The Science of Everything Podcast - Episode 122: Respiratory and Circulatory Systems Part 1

Episode Date: October 31, 2021

The first in a two-part episode covering the circulatory and respiratory systems, including a discussion of the anatomy of the heart, the process of contraction, and the generation and propagation of electrical activity. I also discuss how the heart functions as a pump, the various types of blood vessels, and control of the heart rate by the brain. Recommended pre-listening is Episode 25: Tissues, Organs and Systems, and Episode 26: Human Organ Systems. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

You're listening to The Science of Everything podcast, episode 122, Respiratory and Circulatory Systems Part 1. I'm your host to James Fodor. In this two-part series of episodes, we're going to look at, well, as I said, the respiratory and the circuitory systems. And that's going to include a discussion of the heart and the control of the heart rate, blood vessels, how blood is circulated throughout the body, discussion of the structure and function of blood and chemical and biochemical aspects thereof.

and a discussion of the respiratory system and the function and structure of the lungs, breathing, control of respiration and so forth. Recommended pre-listening for this episode is episodes 25 and 26, so tissues and organs systems and human organ systems, episode 26. Those both be helpful for understanding what I'm talking about here. And that leads me to another important announcement that I need to make regarding the show, which is that some of you may have seen this on the Facebook group if you're a part of that. I ask people about what they think I should do. Basically, there's been an issue that it's come to my attention that some of the early episodes of the podcast, roughly the first 20 or so, or most of the first 20, have not been appearing on everyone's feed,

particularly if you use Spotify, although I think it may be an issue for some other platforms as well. And looking into this, I've worked out the reason for that is because for some reason, many of the earliest episodes were uploaded in M4A format, fire format instead of MP3, which is what I use now. I don't know why I did that, to be honest. It seems like a really odd format to use, but that was years ago. I think there was a reason at the time, because I remember doing something about this.

I had to actually re-upload some of the earliest episodes like 10 years ago now, but I don't remember what the reason was, and it seems to, of course, some issues now. For some reason, Spotify doesn't recognize M4A files. Maybe that will change at some point in the future, but in the meantime, I don't like those episodes being inaccessible, and particularly because I refer to them as prerequisites. You just notice here that I referred back to episodes 25 and 26, which are pretty early,

and there are some other important ones there, like Episode 10 on The Cell, I refer back to it fairly often, or History of the Adam, Episode 8, chemical bonding episode 15. So, unfortunately, it doesn't seem possible for me, to, well, it's not possible for me the way that I use my feed and the service that I use to just change the files to MP3. I have to re-upload them as a separate episode, which means that they'll appear out of order in the feed. And this is the real problem, right? Otherwise, I just fix it. What this means is that I'm going to have to re-upload roughly the first 20 episodes, or at least

most of the first 20 episodes, and they're going to appear in the feed as like new episodes. Now, that's going to be a little bit annoying because I like to have things in order, right? and, you know, that could create some confusion. So be aware that that's going to happen. I'm going to do that shortly after I upload this one, so maybe a few days I'm not exactly sure if I'll do them all at once or spread it out a little bit. Now, they will keep the original numbering.

What I've decided to do is remove all of the old episodes from the feed. I think I may be able to keep episode one, because I think that that's MP3, just sort of a nice, at least it's uploaded at the original date. Most of the next 20, I'll remove from the feed and then re-upload. So they'll keep the episode numbering. So the numbering will be preserved, which is the most important thing because I refer to episodes by their number, not when they were released.

So there shouldn't be any duplicates of episodes, and the numbering should be preserved. The only annoying thing is that the episodes will appear in the feed out of order, which I think is not the worst thing in the world because I suspect most people aren't downloading the entire archive. You're probably just downloading a few that you're listening to and then remove them from your device after you listen to them. At least that's how I do it, and that's how devices typically work. So hopefully it's not too confusing.

If you've already listened to these episodes, then you can just mark them as listen to in your feed or however it works on your advice. I do apologize for this. I know it's kind of annoying, but I figure that this is the best way around this problem. On the other hand, there may be some people who've been listening for a while who haven't gone back to those early ones, and so maybe that's an opportunity for you to listen to those. But yeah, just understand that that's what's happening. That's why all of these will be appearing in the feed. you may have noticed that episode, I think five appeared in the feed because I was doing a test to see how this would work. So that's the reason for that.

Yeah, so after this episode comes out, there's going to be a series of releases of roughly episodes 2 to 20-ish. And those are all existing content. So don't be confused about that. After those are released, then sometime next month will resume regular programming, as they say. So apologize if that's annoying for you, but I've decided that that's the best way to proceed here. All right. that's enough of that. Let's now get started in the first part of this episode in which we're going to talk about the circulatory system and specifically we're going to start with the heart. Essentially, the heart is a pump that circulates blood in blood vessels around the body and the blood delivers nutrients and oxygen to tissues and takes away carbon dioxide and other waste products.

That blood is then separately pumped to the lungs where it picks up oxygen and comes back to the heart where it's then pumped around the rest of the body. That's like the very basic idea. So I'm sort of assuming that, you know, people already know that. I guess we will go through that in more detail. The reason I say that I'm sort of assuming you know it is because I've decided to start at the heart. It's a little bit difficult to know where to start with something like the circulatory system because sort of by nature it's a closed loop, right? So it doesn't really have a start point.

And other sources will start with the blood or they'll start with the blood vessels. You could start at the lungs. I've decided to start at the heart because it's sort of central and it provides a a sort of a reference point from which we can sort of go to and come back. And, you know, it's sort of the heart of the heart of everything, so to speak. So that's where we're starting here. But, you know, beware that there's, the whole thing's a loop, right? So that's not really a starting point. And some things mainly make sense if you, if, after we've discussed all the other aspects of it. So anyway, bearing that in mind, let's talk about the heart. So the heart is a

muscular organ. And it took a while for this to be discovered that the heart is actually a muscle. It's maybe not entirely obvious. So there are three different types of muscle in mammals. There's smooth muscle, skeletal muscle and cardiac muscle. So the heart obviously being cardiac muscle. I will do an episode soon on the muscular system and we'll talk about how skeletal muscle works. So I'm not going to talk here about like the process of the contraction of the heart in terms of how the muscle actually contracts because that will be discussed in that episode because it's fairly similar to skeletal muscle and smooth muscle in terms of the fundamental operation. But yeah, just for the moment, bear in mind that there are three

types of muscle. I think that I did talk about this in the episode 25 on different types of tissue. And cardiac muscle is what makes up the majority of the heart, and its role is to pump blood through the blood vessels of the circulatory system. And it's sort of fundamental to understand how the whole thing works. Basically, in order to get nutrients and oxygen to all of the parts of the body, they need to be a force that pushes them there. And the heart provides that force through a series of rhythmic contractions that pushes the blood out around the body. And then the blood is drawn back to the heart where it is then pushed back around the body again. Obviously there's more to it, but we'll get to those details.

That's the basic idea here. Now the heart is situated at the level of the thoracic vertebrae 5 to 8. You probably roughly know where the heart is. The heart is asymmetrical. So typically the larger sort of swelling of the heart is on the left-hand side, and that's because that's where the left ventricle is, which pumps heart around the majority of the body, and so it needs to be sort of larger and stronger to do that.

Again, we'll get into more detail of that later. There are some people for whom this is different. I can't remember the name of the condition where people actually have the larger section of the heart or more of the heart over to the right-hand side, but that's fairly unusual. For most people, it's sort of swells or it is larger on the left-hand side, but it's sort of center, if you like, is roughly in the middle.

So it's a little bit of a misconception that the heart is on the left-hand side because it's sort of centered roughly around the sternum or like under the sternum, but yeah, it sort of swells over to the left. But often if you have heart pain, it feels like it's on the left-hand side. And the heartbeat is also more readily felt on the left because that's, again, where the left ventricle is, which we'll get into. Let's talk a bit about the structure of the heart. So the heart is contained inside the thoracic cavity, and so it has a series of layers,

that layers of tissue that separate it from the surrounding cavity. So the heart wall itself consists of the, it's mostly the cardiac muscle, right? So that's called the myocardium. Oh, that reminds me. I wanted to mention, I won't always be using the correct anatomical names for everything, right? Because this is not an anatomy lecture. And, I mean, I certainly not super familiar with the names of the Latin names of everything, nor is it very useful, I think, for this podcast. So I'll use the proper names for certain anatomical features, but other times I will just describe them and all their function. So bear that in mind if you're an anatomist out there, you'll have to forgive me for this. But I think it's a better way to do an audio sort of podcast.

Anyway, yeah, so the myocardium is the thickest part of the heart wall and consists of cardiac muscle. Interior to that is a thin layer of epithelium tissue, so squamous epithelium. So that's basically, well, a thin, a thin flat layer of tissue. That's called the endocardium. That's kind of easy because it's on the inside, right? So you've got your endocardium, just a lining of tissue on the inside where the blood is. Then there's the really thick myocardium.

And then exterior to that, there's the outer epicardium. So that's on the outside. And that's a connective tissue which kind of connects the heart to its surroundings. So that's the heart wall. It consists of these three layers of the epithelium, muscle, and then connective tissue. Now, surrounding the heart, or surrounding the heart wall on the outside, side of that is a double membrane sac called a pericardium. So there's the heart wall itself, which has three layers, and then there's a small cavity, and then it's surrounded by another

double membrane, which is the sack that protects the heart and kind of connects it to to the inside of the thoracic cavity. And this layer consists of two separate tissues. I won't mention the names of them, but they're sort of fibrous connective tissues that gives support and helps to protect the heart and anchor it to the thoracic cavity. All right, so that's what the heart is. It's mostly muscle with some connective and epithelial tissue sort of thrown in to protect it. Now, the heart has four chambers. There are two atrial, or each one being an atrium, and two ventricles.

So I said I won't use the technical names for everything, but these are important, right? So four chambers, different types of organisms have different numbers of heart chambers. I'm pretty sure all mammals have four heart chambers. I hope I've got that right, but here I'm going to be focusing on the human heart. I should have clarified that at the outset. When I talk about the circulatory and respiratory systems, I'm talking about human ones, although most of this is going to be applicable to most mammals as well from my recollection. There are different types of organisms that have different numbers of chambers of hearts

and their circulatory systems work differently, but here we're talking about the human one, the mammalian one more generally. So we've got four chambers. We've got two atria and two ventricles, one of each on the left and one on the right. Now, one thing that I want to mention at the outset, if you're using any or have seen or are using diagrams to help follow the episode, or which I recommend doing, by the way, there's a convention in anatomy in which the left-hand side of the body is drawn or represented on the right-hand side of the diagram and vice versa. So it's essentially flipped. I believe the reason for

this is simply that when a physician is looking at or performing an operation on a corpse or a body, then it's sort of flipped from their perspective, right? So the left-hand side of your heart will be on the right-hand side of the surgeon who's operating on you, for example. I believe that's the reason. So that's important because you see a diagram and on the right hand side it will say left ventricle. And you're like, but that's not the left ventricle. You've just got to think about it, right? But you're looking at it from the front, from the perspective of the person whose heart it is, right? It is their left ventricle.

So just don't get confused there. That could be a source of confusion here. Okay, so we're talking about the four chambers of the heart. On the left hand side, there's your left ventricle and the left atrium above that. On the right hand side, your right ventricle and the right atrium above that. Now, the way to think about this is that the atrium, oh, the atria, excuse me, are essentially entry chambers, which I believe is what the term means. So the blood first goes into the atria where it sort of fills up before it enters the ventricle. The ventricles are at the lower part of the heart where the muscle is thickest.

And that's because that's the part of the heart that really does the sort of pumping action that pushes the blood where it needs to go. Okay, now, the heart also has four major valves which separate its chambers from each other. So one valve lies between each atrium and its corresponding ventricle, and then there's also one valve at the exit of each ventricle. So the whole system forms sort of a pathway, right, that you sort of enter into the atrium through a valve. The blood then sort of fills up the atrium, and then at a certain point, it then passes through the valve into the corresponding ventricle. Once it's in the ventricle, when the ventricle contracts and pushes the blood out, it passes out through the valve and then out through the blood vessels to the relevant pathways that it then goes around. So basically there are four chambers and four valves and each chamber has a valve that sort of connects it to and allows the blood to pass

one directionally through to the next chamber. And that's important. The valves are shaped, it's sort of a bit hard to describe this shape, but they're shaped in a way that blood can be pushed through in one direction, but very hard for it to, for blood to go back the other way. There are abnormal heart conditions where that can happen. The blood can be going in directions that it shouldn't be, but basically the valves are structured so that it's sort of a unidirectional flow. You can think of that as sort of a bunch of flaps that are sort of pushed together so that if water can flow or blood can flow in one direction, but it's very hard to flow back in the other direction because it would require sort of pushing the flaps back through

each other and flipping back in the other direction, if you sort of see what I'm in there. If that's not clear, then don't worry too much. But basically these valves ensure that flow is typically unidirectional. So the heart pumps blood in a particular rhythm that's determined by a set of cells that are called the pacemaker cells. And we'll discuss these in more detail later. But there's a set of cells in a particular location called the sinoatrial node, which determines the rate at which the heart is pumping. And that can be then modulated by the brain and other factors.

Again, we'll get into that. But basically, there's a control center of the heart that's telling it at what rate to pump at, and it pumps in a regular rhythm, which then determines the rate of circulation throughout the body. Now, I now need to talk about how the overall sort of system works. So because I've talked about how, you know, blood comes in and blood's pumped around, but I need to explain that how it works, right? We've set up the players, so let's try to explain this in a way that makes sense without needing a diagram. And of course, that being the challenge of this show.

So, again, let's start from the perspective of blood coming in from the body. So again, because this is a circulatory system, there's no sort of starting point, but we can, and I think it's helpful to think about it from the perspective of there is deoxygenated blood that's coming back from the body, having exhausted its oxygen supply. depleted as oxygen supply and it's been filled up with carbon dioxide and other waste products. So this is called deoxygenated blood. So it comes back from the body and comes in through the, it comes in into the heart. So the first thing to understand is that the right side of the heart is where deoxygenated blood comes in. The left hand side of the heart is where oxygenated blood goes

out. So it's like an in-tray and an out-tray if you want to think of it that way. There's an inside and an inside and an outside. The inside is on the right-hand side. That's where deoxygenated blood comes back from the body. It comes in through the blood vessel called the Vena Kava into the right atrium and then from there through the valve into the right ventricle. The opposite happens on the left hand side where you've got oxygenated blood which is located in the left atrium which then passes through the valve into the left ventricle which is then pumped out of a blood vessel to the rest of the body and that blood vessel is called the aorta which you've probably heard of. So again, Vina Kava is the connects the right side of the heart and brings in deoxygenated blood

and the aorta is the big blood vessel which on the left hand side of the heart connects the left ventricle to the rest of the body to provide oxygenated blood. So again, I won't typically make a big deal of using those terms because some people may not be familiar with them, but those are very important Levinna Kava and aorta, so I just sort of mentioned those. The way I remember it is that Vena cava sounds a lot like veins, and the veins are the blood vessels that carry blood back to the heart. So the veins lead back to the vina cava, which then enters into the right-hand side of the heart. So that's the way I remember, but don't worry too much about that. All right.

So far, we've got a picture where we've got de-oxygenated blood coming in on the right-hand side of the heart, first in the right atrium and then into the right ventricle, and then on the left-hand side we've got the opposite. We've got oxygenated blood coming into the left atrium, and then into the left ventricle and then being pumped out around to the rest of the body. But there's obviously a missing step here, right? Because somehow the blood has to go from being deoxygenated in the right ventricle to being oxygenated in the left atrium.

So how do we connect that? And that is, of course, where the lungs come in. The lungs are where oxygenation occurs, where the blood gets its oxygen from. And I'll talk more about the lungs in the second part of this series. For the moment, however, what's important to understand is how the blood kind of gets there and gets back. So when the deoxygenated blood is in the right ventricle and when the heart contracts, it is, that blood is pumped out through pulmonary arteries to the lungs where it picks up oxygen, and then it returns back through the pulmonary veins to the left atrium. So this is the

pulmonary circuit of the circulatory system. It connects the heart to the lungs and is essentially how the blood gets back its oxygenation. This is distinguished from the systemic circulation, is the rest of the body. And the way to sort of think about this is that the systemic circulation is, you know, mostly what the heart's for, right? It's providing oxygen to the body. However, the blood has to get oxygen itself, like it's got to get it from somewhere. It gets it from the lungs. And that happens in the pulmonary circulation. So in some sense, the pulmonary circulation is like the opposite of the systemic circulation. It's doing the opposite thing. Instead of depleting the oxygen of the blood, it's picking up the oxygen. It's also the opposite in another sense, because

arteries are defined as the blood vessels that carry blood away from the heart. heart, whereas veins carry blood back to the heart. Now, in the systemic circulation, arteries are oxygenated. They carry oxygenated blood, which makes sense, right? Because the whole point is to get oxygenated blood from the heart, carry it to the body, and then once the blood has been depleted of oxygen, you know, as it's carried around to the tissues, then that deoxygenated blood has to go back to the heart. In the pulmonary circulation, it's the opposite, right? Because deoxygenated blood has to go from the heart to the lungs and come back as oxygenated blood. because obviously it's picking up the oxygen.

So the pulmonary circulation is the only place typically where you'll have arteries carrying deoxygenated blood and veins carrying oxygenated blood. And that can be a little counterintuitive, again, because you typically think of arteries, oh, they carry the oxygenated blood away from the heart. But in the pulmonary circulation, arteries still carry blood away from the heart,

but there it's actually deoxygenated blood. So you've got to separate sort of mentally the systemic circulation, which is sort of mostly what the heart is doing, and that's what we think of it is doing. from the pulmonary circulation, which is where the blood is picking up the oxygen that it needs to get around to the rest of the body. So let's go through that again, right? So the deoxygenated blood comes back from the systemic circulation, from the rest of the body, and through the vina cava, enters into the right atrium. Then, you know, in the right stage of the contraction,

it passes through a valve into the right ventricle. It's still deoxygenated at this point. then when the heart contracts, the deoxygenated blood is pushed through the pulmonary arteries, through another valve, into the pulmonary arteries, and thence to the lungs where it picks up oxygen. The now oxygenated blood comes back to the heart through the pulmonary veins. Again, it's oxygenated in the veins because this is still part of the pulmonary circulation. But once it comes back into the left atrium, it's back in the heart now, and the pulmonary circulation part is finished. We now have oxygenated blood in the left atrium at the right. At least atrium, at the right stage and contraction it passes through another valve into the left ventricle

and then at the right stage of contraction it is then pumped through the final valve out through the aorta and to the circulatory or systemic circulation and then carries oxygenated blood throughout the body where it depletes its oxygen comes back through the veins through the vina caver into the right atrium where it passes into through the valve into the right ventricle and then back through the pulmonary arteries to the lungs and then it picks up oxygen and then comes back through the pulmonary veins into the left atrium and then to the left ventricle and then it's pumped out through the aorta round to the systemic circulation and then it comes back and the cycle goes around right so hopefully you get the idea there so it's two cycles in one two

circulations in one systemic where it depletes oxygen to the body and then pulmonary where it picks up the oxygen from the lungs the left side of the heart is responsible for the oxygenated portion of the blood coming from the lungs and then pumped out to the body, whereas the right-hand side of the heart is responsible for the de-oxygenated part of the blood, comes back from the systemic circulation, and then it's pumped out to the lungs to pick up the oxygen. So that's how the sort of division of labour is divided up in the two sides of the heart. And that's also why the left hand, the left side of the heart is typically the larger, because the left ventricle needs to be bigger and stronger than the right ventricle, because the right ventricle just has to pump the blood

from the heart to the lungs, which is not that far. Whereas the left ventricle has to pump the blood from the heart to everywhere else, including the brain and down to your toes everywhere. So it has to be a lot stronger. It needs higher pressure, essentially, a greater force to do that. So that's why there's the asymmetry there, because they have different jobs, and basically the left ventricle has the harder job,

if you want to think of it that way. Right, so there's the structure of the heart and how it sort of works with respect to the basic circulation. Now, I need to talk about the process of heart contraction, because I've been, I mentioned that a number of times. They've said like at the right phase of contraction, but I haven't sort of explained how that works. So that's the next step.

So the heart contracts, not all at once, but there's a series of phases that it goes through in the process of contraction. This is necessary for it to work in order. There's a series of motions or moves or maneuvers or stages that it has to process through, otherwise, you know, that's not going to work properly. This process of contraction is called a cardiac cycle, the sequence of events that occurs when the heart contracts and then relaxes with every heartbeat.

The basic idea of the cardiac cycle is that both of the ventricles will need to contract. The right ventricle will need to contract to push blood to the lungs, the left ventricle will need to contract to push oxygenated blood throughout the body. I mentioned before that there are special cells which are responsible for producing and maintaining the cardiac cycle. Now, these cells are located at the sinaiatrial node, which is in the right atrium, and they continually depolarize and then repolarize at a regular rhythm, so as to produce a series of action potentials, which then are propagated throughout the heart.

Effectively, you can think of this as a three-stage process. It's a little bit more than that, but for our purposes here, I think three stages will be helpful. So the first stage is when the cardiac cells in the synioratial node depolarized, so they produce an action potential. That activation then spreads to another site called the atrioventricular node. When that site depolarizes and activates, that then leads to contraction of the atria. the second stage, basically propagation of the activity to the AV node and contraction of the atria, left and right at the same time. The third stage is when the electrical activity is then passed

from the AV node down through particular fibers called Pukingji fibers, which are sort of like in the located roughly in the middle of the heart, and then is propagated out along the bottom of the heart to the muscles surrounding both of the ventricles, because the ventricles at the bottom at the top, right? So you get the activity kind of comes from the top, down the middle, and to the bottom and then around out the sides. And as that activity spreads around the ventricles, it triggers the contraction of the ventricles. And then those relax, and then the cycle is repeated, where the cells at the SA node excite again, and that triggers the excitation of the AV node, which triggers the contraction of the atria, which then propagates activity down through

and around, through the middle of the heart, around the ventricles, and then triggers the contraction of the ventricles. This cycle repeats regularly over and over every time your heart beats, or at least if your heart's working properly. And it's remarkable because it has to do this, you know, continually for however long you live, right? You can't go without your heart beating on a regular basis because then you'll lose consciousness and eventually die if it's not restarted. So it's a remarkable machine that can beat continually for 70, 80, 90 years with no maintenance or, you know, with no time taken down for certain maintenance or anything like that. It's quite insane. Now, a little bit more explanation as to how this electrical activity works.

Muscle cells are different to neurons. When we talk about electrically excitable cells, We typically think about neurons, but here we're not talking about neurons. We're actually talking about cardiac muscle cells. And they're electrically excitable as well. They produce action potentials, but they're different to action potentials that are produced from neurons, as we'll talk about in a moment. Another important difference is that the muscle cells are all connected together by special structures called gap junctions, which I may have mentioned in a previous episode about cell structure and function.

I can't remember. And basically they allow the action potential, the ions, to directly move from one cell to the other. So this ensures that the whole process is much faster and more reliable than it would be if there was a series of synapses that say you have in the nervous system. So the cardiac muscle cells are all directly connected to each other via these gap junctions, which basically like perforations in the membrane, literally gaps that the ions can pass through, which connects their cytoplasms directly to each other. Now it still takes time for that to propagate, right, so that the entire heart doesn't contract

at once, but it does contract fairly quickly sort of in succession as the activity spreads and propagates from the SA node through the AV node and then down through the middle and around the ventricles. The process here is also a little bit different to neurons because as I mentioned, cardiac cells fire action potentials, or they're electrically excitable in much the same way that neurons are. And you can see past episodes where I've talked about how that works in neurons. I won't go through that here. But there is an important difference. So typically what happens in neurons when they're electrically excited is that there's the sodium channels open. So there's an initial influx of sodium because sodium is typically in low concentration inside the cell. and higher concentration outside. So first of all, what will happen when the certain voltage is reached is that the sodium cells will flood inside. Sodium is positively charged, so that increases the

internal potential of the cell. It increases the membrane potential. That's called depolarization, because it's going from initially negative to a positive polarity as the sodium rushes in. But then the sodium channels will close, and the potassium channels will open. Potassium is the opposite to sodium. It's highly concentrated inside the cell, but lower concentration outside. So when the potassium channel is open, that's selective to potassium, now the potassium rushes out. Because potassium is positively charged, now the inside of the cell loses that positive charge. And so it repolarizes. It goes from having a positive potential to a more negative potential again.

And it overshoots a little bit and then sort of returns back to its resting potential, which is like negative 60 or something like that. I mean, depends on the exact type of cell. So basically, the resting potential is negative. It depolarizes into a positive polarity. because of sodium, and then when potassium is allowed to flow out, it repolarizes back to the negative potential. That's how action potentials work in neurons, and more or less how they work in cardiac tissue as well, cardiac cells. But there is a difference. And that difference is that instead of a quick

upward tick and then a fairly rapid repolarization, instead of that, you get this long plateau phase in muscle cells, which does not happen in neurons, where basically the cell is held at a depolarized like positive potential for a few hundred milliseconds instead of going pretty quickly back down to the negative resting potential. It's held at a depolarized level for in the scheme of things a fairly long time. And that works through the operation of calcium channels, which essentially slowly open and then slowly close in a way that allows that potential to stay higher. Calcium is positively charged and so is a way of maintaining that depolarization at the positive level. Remember, normally it's negative. So the calcium is stored in the cyclone.

Plasmic reticulum, which is a structure that's found in muscle cells that holds calcium. Calcium also plays an important role in muscle contraction, but we'll get to that part later. But here, basically, that calcium is slowly released into the cytosol, which maintains the depolarization for longer than it normally would, until the calcium channels close and its calcium is eventually pumped back into the cycloplasm reticulum, and then finally the cell can repolarize and return to resting potential. So that long plateau phase, thanks to the operation of the calcium ions, is very important because that is the phase where muscle contraction occurs. And it's important that there's a sort of extra time there because otherwise you could have

issues of the muscle contraction occurring at the wrong time relative to when the cells are repolarizing. So basically that long plateau phase helps to keep everything in the right order and keep things synchronized because muscle contraction essentially takes longer than typical neurons take to fire. So things would get out of sync if you had regular very rapid firing of action potentials like you can have in the nervous tissue. So instead you have this longer plateau phase which helps to keep things synchronized and keep things in order. Now, as I mentioned, I'm not going to explain here how the muscle cells contract because I'll explain that when we get to skeletal muscle. It's fairly similar in the overall outline.

Basically, we've got these fibers that sort of move into each other and therefore shorten the muscle, which causes it to sort of tighten it and contract. and that's mediated through the calcium ions that I mentioned before, but again, we'll get into that later in a future episode. For our purposes here, it's just important to understand that that process is mediated by the electrical excitation, again, that starts in the SA node, moves to the AV, and then down to the ventricles, and then contracts the ventricles. So the atria contract first, and then the ventricles, and then there's a relaxation period, and then you repeat the cycle. So overall, this leads to a fairly regular rhythm, well, very regular, actually, a rhythm of the heart, and this is what elic at two, carefully control where the blood is going and at what times. So here's how it works. First, there's activity in the SA node, which is spontaneous depolarization, which sort of is the initiation of the process. That then travels to the AV node, which depolarizes and spreads the activity around both of the

atria and causes the atria to contract. So the muscle around the atria, the top parts of the heart to contract. What that does is it forces blood from the atria, where it's been gradually filling in the atria. It forces the blood into the ventricles. So that's the purpose of atrial contract, it doesn't actually push blood around the body, it pushes it from the atria into the ventricles. At that time, the valves that connect the atria to the ventricles are open, obviously, otherwise the blood wouldn't be able to get into them. But the valves that connect the ventricles to the rest of the body, to the pulmonary circuit and to the systemic circuits, those are closed.

They need to be because otherwise blood would start to be pushed out before it's ready. So it would be pushed from the atrium into the ventricle and then out either to the lungs or to the rest of the body. it's not ready for that yet, right? So the right valves need to be open at the right time. So it kind of makes sense, right? I mean, you want the blood to go from the atria to the ventricles, so you've got to open the valve between the atria and the ventricles and close the valves that separate the ventricles from the rest of the circulatory system. Okay, so at this stage, we've pushed the blood into ventricles. So this is both the deoxygenated blood in the right ventricle and the oxygenated blood in the left

ventricle. So after the atria has contracted, then the next stage is contraction of the ventricles, as the electrical depolarization sort of filters down through the middle of the heart and around the bottom side where it sort of surrounds the ventricles, the ventricles then contract. At this stage, the valves that connect the atria to the ventricles are closed. That happens around just as a contraction starts. There's a very precise order into it, which I won't try to describe here, is exactly when these different parts happen and exactly what the stages of contraction is. I'm not going to try to describe that here. I'm just giving the general idea. And the general idea is around when the ventricles contract, the valves that connect the ventricles to the atrial close.

Obviously, it has to, right? Because otherwise, it would push the blood back into the atria, which defeats the whole purpose, right? So those valves need to close. But the valves that connect the ventricles to the rest of the body, to the pulmonary circulatory system in the case of the right ventricle and the systemic circulation in the case of the left ventricle, those now need to open. And those open, I believe it's slightly after the ventricles begin to contract because there's a sort of a two stage in that contraction process. But I'm not going to belabor that point here. Basically, the ventricles start to contract. And then towards the end of that, the valves connecting them to the rest of the circulatory system, the rest of the circuit system,

open up. And then the blood is ejected from the ventricles, or most of it is ejected from the ventricles to either the lungs in the case of the right ventricle or the systemic circulation, the rest of the body in the case of the left ventricle. And that's called the ejection phase, where the high pressure that's been built up by the contraction of the ventricles, squirts the blood out around the circulation of the rest of the body. After that, there's a relaxation phase where all of the muscles relax and we sort of reset. And it's at that point, or just after that point, where the atrium start to refill with blood that's come back,

either from the lungs or from the rest of the body. So the atria start to fill up, and then we go back to the next phase, which is the valves that connects the ventricle and the atro opens up, and the ventricles start to fill. So again, we'll just recap that again. Basically, what happens is blood starts to come back from the lungs and from the systemic circulation, starts to fill the atria. Then the valves open up starts to fill in the ventricles.

Atria contract pushes the blood into the ventricles and out of the atria. Then the valves between the atria and the ventricles close. The ventricles start to contract. As they build up pressure, the valves connecting the ventricles to the pulmonary and the systemic circular system's open up. blood is squished out of the lungs through the pulmonary circulation and also through the systemic circulation, picking up oxygen in the case of the pulmonary circulation and bringing it back to the heart, and then in the case of the systemic circulation, it depletes oxygen around the systemic circulation and eventually it finds us way back to the heart. So that's the overall process. It's a two-stage

contraction process, atria contract, ventricles contract, and then there's a relaxation phase. The famous lub-dub sound of the heart essentially corresponds to this two-stage contraction process. there's more specific events that this can be associated with. I believe one of the contractions associated with one of the opening of the valves, and I think it's mostly the left side of the heart that's heard because that's sort of stronger. But again, just for simplicity here, you can think of it as lub dub is corresponding to the lub sound. The first part of the phase would be the atria contracting, and the second phase would be the ventricles contracting.

So that's how it sort of all works. The heart needs to regularly control its contraction, which is done through the sinoatrial as I mentioned before, so is to ensure that this series of excitation and contraction happens at a regular rate in the right order continually, until you die. And so it's quite remarkable how it all sort of fits together. Now, before we finish up this episode, I did want to talk a little bit about the control of the heart rate, because I've been mentioning that it kind of happens consistently at the same rate until you die. I mean, that's normally true, right? But obviously, the heart rate can change during exercise, for example, or if you hold your breath

or hyperventilate or whatever. So resting heart rate might be around 70, per minute as a rough average and it can increase dramatically up to you know 200 beats per minute at the high end. In addition stroke volume increases. So stroke volume is the amount of blood that's pumped by the heart and that's not the same all the time either. You can effectively think of that as like the intensity of the contraction. So the heart can regulate the amount of blood that it circulates by both increasing the rate at which it beats which can basically like triple and also the volume that it pumps in each stroke which can go from about 60 milliliters to

200 milliliters at the sort of high end. And if when you combine both of those, they allow for a roughly 10fold increase in cardiac output during exercise, which is pretty insane. Like obviously, you need a lot more oxygen to your tissues, particularly your skeletal muscles during intense exercise. Of course, this varies a lot depending on your physical fitness. So cardiac activity, intense cardiac activity that elevates your heart rate and your breathing rate will over time lead to essentially a strengthening of the heart muscle. And that typically, what it does is that it will lower your resting heart rate. So, you know, people who are fit have a lower resting heart rate. I think it can go like 40 beats per minute or something like that. But that will have a higher

stroke volume at rest, but then also when they gauge in exercise. So it's kind of an interesting thing, basically the heart strengthens its capacity to have a very high stroke volume. And it compensates for that during resting activities when you don't need that additional stroke volume by reducing the beats per minute. And the upside to that is that when it's needed, you can can still crank up that heart rate to about the same maximum level, but now you've got higher stroke volume. And so the total amount of oxygen that you can get to your tissues is greater, and therefore you can move faster. You can move longer. That's sort of long-term control. What about sort of of a shorter term? Like how does your heart kind of know how much you're exercising, so to speak,

or how does it know what activity to engage at? Well, the cardiovascular center is a region in the medulla obligata, which is part of the brain, the brain stem technically. It's sort of the lowest level of the brain, evolutionarily and I guess anatomically as well, just above the spinal cord. And it's responsible for regulating heart rate and stroke volume. I mean, the medulla or magata has a lot of functions, but the cardiovascular center within that is responsible for regulating heart rate and the stroke volume. Now, it's important to know or to understand that the heart doesn't need the brain to tell it to contract.

The sinus atrial node will do that by itself, right? So that's why, you know, when hearts are transplanted, that they'll keep beating. The heart will keep beating if you disconnect it from the brain. It's connected to the cardiovascular center, and that innovates the SA node, so the sinus actual note, as well as the AV node. So it can basically tell the heart to change the rate at which is contracting, but the heart doesn't need those signals to contract.

Those are modulatory signals. They don't produce the contraction itself. Essentially that that cardiovascular center in the medulla receives input from the skeletal muscles, indirectly through the spinal cord, and that's how it is able to get information about, you know, how much oxygen is needed to be produced to the tissues, or actually I think is more relevant how much carbon dioxide is building up into tissues. That typically regulates the heart rate more, I believe, than the actual amount of

oxygen. There are also bar receptors in the aorta and some of the other arteries surrounding the heart, and these detect changes in pressure, which send signals to the cardiovascular center to basically indicate whether the pressure is lower or higher than it should be. And so that's one mechanism. There are also chemoreceptors which detect concentrations of oxygen, carbon dioxide, pH, and other facets of, you know, what the blood should be and sends those signals to the cardiovascular center as well. So the cardiovascular center is integrating a large number of inputs. It's not just getting input from the skeletal muscles. So it does get that input directly from motion of muscles, but also, as I said, bar receptors which detect pressure, chemo receptors, which detect levels of oxygen and carbon dioxide.

So there's quite a lot of information that's integrated there. And, you know, if there's carbon dioxide building up, that'll produce signals are like, oh, we need to bump up the heart rate. And so that will send a signal to the relevant nodes of the heart, which will then cause them to depolarize more rapidly than they typically would. And thus you get, as well as to increase the stroke volume, and then that increases your cardiac output. So the cardiovascular center in the medulla is connected to the heart, specifically the sonary on the anterior anterior ventricular nose,

which control the rate of contraction. It's connected to them via the sympathetic cardiac nerve. So that's part of the sympathetic nervous system. If you recall, the peripheral nervous system is divided into sympathetic and parasympathetic. Sympathetic is basically, it's increasing breathing, increasing heart rate, sweating, things like that, reducing digestion. Whereas parasympathetic is the opposite. That's kind of rest and digest, slow down, slow heart rate, increased digestion, things like that.

So there's separate pathways for each of these. So basically there's a break and there's an accelerator to the heart, both controlled by the cardiovascular center. One is through the parasympathetic nervous system via the vagus nerve, basically acts as a break. The other, through the sympathetic nervous system via the sympathetic cardiac nerve, is the accelerator. So basically what's happening to some of that together is the medulla, or specifically the cardiovascular center within the medulla, is integrating all of these signals from skeletal muscles, from barrow receptors, which detect pressure, from chemoreceptors, which detect the amount of oxygen and carbon dioxide and so forth in the blood.

And those are integrated and sent up to the medulla, and that information is processed so as to modulate the strength of the signal sent via the parasympathetic, which is like the brake or the sympathetic, which is like the accelerator, in terms of modulating heart rate. Now, there's one final aspect that I wanted to mention before we finish up this episode, and that is blood vessels, because I've been talking about blood vessels quite a bit, but I haven't really said much about sort of what they are or how they work. So blood vessels are sort of like the plumbing of the cyclotrisism, if you like. They're essentially pipes that carry blood around the body. there are five main types of blood vessels, the arteries which carry blood away from the heart,

arterials, which are essentially just small arteries, the veins which carry blood to the heart, the venules, which are sort of the smaller versions of veins, and then there are the capillaries, which are sort of in the middle, right? The capillaries is where the exchange of water and oxygen other chemicals between the blood and tissue actually occurs. So capillaries are the sort of the smallest and also most numerous of their blood vessels. Like the, you can think of them as sort of local roads, which actually leads to the houses and buildings, whereas the veins and arteries are kind of like the big highways, where the blood is transported sort of on mass across different parts of the body. And then there's sort of smaller secondary roads, which are the venules and the arterials,

sort of in the middle. Now, the structure of the blood vessels depends on their function, right? So capillaries are very different to veins, which are different to arteries. So if we talk about veins and arteries first, arteries have to be the toughest and stronger. of all, but at least typically, you know, other things being equal. The reason is because arteries carry blood away from the heart, and for the most part, again, talking about in the systemic exchange, they carry high-pressure oxygenated blood, high pressure because it's come straight from the left ventricle, which exerts the highest force pressure on the blood pumping it around the body. So that means the arteries, especially nearer to the, nearer to the aorta,

need to be quite tough. So they need to be reinforced with more smooth muscle and tough connective tissue. Whereas veins typically, certainly in the systemic system, hold lower pressure deoxygenated blood, which is trickling back to the heart. And therefore they don't require as much padding as much reinforcement. So both arteries and veins effectively consist of layers of epithelial muscle or smooth muscle and connective tissue wrapped around in a number of layers to give reinforcement and protection and shape to it. But, yeah, typically arteries will be more strongly reinforced and more robust than veins because they need to absorb those pressure coming from the left ventricle.

Now, capillaries are quite different. Capilaries consist of just a single layer of endothelial cells which surround the capillary. And then there may be a membrane that surrounds that. Basically, the capillaries are a site where the blood, oxygen, and carbon dioxide is allowed to diffuse directly through those thelial cells and interact with the surrounding tissue. And we'll talk about how that works in more detail in the next episode. But obviously, there needs to be a lot of thinner, right? Because that exchange of nutrients has to occur directly, sort of cell to self, through diffusion.

And so that can't be very thick because then that wouldn't be able to happen. Now, there are many different names for different parts of the circulatory system, depending on sort of where the blood vessels are and where they're going. The main two distinctions are important to understand of the pulmonary circulation, which goes from the heart to the heart, and then there's a systemic circulation, which goes from the heart to the rest of the body, carrying oxygenated blood, and then bringing deoxygenated blood back to the heart to be pumped back to the lungs. There are other distinctions as well.

For example, there is the hepatic artery in the hepatic vein, which take blood to the liver and some of the surrounding organs as well. There's the renal branch which takes blood to the kidneys, so the renal lighter in the renal vein. There's also the carotid artery, which carries, and corresponding veins, which carry blood to the head and the brain. Disruptions of blood supply there are common cause of strokes. there's the brachial arteries which serve the arms, femoral arteries which serve the legs, and finally the heart has its own supply of blood, which comes through the coronary circulation,

so the coronary artery. And that may be interesting to note that the heart doesn't actually get supplied with nutrients or oxygen via the blood that's inside the heart. It requires its own circulation. And a disruption to the coronary arteries is a common cause of heart attacks. All right, so that concludes what I wanted to talk about in today's episode. In the second part of the series I'll talk about blood and how it carries oxygen and also blood typing and how blood is uh gives up as oxygen and takes it up when it needs to so we'll give a bit more of the biochemical detail there also talk about the respiratory system so lungs um and um control of respiration and lung capacity and things like that so if you enjoyed this episode please consider leaving a

favourable review of the podcast on an aggregator of your choice um iTunes or others are all much appreciated. You can contact me directly at Fods12 at gmail.com. That's FODDS12 at gmail.com. If you'd like to make any requests or suggestions or just give feedback. If you would like to support the show financially, you can do so via Patreon or making one off donation via PayPal. The links to those should be in the description of the episode or you can just sort of Google the Science of Everything podcast. You should be able to find that.

I greatly appreciate all my patrons. It helps me to devote more time to this show and keeps me regular with the schedule. as well, so I strongly appreciate that. Thanks everyone very much again for listening, and just a reminder that there will be some episodes appearing in the stream from earlier shows that I'm repudding up again so as to ensure that they're accessible to everyone, as I discussed at the start. So just bear in mind that that will be happening. Feel free to sort of skip those if you've already listened to them. And after that, the second part of this will be coming up in a few weeks. Thanks again, and talk to you next time.