Lesson video

Hello and welcome to today's lesson.

My name is Mr. Swains and I'm really looking forward to teaching you today.

We're going to be looking at the relationship between heart rate, stroke volume, and cardiac output.

And we'll be exploring what these different terms mean, how they're measured, their relationship with each other, and what happens when we start to exercise.

I'm sure you've heard about heart rate before and perhaps you've even measured your own heart rate to understand what happens when we exercise and why.

But it's less likely that you've come across those terms of stroke volume and cardiac output, even though they hold the key to the whole picture around blood distribution and how the cardiovascular system works.

So that's what we're gonna explore in detail today.

So today's lesson is called the relationship between heart rate, stroke volume and cardiac output.

And it comes from the anatomy and physiology, the cardio respiratory system unit.

By the end of this lesson, you'll be able to explain the relationship between heart rate, stroke volume and cardiac output.

So I'm wondering if you have an idea of what stroke volume or cardiac output are, and if you know what the formula is that connects the three of them.

I'm making an assumption there that you know what heart rate is, so have a little think.

So the key words for today's lesson unsurprisingly, are heart rate, stroke volume, and cardiac output.

So again, unsurprisingly, our lesson today will be split up into three parts.

So the first part will give us a chance to explore and describe and calculate heart rate.

Then we'll move into the second part where we do the same and are able to describe stroke volume.

And then in the third part, we'll figure out how to calculate cardiac output.

I hope you're ready.

Let's get started.

So have you ever wondered why your heartbeats or what the relationship between your heart rate and your pulse is? Have a little think.

So heart rate or HR for short is the number of times your heart beats permanent.

It's measured in BPM or beats per minute and it shows how hard the heart is working and therefore, it's affected by the amount of physical activity you're doing.

So right now I'm sat still, my heart rate will be much lower than if you've perhaps been running around before sitting down to watch this video.

It's affected by stress.

So if you're feeling stressed or perhaps you know members of your family that have been feeling stressed at different times, they will have an elevated heart rate and it's also affected by your overall health.

So healthier people typically have a lower heart rate for any particular intensity.

So for example, at rest their heart rate will be lower.

In contrast, pulse refers to that physical sensation of the heartbeat.

So we often talk about taking our pulse and that's where we can feel our heart rate but not at our heart.

So it can be felt through the walls of arteries and you could see me there illustrating it being measured at the carotid artery, which is in your neck, but you can also measure it in your radial artery in your wrist.

Now I find that one much harder to find, particularly when I'm at rest, whereas when I've been exercising and my heart is really beating, I guess faster, but also more forcefully, then I do find it easier to find it in my wrist.

So if ever you're trying to find your heart rate and you think, I can't find it, I can't find it, try doing a bit of exercise and then you'll find how to locate it better because it'll be beating more forcefully due to the effects of exercise.

So what do you think a normal resting heart rate is for an average person? Any thoughts? So we've got a nice illustration here of someone taking their heart rate with these two fingers in the carotid artery.

Really important, you don't do it with your thumb because you also have a pulse in your thumb, so that might confuse you.

But to get an accurate measure of your resting heart, it's best to sit down in a calm or quiet place and take your pulse for 30 seconds.

Now let's remember that your heart rate is measured in BPM, beats per minute.

So therefore if you were to take your pulse for 30 seconds, you need to multiply that score by two to make it out of 60 seconds, IE, beats per minute.

You might find it easier to actually take your pulse for the whole 60 seconds.

So set a stopwatch for a 60-second countdown, hit go when you found your pulse, count the heartbeats, and then when your alarm goes off, you'll stop counting and we'll explore later why that counting for a whole 60 seconds isn't ideal when you do an exercise and perhaps lots of you have got smart watches or devices that are able to measure your heart rate for you as well.

So what do you think your resting heart rate is? Why not have a go at taking it now? Pause the video, take your heart rate for 30 seconds and multiply it by two.

Okay, let's have a quick check.

Which two of the following are a definition of heart rate? Is it A, the number of heartbeats per minute? B, the amount of blood ejected from the heart per beat? Is it C the number of times the heart beats per minute or is it D the number of times your lungs expand in a minute? Have a think.

And remember, two of these answers are correct.

And of course, yes, it's A, number of times the heart beats per minute and C, the number of times the heart beats per minute measured in BPM.

Okay, so what do you think happens to heart rate then if you were to start to do some exercise? So here's an example of a graph and we've got heart rate plotted up the vertical axis and we've got time across the horizontal axis.

So from naught minutes up to 20, 25 minutes on this example.

What do you think? Well, stands to reason that your heart rate would increase if you started to do some exercise.

So maybe your resting heart rate was about 60 or 70 beats per minute.

So here we go.

We've plotted a line on this graph of that heart rate increasing.

So the longer you exercise, the more it increases.

Do you think that's right? Alex is asking us that question.

Would it just continue to increase and increase the longer you exercise for or if you were gonna go and run a marathon or an ultra marathon, would your heart rate just keep going up and up and up? And of course, no, it wouldn't.

So the heart's a really clever organ in the body.

It's controlled by your brain and your brain figures out the best way to increase your heart rate to an optimal level based on that exercise that you're doing.

And we'll find out later how that connects really closely to stroke volume as well.

So in fact, if you were going to do a 12 minute run, but that 12 minute run didn't start until four minutes on this graph, and you'll see in a minute why I'm explaining this as the run starting at four minutes rather than the run starting at zero minutes.

Because interestingly, and you'll be aware of this, we have this anticipatory rise, so this almost flood of adrenaline into our body that kickstarts that increasing heart rate before we absolutely need it.

And this is part of our evolutionary mechanisms of fight or flight and it's almost comes from that idea of needing to run away from a predator.

So the minute you perhaps see that predator, adrenaline is released into the body and it's like boom, let's increase that heart rate so that you're already starting to circulate more oxygen and nutrients around the body so that you're gonna be able to run fast away from that predator.

And then we see, so that happens just before four minutes, then we see a steady increase in heart rate up until in this example about a 150, 155 beats per minute.

And at that point, this particular individual on this run had got to a steady state, their heart rate had increased to a level where it was providing enough oxygen for the body to keep working aerobically.

And you see that continue all the way through to 16 minutes when this 12 minute run ends.

So remember when we've got four minutes of rest at the beginning, that anticipatory rise, and then 12 minutes of running, which brings us the 16 minutes on the graph.

And at that point you're gonna have, instead of the heart rate dropping immediately straight back to that resting level of let's say 60 beats per minute, it slowly decreases down and we call that repaying the oxygen debt or a recovery after exercise, and then it'll get back to its resting levels and actually the fitter you are, the faster it gets back to resting levels.

So I've just annotated this graph a little bit for you to help see that resting heart rate at the beginning, then that anticipatory rise, they reckon your heart rate goes up by, that's four or five beats per minute just before that onset of exercise.

And then it'll continue to climb up until a point where the oxygen being supplied to your muscles is sufficient for the pace or the intensity that you are running at or cycling at or swimming at or exercising at.

And that'll run all the way through to the end of exercise, at which point the demand reduces immediately, but actually you continue to pump more blood around the body to flush things like lactic acid out from your working muscles and clear that carbon dioxide out as well and replenish it with some fresh oxygen supplies ready in case you decide to go out for a run again.

So let's have another quick check.

Which of the following graphs could represent a heart rate whilst resting? Is it A, going vertically up? Is it B, coming down or is it C, sitting horizontally there? That's right, well done.

It's C.

So sitting at that fairly steady heart rate, so for example, if you went to sleep listening to me, your heart rate will actually decrease further and sit at quite a low level.

And it's always I find fascinating to take my pulse and see how low can I get it? IE how much of a resting state can I get myself in? Okay, so a little bit of wider learning here.

Research into heart rate variability or HRV is a really growing area of research and what we are finding is, although it's beyond the scope of your exam for GCSEPA, I think you'll want to understand a bit more about this because higher variability in the time between heartbeats is often a sign of really good heart health and fitness.

So if you've got some sort of smartwatch or fitness tracking device, it might well be taking your heart rate, and therefore it will be telling you your heart rate variability.

And that's a really interesting one to track because you want a higher heart rate variability, IE more variation in that heart rate score, that shows a really good heart health and fitness and suggests that you live in a good, healthy, active lifestyle.

A lower heart rate variability on the other hand, may indicate signs of stress or heart problems, but remember this is beyond the scope of your exam.

So no need to remember that.

I just thought you'd find it interesting.

And there's that note again that smart watches are able to monitor that heart rate variability and perhaps on your smartphone, you'll be able to look at and monitor your heart rate variability over time, perhaps over the years.

I wonder what happens to your heart rate variability during the stresses of exam season, for example.

Okay, time for me to get you to do your first practise task of this lesson.

So what I'd like you to do is draw a heart rate response to a game of let's say football.

And that game of football starts at 10 minutes on this graph and it lasts for 30 minutes.

Don't forget, you're gonna need to draw on that graph, resting heart rate, that anticipatory rise and then perhaps what happens if you go sprinting for the ball or if there's a slower period of play.

And then what happens during recovery, which will start at 40 minutes on this graph.

And you may even find that you need to annotate your graph a little bit to say, look, this is where sprinting for the ball happened.

This is where a slower period of play happened.

Pause the video now and come back to me when you're ready.

Okay, let's see what you came up with.

Is it a graph a little bit like this? And of course yours will vary because you'll have had different thoughts around that sprint, which causes a spike in heart rate to go up or maybe there were multiple sprints, so multiple spikes.

We can see here that this performer had a really intense and quite long lasting run, about 30 minutes and then their heart rate dropped back down again.

Whereas just before they had that sprint, there were a period of perhaps quite sedentary play.

Maybe the ball was nowhere near them on the field, so they were perhaps fairly static.

All right, so your graph should include that resting heart rate of perhaps anywhere between 50 and 80 beats per minute.

A trap that some people fall into is they draw that heart rate starting at zero.

And of course our heart is always beating even when we're asleep.

Then we've got that anticipatory rise.

Maybe yours was shown a bit smaller than mine is, and then varying heart rate during different periods of kind of sprinting or walking or standing on the football pitch when the ball's nowhere near you.

And then that recovery heart rate.

And in fact, you might even want to look at the difference in heart rate between a goalkeeper and a striker, for example, or maybe even bigger differences would be seen between that goalkeeper and a midfielder who's doing lots of running all the way through the game.

Okay, so time to move on to the second part of this lesson where we're gonna dig into stroke volume.

Can you remember what stroke volume was? Well, so each time your heart beats, it squeezes blood out into the ventricles and around the body.

So in each heartbeat, each squeeze, blood is ejected out into the ventricles and the amount of blood squeezed out or ejected from each ventricle.

So from the left ventricle and from the right ventricle in each beat is known as stroke volume or SV for short, is measured in millilitres.

So in the same way that perhaps your drinks are measured in millilitres, so is stroke volume.

So I'm wondering, well in fact, June is wondering what do you think is a typical stroke volume during rest and what factors affect stroke volume? Have a little think.

Okay, before we get into that, it's interesting also to note that when the heart contracts, it doesn't squeeze all of the blood out into the arteries, particularly at rest because it's not contracting as forcefully, it just squeezes some of that blood out and then lets it fill again.

Squeezes some of it out, lets it fill again.

But even if you are doing a maximum effort, you know, for example a marathon run and it's squeezing really forcefully, it still won't eject absolutely every last drop of blood out of the heart.

And in fact at rest, it'll squeeze out about 70 millilitres in a healthy adult.

So for me, perhaps about 70 millilitres of blood is ejected in each beat and that stroke volume increases if you start exercising.

So your heart's gonna squeeze more forcefully and it also increases if you train regularly and get fitter.

So actually I'd like to think that my resting stroke volume is a bit higher than 70 millilitres, maybe more like 90 millilitres.

But of course it's really hard to measure this, isn't it? We're not going to cut me open and squirt the blood out in one beat and measure it.

So actually it's measured in a different way.

And again, not needed at GCSE level.

So let's just have a little look for comparison.

70 millilitres is about quarter of a normal can of Coke or Sprite or Pepsi.

So it's mad, isn't it, that a quarter of a can of Coke's worth of volume is squeezed out of the heart in every beat, even when we're just at rest.

It goes to show just how important our cardiovascular system is and that heart keeps doing that for our whole life.

So let's have another quick check for understanding, true or false.

Improved cardiovascular fitness leads to an increased stroke volume.

Is that true or false? And why do you think that is? That's right.

It's true, because a bigger and stronger heart is able to work more efficiently by pumping out more blood per beat, which means the heart doesn't need to beat as many times per minute to circulate the same amount of blood to the body.

And that's why if you get fitter, and therefore your heart gets bigger and stronger called cardiac hypertrophy, so I've got a bigger and stronger heart, it can squeeze out more blood per beat.

So it doesn't need to beat as many times per minute to get that same amount of blood around the body per minute.

So that's great that our heart rate at rest can decrease because that puts less strain on our heart and is one of those positive outcomes of being fitter and healthier.

Let's have a little look.

What happens during exercise then? So I've already said that the heart contracts more forcefully and squeezes more blood out per beat.

So the harder you work, the higher your stroke volume.

And in fact, if you are an endurance trained athlete, so a supreme endurance trained athlete, your stroke volume might go up as high as 200 millilitres per beat during really strenuous exercise.

That's mad, isn't it? So that's volume of well over half of a can of coke per beat.

For the rest of us who are perhaps not as endurance trained, our stroke volume still goes up during strenuous exercise, but it wouldn't go up as high as that.

So let's say maybe 120, 150 millilitres per beat, but still, really significant amount of blood being pumped out of each side of the heart, every beat.

So another quick check, which of the following statements is correct? Is it A, stroke volume decreases from about 120 millilitres at rest when you start exercising? Or is it B, endurance trained athletes have a lower stroke volume? Or is it C, stroke volume increases from about 70 millilitres at rest in an average person? Or is it D, stroke volume is the amount of blood pumped out of the heart per minute? What do you think? Well done.

That's right.

So stroke volume increases from about 70 millilitres at rest.

And in fact, you don't need to remember that number.

I just find often a bit of relative scores and numbers for things helps me remember them.

So onto our next practise task, I'd like you to use this word bank to fill the gaps in the sentence.

So stroke volume is the amount of blood pumped outta the heart by each what with each beat? A typical resting value is what? If you take part in aerobic exercise, like a long distance run, it could increase to over what? And training could result in a stroke volume during maximal exercise of 140 millilitres or more.

And I said, didn't I that an endurance trained athlete might go as high as 200 millilitres.

Training results in more what heart and hence, at what stroke volume? This in turn enables our resting heart rate to be what? Pause the video now whilst you fill the gaps and then let's see how you've got on.

Okay, so hopefully yours looks like this.

So stroke volume is the amount of blood pumped outta the heart by each ventricle with each beat.

So the left ventricle and the right ventricle are both pumping out 70 millilitres.

So a typical resting value of stroke volume is 70 millilitres.

If you take part in aerobic exercise, like a long distance run, it could increase to over a hundred millilitres and training could result in a stroke volume during maximal exercise of 140 millilitres or more.

Training results in a more efficient heart and hence, a higher stroke volume.

This in turn enables our resting heart rate to be lower.

So again, lots of people and maybe you've done this, will compare their resting heart rate over time to see if they've been getting fitter.

So a nice indication of the training benefits over time are what does it do to your resting heart rate? Okay, onto the third part of this lesson then.

So we're gonna explore and calculate your cardiac output and how to do that.

So cardiac output, which is in the formulas often shown as a Q, in fact it strictly speaking with a little dot over the top of it because that represents volume.

So it's the volume of blood pumped out of the heart per minute and it's in fact, knocked out of the heart but out of each ventricle.

So let me say that again.

Cardiac output is a volume of blood pumped out per minute by each ventricle of the heart.

So I wonder if you can start to figure out how might you calculate cardiac output if you knew heart rate and you knew stroke volume? That's right, it's calculated as the product of heart rate times stroke volume, and therefore it's measured in litres per minute.

Nice little formula there and one that you'll definitely want to remember.

So cardiac output equals heart rate times stroke volume.

Let's have a little look and work through some examples of that to calculate cardiac output for some different people.

So if we know that we can calculate heart rate, sorry, cardiac output by knowing their heart rate and their stroke volume and that uses this formula.

So if somebody's heart rate was 72 beats per minute and their stroke volume was 70 millilitres, what does that mean their cardiac output would be? Well of course we do 72 multiplied by 70, which equals a little over 5,000 millilitres per minute and of course makes more sense to talk about it in litres.

So that's approximately equal to five litres per minute.

So that's really interesting isn't it? At rest, most of us will have five litres of blood circulating around any part of our body per minute.

And in, fact we have about three and a half litres of blood in our body.

So every bit of blood will go one and a half, not quite one and a half times around the body per minute.

That's amazing, isn't it? How much is going on behind the scenes that we're not even aware of? Well what about if we started to do some exercise then? So using that same formula, can you have a go at calculating what someone's cardiac output would be if their heart rate was 180, now that means they're exercising pretty hard, and therefore their stroke volume is also perhaps up towards its limit at 120 millilitres.

What do you come up with if you multiply those two figures together to get this during exercise cardiac output? Well hopefully you got 21,600 millilitres per minute.

So about 20 litres of blood are pumped around the body per minute when you're doing heavy exercise.

And in fact, it can go even higher than that.

And in endurance trained athlete, so if you've seen someone win the marathon, that sort of athlete can get theirs up to as high as 40 litres per minute.

And again, remember I said earlier you have about three and a half litres of blood in your body.

So during maximal exercise in an endurance trained athlete, every little blood cell is going around the body 10 times a minute.

Crazy, right? The body's amazing.

Let's do another quick check.

So true or false, when stroke volume increases cardiac output decreases.

Is that true or false? That's right, it's false.

So cardiac output or Q is the combination of heart rate multiplied by stroke volume.

So whenever stroke volume increases, your cardiac output will too.

Okay, so earlier we looked at heart rate response to exercise.

So let's have a little look at cardiac output response to exercise.

So Jacob's wondering, what do you think happens to cardiac output during that 12 minute run that started at four minutes that we looked at earlier? Perhaps refer back to the graph that you drew and here we go.

So it starts at about five litres per minute at rest because we know that's the circulating cardiac output and the same as heart rate increases, but because it increases by heart rate times by stroke volume, that steady state increase goes up to somewhere between 20 and 30 litres per minute, up until 16 minutes the end of the race, and then it will gradually decline again back to the resting level.

So let's have a look at one of these together.

What would it look like for an average adult if they were doing 400 metre sprint every four minutes for four repetitions? So let's have a look at this graph down the bottom left and we can see that graph showing five litres per minute at rest, then a little bit of an increase in cardiac output, then a huge increase as they do their first 400 metre run or sprint.

Then it recovers back towards resting level, but not as far as that.

Then a second sprint, then a third sprint, then a fourth sprint.

And you can see there that there's a little bit of cardiovascular drift.

So a little increase in cardiac output each time and it doesn't recover quite as well as it did the time before.

So I'm wondering based on that whether you can figure out how would cardiac output look different for a well-trained individual compared to that average adult on the left hand side.

What do you think? Hopefully you've come up with something a bit like this.

So similar resting cardiac output is the same about five litres per minute, but then it goes up instead of to only sort of 20/25, it goes up to perhaps more like 35 litres per minute for those four repetitions.

And it also recovers much better back down to towards resting level because obviously that well-trained individual is able to cope with those 400 metre sprints far better than someone that doesn't do that regularly.

Now the question for you, how does being well-trained affect the time taken to compete? Sorry, to complete each of those 400 metre sprints.

That's right.

This person would also be much faster.

So maybe they're able to do these 400 metre sprints at, you know, perhaps 45 seconds per repetition, whereas the average adult would be well over a minute per repetition.

Okay, for your next and final practise task of this lesson, I'd like you to have a go at measuring your heart rate at rest.

Maybe you've had a go at that already, but then next I'd like you to complete the following exercises and measure how your heart rate responds, and therefore what is your heart rate straight after marching on the spot for 60 seconds, jogging on the spot for 60 seconds and 20 burpees or a similar high intensity exercise that you can do.

I've got a top tip for you here instead of taking your pulse for 30 or 60 seconds after you've done that bout of exercise, because we've seen haven't we, that heart rate decreases during recovery.

And if you are super fit, I'm sure you are doing GCSEPE, then your heart rate will respond much, sorry, recover much faster after exercise.

So what I suggest you do is actually take your pulse for just 10 seconds, but then you'll need to multiply it by six, won't you to make it beats per minute because there's six lots of 10 seconds in a minute.

If that sounds a bit hard for your maths brain, then an alternative is you could take your pulse for six seconds and multiply it by 10, IE add a zero to the end of it.

But that sometimes gets a little bit less accurate.

Once you've taken your pulse at rest, after marching, after jogging and after some burpees, I want you to draw a bar chart to represent that heart rate response to these different types of exercise.

And then finally, you'll have done some exercise, which is great to get your juices flowing and to get you able to think and concentrate better.

I want you to then have a go at explaining the relationship between heart rate, stroke volume, and cardiac output during exercise.

Pause the video now whilst you do this extended task and then come back to me when you're ready.

Okay, so hopefully you've come up with a graph similar to this.

So we can see resting heart rate.

I know somewhere between 60 and 80 beats per minute.

Then that heart rate when marching on the spot might be heading up towards about 80 beats per minute.

Jogging on the spot up towards a hundred, maybe a little bit over a hundred beats per minute.

And then that burpees, would be right up at perhaps 120, 150 beats per minute.

Is that what you came up with? And then onto the thinking bit, explaining the relationship between heart rate, stroke volume, and cardiac output during exercise.

You might have said heart rate is the number of times your heart heart beats per minute.

Stroke volume is the amount of blood pumped out of the left ventricle per beat.

Cardiac output is the amount of blood pumped per minute and is calculated by multiplying stroke volume by heart rate.

And then finally, hopefully you've recalled that during increasingly strenuous exercise, all of those measurements increase, which provides more oxygenated blood to the working muscles.

Let's have a quick summary of today's lesson.

So heart rate is the number of times the heart beats per minute and a typical resting heart rate if I took mine now, might be around 72 beats per minute, anywhere between 60 and 80 really.

Stroke volume is the volume of blood pumped out of the heart by each ventricle with each beat and it's measured in millilitres or ML.

A typical resting stroke volume is 70 millilitres.

And then cardiac output, which is calculated as heart rate times stroke volume is the volume of blood pumped by the heart per minute.

A typical resting cardiac output is five litres per minute.

And then we've been able to plot some heart rate graphs to show how exercise intensity over a time period.

I hope you've enjoyed today's lesson and I look forward to seeing you next time.