Hi everyone and welcome to Learn A-Level Biology for Free with Miss Estrick. In this video I'm going to go through the control of the heart rate, so make sure you are subscribed if you aren't already to keep up to date on all the videos. So control of the heart first of all links to the control of the stages in the cardiac cycle, which you learnt in year 12. So you would have already known that cardiac muscle is myogenic, meaning it can contract and relax on its own accord.
doesn't require a stimulus. However, the rate at which it contracts and relaxes is controlled by the nervous system. So we'll be coming on to that. And what we're going to be looking at is how we have the different stages that the different chambers of the heart contract in this cycle is controlled. So what is it that triggers atrial systole?
so the contracting of the atria, ventricular systole or systole which is the contracting of the ventricles and then we have a diastole or distally and this is when ventricles and atria both relax. And if you can't quite remember this I'll link up here my video to the cardiac cycle so you can revise that first. So it's all to do with key structures in the heart that you haven't actually learned about yet.
So the first one is what we call the sinoatrial node and this is located just here in the right atrium and it's also known as the pacemaker and it's a group of cells or in other words a tissue in the right atrium which can release a wave of electricity which we call depolarization and when electricity or depolarization hits cardiac muscle, it causes it to contract. So that's why it's called the pacemaker, because the SAN is the cell that can release this wave of depolarization and start off the muscle contractions. There's also the AVN, which is the atrioventricular node.
And this is located in between the atria and the ventricles, as the name suggests. and it's within the border of the left and the right hand side. So although I've drawn it here, it might actually be slightly further over within this diagram.
You then have your bundle of his and these are conductive tissues which run down the septum of the heart and up the walls of the two ventricles. And lastly, the perkyne fibres, which you might see written as Purkinje fibres in some textbooks. AQH tend to use the phrase perkyne, so perkyne fibres, and these are conductive tissues that go all the way through the walls of the ventricles.
So those are the key tissues involved, but how they actually control the cardiac cycle is what we'll have a look at. So step one, the SAN, releases a wave of depolarization across the two atria. And that is what causes atrial systole.
Both atria will contract. The next step is the AVN releases another wave of depolarization. Now this doesn't go directly down into the ventricles because there's actually a layer of non-conductive tissue that separates the atria and the ventricles. And it's not one that you can see, it's just the layer of tissues that separates those top and bottom chambers is insulating.
And that's why the bundle of his is important because as the AVM releases this second wave of depolarization, the depolarization wave cannot go directly downwards so it has to go through this conductive tissue in the septum which is what the bundle of His is. So it travels down the septum and the bundle of His then moves up the outer walls of the ventricle and that can then pass the wave depolarisation finally through the perkyne fibres which are then going to branch into all of the walls of the ventricle. So as a result what that then means is you'll actually have in your ventricles it's the apex of the heart of the ventricles which will contract first an apex just means the tip because it has to travel down the bundle of his so you'll have some contraction in the septum but then the apex and these outer walls contract first and then the percon fibers call the rep cause the rest of the walls to contract and that's really useful because if you think about getting toothpaste out of a toothpaste tube to get the maximum amount of toothpaste out you should squeeze from the bottom and push all the way up If you squeeze in the middle or at the top, you're not going to get as much toothpaste. And that's exactly the same with the heart. Squeezing at the bottom and then moving that contraction all the way up forces out the maximum amount of blood from the heart.
Now, you could also be asked, why is it an advantage that we have this non-conductive tissue, which actually causes a very slight delay between the atria contracting, and the time it takes for the depolarization to get to the ventricles and therefore the ventricles contracting. And the reason for that is it allows enough time for the atria to fully contract, pump all of the blood in the ventricles so they are then full before they contract. So it's only a very slight delay, it's milliseconds, but that is all it takes to give enough time for the right atria to fully empty.
So that is what controls the cardiac cycle, but how quickly the SAN releases that wave of depolarization is controlled by the nervous system. And it's a part of the nervous system called the autonomic nervous system. And what that means is it's automatic, it's subconscious, you do not think about making it happen, it will automatically happen.
And the coordinator center is the medulla oblongata in the brain. So that is the part of the brain which controls the heart rate and we can see here in the picture where the medulla oblongata is and there are nerves connecting to directly to the SAN in the heart and what that means is it's able to control how quickly the wave of depolarization is released from the SAN. And there's two different routes that can be taken depending on whether you want the SAN to be releasing waves of depolarization more rapidly to increase the heart rate or more slowly to decrease the heart rate. So the sympathetic nervous system or the sympathetic nerve we can see here is shown in purple. And any impulses sent down the sympathetic nervous system will actually have the effect of increasing the heart rate.
You also have the parasympathetic nervous system and that has the opposite effect. Any impulses sent down the parasympathetic nervous system will trigger the SAN to release the wave depolarization more slowly and therefore it decreases the heart rate. So we'll have a look at a few examples of how this links to homeostasis.
And the two key examples you need to know about are how the heart responds to changes in pH of the blood and your blood pressure. So those are the two stimuli which will then trigger whether the impulse goes down the parasympathetic or the sympathetic nervous system. So this is to do with nervous response. So you still need to think about how you have your stimulus, which we have here. Stimuli are detected by receptors and if it's to do with the pH of the blood, it's a chemical, so it's a chemical receptor or chemoreceptor.
If it's to do with the blood pressure, it's a pressure receptor, also known as a baroreceptor. The location of these two receptors is the same. They're found in the aorta, but also the carotid artery. And that is the artery which branches.
out of the aorta to the rest of the body. So just in general what might cause these changes or these stimuli and why we have to respond. So changes in pressure that could be due to stress, anxiety, diet, genetics. If your blood pressure is too high it can cause damage to the linings of the walls of the arteries that can lead to blood clots and then potentially heart attack or stroke. So it's very important that these mechanisms are put in place to reduce the blood pressure.
If the blood pressure is too low, That might mean that there's insufficient supply of oxygenated blood to respire in cells, but also insufficient removal of waste. And that could then build up toxins. So response to pH, this links to the idea of enzymes. So the pH of your blood will decrease during times of high respiration.
So for example, exercise. And that's because carbon dioxide is a product of... aerobic respiration. Also lactic acid is a product of anaerobic respiration. So these two acids can build up in the blood quite rapidly if the heart rate doesn't increase to get the blood to the lungs for the carbon dioxide to be removed or to get the blood to the liver to get the lactic acid broken down.
And if those acids aren't removed rapidly enzymes could denature and proteins within the blood can denature like hemoglobin. So it's very important that the responses to increase the heart rate to be able to remove those acidic molecules. So again this is much like the synapses topic I did, it's another long answer, quite text heavy bit of theory where you could be asked for a four or five mark question to go through the whole process. So I've split it into the flow diagram you might be more familiar with, the idea of a stimulus detected by a receptor, what is the coordinator that coordinates the response, the effector that actually will implement the response and what is the response. So you can then see where the marks are broken down.
So the first thing I'm looking at, first example, is an increase in pressure. So the receptors you'd get a mark for pointing out would be the pressure receptors or baroreceptors and for pointing out they're in the walls of the aorta and the carotid artery. And what happens is if the blood pressure is too high, that actually stretches the blood vessels and that in turn stretches the pressure receptors and that is what then triggers the action potential along the sensory neuron.
So the coordinator sensor centre is in the brain, that would be your medulla oblongata in the brain. And what happens is more impulses are sent to the medulla oblongata, and then you'll have more impulses sent along the parasympathetic nervous system to the SAN, which will then decrease the frequency of electrical impulses. And that then means that... We'd have the effector is the cardiac muscle, the SAN tissues as well, releasing fewer ways to do polarisation and the response is the reduced heart rate. So I've just noticed throughout there are a few typos here.
So sorry about that. But hopefully listening at the same time, you've spotted what the correct version should be. What I did want to point out is the most common reason people miss marks is in this section here. You need to be pointing out that it's more electrical impulses going to the medulla oblongata.
More impulses are going along the parasympathetic nervous system. So you get more impulses at the SAN. Because you will continually have impulses, but you'll only get a change in response if you have more impulses.
So you would have to have the more in your answer. Now decrease pressure, same idea, but opposite. So still the same receptors, but this time if the blood pressure is lower, they're not being stretched. You still get more electrical impulses since the medulla oblongata, but this time it will trigger more impulses along the sympathetic nervous system.
So more impulses reach the SAN. And this is where our effector is. It's the cardiac muscle, in particular the SAN tissues within that cardiac muscle.
and because they're now receiving more impulses from the sympathetic nervous system, the response is an increase in heart rate. So lastly we'll have a look at one example of pH. So decrease in pH, so that would mean it's becoming more acidic, so there must be more carbon dioxide or lactic acid. This is detected by chemoreceptors in the wall of the aorta and the carotid artery. So what will then happen is more electrical impulses are sent to the medulla oblongata. Again, you have to have that more.
And then more impulses are sent. via the sympathetic nervous system to the SAN, and that will increase the frequency of electrical impulses. So the effector is still the cardiac muscle, in particular the SAN tissues.
Because they're receiving more impulses from the sympathetic nervous system, it's going to fire that wave of depolarisation more frequently. The response then is increased heart rate to deliver the blood to the lungs, to remove the carbon dioxide more rapidly. So the key here is more electrical impulses and making sure you're stating whether it's a sympathetic or parasympathetic nervous system.
So that is it for controlling the heart rate.