart. So these capillaries-- in the capillaries we get oxygen. Now we're going to go back to the heart. Hope you can see what I'm doing. And we're going to enter the heart on this side. You actually can't even see where we're entering the heart. We're going to enter the heart right over here-- and I'm going to go into more detail on that. Now we have oxygenated blood. It's red. And then that gets pumped out to the rest of the body. Now this is the interesting thing. When we're talking about pulmonary arteries and veins-- remember, the pulmonary artery was blue. As we go away from the heart, we have de-oxygenated blood, but it's still an artery. Then as we go towards the heart from the lungs, we have a vein, but it's oxygenated. So that's this little loop here that we start and I'm going to keep going over the circulation pattern because the heart can get a little confusing, especially because of its three-dimensional nature. But what we have is, the heart pumps de-oxygenated blood from the right ventricle. You're saying, hey, why is it the right ventricle? That looks like the left side of the drawing, but it's this dude's right-hand side, right? This is this guy's right hand. And this is this dude's left hand. He's looking at us, right? We don't care about our right or left. We care about this guy's right and left. And he's looking at us. He's got some eyeballs and he's looking at us. So this is his right ventricle. Actually, let me just start off with the whole cycle. So we have de-oxygenated blood coming from the rest of the body, right? The name for this big pipe is called the inferior vena cava-- inferior because it's coming up below. Actually, you have blood coming up from the arms and the head up here. They're both meeting right here, in the right atrium. Let me label that. I'm going to do a big diagram of the heart in a second. And why are they de-oxygenated? Because this is blood returning from our legs if we're running, or returning from our brain, that had to use respiration-- or maybe we're working out and it's returning from our biceps, but it's de-oxygenated blood. It shows up right here in the right atrium. It's on our left, but this guy's right-hand side. From the right atrium, it gets pumped into the right ventricle. It actually passively flows into the right ventricle. The ventricles do all the pumping, then the ventricle contracts and pumps this blood right here-- and you don't see it, but it's going behind this part right here. It goes from here through this pipe. So you don't see it. I'm going to do a detailed diagram in a second-- into the pulmonary artery. We're going away from the heart. This was a vein, right? This is a vein going to the heart. This is a vein, inferior vena cava vein. This is superior vena cava. These are veins. They're de-oxygenated. Then I'm pumping this de-oxygenated blood away from the heart to the lungs. Now this de-oxygenated blood, this is in an artery, right? This is in the pulmonary artery. It gets oxygenated and now it's a pulmonary vein. And once it's oxygenated, it shows up here in the left-- let me do a better color than that-- it shows up right here in the left atrium. Atrium, you can imagine-- it's kind of a room with a skylight or that's open to the outside and in both of these cases, things are entering from above-- not sunlight, but blood is entering from above. On the right atrium, the blood is entering from above. And in the left atrium, the blood is entering-- and remember, the left atrium is on the right-hand side from our point of view-- on the left atrium, the blood is entering from above from the lungs, from the pulmonary veins. Veins go to the heart. Then it goes into-- and I'll go into more detail-- into the left ventricle and then the left ventricle pumps that oxygenated blood to the rest of the body via the non-pulmonary arteries. So everything pumps out. Let me make it a nice dark, non-blue color. So it pumps it out through there. You don't see it right here, the way it's drawn. It's a little bit of a strange drawing. It's hard to visualize, but I'll show it in more detail and then it goes to the rest of the body. Let me show you that detail right now. So we said, we have de-oxygenated blood. Let's label it right here. This is the superior vena cava. This is a vein from the upper part of our body from our arms and heads. This is the inferior vena vaca. This is veins from our abdomen and from our legs and the rest of our body. So it it first enters the right atrium. Remember, we call the right atrium because this is someone's heart facing us, even though this is on the left-hand side. It enters through here. It's de-oxygenated blood. It's coming from veins. the body used the oxygen. Then it shows up in the right ventricle, right? These are valves in our heart. And it passively, once the right ventricle pumps and then releases, it has a vacuum and it pulls more blood from the right atrium. It pumps again and then it pushes it through here. Now this blood right here-- remember, this one still is de-oxygenated blood. De-oxygenated blood goes to the lungs to become oxygenated. So this right here is the pulmonary-- I'm using the word pulmonary because it's going to or from the lungs. It's dealing with the lungs. And it's going away from the heart. It's the pulmonary artery and it is de-oxygenated. Then it goes to the heart, rubs up against some alveoli and then gets oxygenated and then it comes right back. Now this right here, we're going to the heart. So that's a vein. It's in the loop with the lungs so it's a pulmonary vein and it rubbed up against the alveoli and got the oxygen diffused into it so it is oxygenated. And then it flows into your left atrium. Now, the left atrium, once again, from our point of view, is on the right-hand side, but from the dude looking at it, it's his left-hand side. So it goes into the left atrium. Now in the left ventricle, after it's done pumping, it expands and that oxygenated blood flows into the left ventricle. Then the left ventricle-- the ventricles are what do all the pumping-- it squeezes and then it pumps the blood into the aorta. This is an artery. Why is it an artery? Because we're going away from the heart. Is it a pulmonary artery? No, we're not dealing with the lungs anymore. We dealt with the lungs when we went from the right ventricle, went to the lungs in a loop, back to the left atrium. Now we're in the left ventricle. We pump into the aorta. Now this is to go to the rest of the body. This is an artery, a non-pulmonary artery-- and it is oxygenated. So when we're dealing with non-pulmonary arteries, we're oxygenated, but a pulmonary artery has no oxygen. It's going away from the heart to get the oxygen. Pulmonary vein comes from the lungs to the heart with oxygen, but the rest of the veins go to the heart without oxygen because they want to go into that loop on the pulmonary loop right there. So I'll leave you there. Hopefully that gives-- actually, let's go back to that first diagram. I think you have a sense of how the heart is dealing, but let's go look at the rest of the body and just get a sense of things. You can look this up on Wikipedia if you like. All of these different branching points have different names to them, but you can see right here you have kind of a branching off, a little bit below the heart. This is actually the celiac trunk. Celiac, if I remember correctly, kind of refers to an abdomen. So this blood that-- your hepatic artery. Hepatic deals with the liver. Your hepatic artery branches off of this to get blood flow to the liver. It also gives blood flow to your stomach so it's very important in digestion and all that. And then let's say this is the hepatic trunk. Your liver is sitting like that. Hepatic trunk-- it delivers oxygen to the liver. The liver is doing respiration. It takes up the oxygen and then it gives up carbon dioxide. So it becomes de-oxygenated and then it flows back in and to the inferior vena cava, into the vein. I want to make it clear-- it's a loop. It's a big loop. The blood doesn't just flow out someplace and then come back someplace else. This is just one big loop. And if you want to know at any given point in time, depending on your size, there's about five liters of blood. And I looked it up-- it takes the average red blood cell to go from one point in the circulatory system and go through the whole system and come back, 20 seconds. That's an average because you can imagine there might be some red blood cells that get stuck someplace and take a little bit more time and some go through the completely perfect route. Actually, the 20 seconds might be closer to the perfect route. I've never timed it myself. But it's an interesting thing to look at and to think about what's connected to what. You have these these arteries up here that they first branch off the arteries up here from the aorta into the head and the neck and the arm arteries and then later they go down and they flow blood to the rest of the body. So anyway, this is a pretty interesting idea. In the next video, what I want to do is talk about, how does the hemoglobin know when to dump the oxygen? Or even better, where to dump the oxygen-- because maybe I'm running so I need a lot of oxygen in the capillaries around my thigh muscles. I don't need them necessarily in my hands. How does the body optimize where the oxygen is actually delivering? It's actually fascinating.