Lesson video

Hello, and welcome to today's lesson.

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

We're gonna be looking at the short-term responses of the body to exercise.

As you know, exercise, including playing a sport, puts increased demand on the body to be able to perform at our best.

Today, we're gonna be breaking that down into the different body systems, and looking at them in turn.

So today's lesson is called The Short-Term Responses to Exercise and Changes in the Body, and it comes from the Anatomy and Physiology: The Short and Long-Term Effects of Exercise unit.

By the end of today's lesson, you'll be able to identify the short-term effects of exercise on the body, and apply them to sporting examples.

So I wonder, do you already know some of these acute changes that happen in the body that help us cope with the demands of exercise? And do you know why these changes occur? The keywords for today's lesson are lactic acid, fatigue, rate of recovery, and redistribution of blood.

Okay, so for today's lesson, I've split it into three sections.

The first section, we'll unpick those muscular system responses to exercise, then we'll look at the cardiovascular system responses, and then thirdly, at the respiratory system responses to exercise, and we know they all combine to enable us to cope with that exercise.

So let's get going then.

So when we start to exercise, there's a number of immediate changes that happen at the muscular system.

I wonder.

Well, in fact, Lucas is wondering, isn't he? Can you name and describe any of these changes? Be great to pause the video now, and jot down any thoughts that you have.

Okay, let's summarise them then.

So the short-term changes that happen at the muscular system that are most important to remember are that, obviously, at the muscles, we've got that increased demand for oxygen, or O2, because the muscles are contracting, and we know for muscles to contract, they need energy, and their predominant source of energy is oxygen and glucose.

So our muscles require more oxygen, but they also produce more carbon dioxide, or CO2, so we know that that's one of the waste products of aerobic respiration, and we're gonna need to get rid of that.

We also know that particularly if we're working at a high intensity, there's a production of lactic acid in the muscles, and that lactic acid is a fatiguing byproduct, so it causes our muscles to ache, and eventually we'll need to either reduce the intensity or stop working completely.

And then, also, we have a massive increase in temperature at the muscles, and I'm sure you've felt that, and in fact, if it's a cold day and you're outside, maybe you're in a PE lesson and you're outside in the middle of January and it's freezing cold, well, one of the best ways to keep warm is to keep moving.

So by jogging around, running around, doing some exercise, getting thoroughly involved in that lesson, you'll find that the muscle temperature increases, and you feel the effects of the cold far worse.

The worst thing you can do is sit still.

Let's explore those in a bit more detail.

But before we do that, let's have a quick check.

Which of the following is not a response to exercise that is related to the muscular system? Is it A, an increased muscle temperature? Is it B, an increase in lactic acid at the muscles? Is it C, increased carbon dioxide required by the muscles? Or is it D, increased oxygen consumption by the working muscles? Have a little think.

That's right.

It's C, isn't it? Because actually, there's an increased carbon dioxide produced at the muscles, not required.

We don't want carbon dioxide.

In fact, we want to get rid of it after it's been produced.

Okay, so Laura's got a question for us.

She's wondering, why is there an increased demand for oxygen at the muscles? And I touched on this very briefly, but to dig deeper, we know that aerobic respiration requires oxygen combined with glucose to provide that energy for exercise.

So the more you exercise, the more oxygen your muscles will require.

The higher the intensity that you exercise at and the longer the duration, the more oxygen you will consume.

But if exercise is at a really high intensity, for example, sprinting, then you're not providing enough oxygen for your muscles to fully break down that oxygen.

So we build up what's known as an oxygen debt, and that's why after exercise, particularly sprinting exercise, we end up out of breath for a while, because we're trying to repay that oxygen debt with extra oxygen consumption, and our heart rate stays higher to circulate more oxygenated blood around the body, to reoxygenate the muscular system.

Okay, so Laura's got another question for us.

Why is there an increased production of carbon dioxide at the muscles? Again, I touched on this briefly, but carbon dioxide is that waste product of aerobic respiration.

So the harder you exercise, the more carbon dioxide you will produce in the muscles, and the more you produce, the more you'll need to get rid of, and that's why the muscular system works really closely with the cardiovascular system to pump that carbon dioxide in the blood back to the heart and then back to the lungs, and then the respiratory system works really hard to get that carbon dioxide out of the body and then fresh oxygen into the body.

Okay, another question from Laura.

She's got lots of questions today, hasn't she? So why is lactic acid produced at the muscles? I wonder.

And that's right.

It's a fatiguing waste product of anaerobic respiration.

So when we exercise at a high intensity, so, for example, sprinting, or maybe lifting heavy weights, or against a really hard resistance, maybe running uphill, that lactic acid is produced.

And when that lactic acid is produced, it forms lactate in the muscles, and then exits into the bloodstream, and continues to cause us problems, really, if we want to keep performing.

So if too much of this lactic acid builds up, we're forced to slow down or stop exercising completely, and actually that number for most of us is four millimoles per litre of blood.

So if we go over that onset of blood lactate accumulation and into that 4+ millimoles of lactate per litre of blood, then we start to need to really consider, are we gonna slow down or are we gonna take our body to breaking point, and end up being forced to stop exercising? Okay, another question from Laura.

So why do these muscles increase in temperature then when we exercise? And I mentioned this earlier, didn't I? One of the great things about if you're outside in the cold is to keep moving to keep warm.

But why do we produce so much increase in body temperature when we exercise? Well, it's because of these chemical reactions that are taking place in our muscles.

So in a thing called the mitochondria, so that's I guess the power plant in your muscle cells, what's going on there is it's using oxygen and glucose, and it's breaking it down a bit like a chemical factory.

And when it does that, yes, it produces the energy for your actin and myosin filaments to slide over each other for muscular contractions to happen, but it also, they're not that efficient, our bodies, and it produces lots of waste heat.

So that energy it's producing to enable movement is only about 20% efficient, and 80% of it goes into heat, which warms up our muscles.

And of course, that's a bit of a problem, because we've got to get rid of that waste heat.

Nice way to think about this is perhaps like a firework going off inside your muscles.

And one of the reasons we sweat is to help get rid of that excess heat, and it's also why we get red skin when we're exercising, because our body sends more blood to the surface of the skin to try and help radiate that heat out.

But of course, we'll look at that in more detail in the second section, when we're looking at the cardiovascular responses to exercise.

But right now, we've gotta remember that muscles increase in temperature because of those chemical reactions that are happening within them, and as I've just said there.

So that increased blood flow gets more oxygen to the muscles, but it also increases temperature further.

'Cause the more you're pumping blood around, the more, again, you're generating heat.

So let's do another quick check.

True or false? Our muscles only get warmer if we exercise in hot environments.

Is that true or false? That's right.

It's false.

Do you know why though? Correct.

It's about those chemical reactions, the fireworks that are going off in our muscles, and they produce energy for movement, but they also give off a lot of heat, almost like a waste product.

So hence one of the best ways to keep warm is to do some exercise.

Okay, for your first task in today's lesson, I would like you to describe the four main changes that we've just outlined that happen at the muscle site as a result of starting to exercise.

So those short-term responses to exercise specifically at the muscular site.

Pause the video now, and come back to me when you are ready.

Well done.

So hopefully, you've come up with something similar to this.

So the first part of this, and you might have done 'em in any order, but muscle temperature increases as a result of those chemical reactions taking place at the muscles.

Second thing is we have an increased demand for oxygen.

So there's increased oxygen consumption to provide energy for aerobic respiration.

Thirdly, we have increased carbon dioxide produced, and that's one of the byproducts of aerobic respiration.

And then, particularly if we're working at a high intensity, there's an increase in lactic acid accumulating in the muscles, and that is a fatiguing byproduct that will eventually cause us to need to slow down or stop exercising completely.

Remember I said, didn't I, that once it crosses that threshold of about four millimoles of lactic acid per litre of blood, that's when it really starts to cause this problem, and sometimes you'll hear about marathon runners saying that they hit the wall.

That means they were running too fast, and that lactic acid crossed over that threshold, and they couldn't keep going at that pace.

That takes us nicely, and this connects, doesn't it, into that second body system of the cardiovascular system.

So when we start to exercise, there's a number of immediate changes that happen to the cardiovascular system as well.

And in fact, we explored briefly, didn't we, that some of these happen just before we start exercising, thanks to that adrenaline release.

So Lucas is wondering, can you name and describe any of these changes at the heart, blood, and blood vessels? Pause the video now.

Have a quick go.

Okay, let's have another quick summary then before we delve into the detail of these.

So increase in heart rate.

That's the obvious one, isn't it, and the most important one that you remember in connection with all of this.

So the minute you start to exercise, your heart rate goes up, but also your stroke volume goes up, so your heart beats more forcefully in each beat, so it gets more blood out per beat.

And as a consequence of those two increases, you get an increase in cardiac output, so total quantity of blood circulating in the body.

And then, also, I mentioned briefly in the last section that there's an increased blood flow to your working muscles and also actually to the surface of the skin to help you cool down, and this is called the vascular shunt mechanism.

So the redistribution of blood is known as the vascular shunt mechanism because blood vessels, vascular, are shunting more blood to a different part of the body, and we'll explore that in a bit more detail in a moment.

So looking first at that heart rate response then, did you know that a typical teenager or adult will have a resting heart rate of about 72 beats per minute? And in fact, anywhere in that range of 60 to 100 beats per minute is perfectly normal.

And actually, newborn babies have a much higher resting heart rate, and that's because they've got a much smaller heart.

But also, as we transition to children, to teenagers, and then into adults, generally, our heart rate reduces.

It slows down, and that's because it becomes more efficient at pumping blood, more able to pump more out per beat, so it doesn't need to beat as many times per minute.

So Alex is here this time with a question.

What happens to your heart rate when you exercise? And Aisha is wondering, what about when you're sleeping? And what about if you were a super fit athlete? What other changes might happen? Can you quickly recall them? So what is your heart rate measured in? That's right.

It's beats per minute.

And secondly, what is a typical range for resting heart rate? That's right.

72 beats per minute tends to be the one that we all quote.

But actually anywhere between 60 and 100 is perfectly normal.

So let's look at some of these factors that increase your heart rate, just for a bit of wider knowledge.

So physical activity or exercise, that's really clear, isn't it? That increases your heart rate, and that's the main topic we're gonna explore.

But also, adrenaline release due to stress, anxiety, or excitement, we call that the fight or flight response, also increases your heart rate.

Coffee and stimulants, but also some medications increase your heart rate.

Body temperature increases your heart rate 'cause you're trying to get rid of that excess heat to maintain homeostasis, or you know, a steady state.

Dehydration increases your heart rate.

So it's really important that you stay hydrated.

Keep drinking.

Illness causes an increase in heart rate, and you might have felt that sometimes.

That's why the doctor or your parents or guardians might take your heart rate, take your pulse when you're ill.

So these are the things that cause your heart rate to increase.

But what causes your heart rate to decrease? Izzy is wondering.

Pause the video now and see if you could jot down some ideas.

Okay, let's look at them then.

So resting.

That decreases your heart rate, and a nice fun game you can sometimes play is just sit at rest, try and get as calm as you can, breathe some deep breaths, see how low can you get your resting heart rate.

Sleep.

That gets our heart rate really low.

And if you wear some sort of smart watch, you know, perhaps an Apple Watch or a Fitbit, or a Garmin, that will be able to tell you what your resting heart rate or your sleeping heart rate was overnight.

Really interesting to see.

Improved fitness reduces your heart rate, and we'll delve into the detail of that in the next lesson, where we look at the long-term adaptations to training.

But yeah, increased fitness reduces your heart rate, and that's why quite often people get a bit competitive.

They compare resting heart rates with each other to see who's got the lowest, and does that mean they're fitter? Maybe a little bit.

Relaxation and meditation are ways to reduce your heart rate.

Some other medications.

So some medications make your heart rate go up, others make it go down, and beta blockers is an example of a medication that makes your heart rate go down.

Hypothermia.

So being too cold reduces your heart rate, and it could do to a dangerously low level.

And then there's some other medical conditions.

So one of them is called bradycardia.

So bradycardia is a term used for when your heart rate is below 60 beats per minute, and often as athletes, so certainly for myself, I'll try to keep my resting heart rate below 60 'cause I know that that's a sign for me that I've got a healthy heart.

It's squeezing a lot of blood out per beat, and it's able to, therefore, beat less times at rest.

But there is also a medical condition.

If I went into hospital or if a normal person perhaps that has a resting heart rate of about 72 beats per minute, if theirs was below 60, that would suggest perhaps there's something wrong.

So we need to be careful looking at that resting heart rate, and how low we want it to go.

I always remember a quote of Paula Radcliffe, the famous long distance runner, having a resting heart rate of about 36 beats per minute, so incredibly low as a resting heart rate.

And then ageing reduces your heart rate.

So this is a formula you'll want to remember.

Your maximum heart rate is 220 beats per minute minus your age.

And then deep diving also reduces your heart rate.

But these aren't things that we hugely need to remember now.

So let's see if we can recall another few quick answers as a check.

What's the effect of improved fitness or better sleeping on your resting heart rate? That's right.

It reduces, and this is known as bradycardia if it goes under 60 beats per minute.

Secondly, what's the effect of stress or adrenaline release on your heart rate? That's right.

That causes it to increase.

And then thirdly, how do you calculate your maximum heart rate? That's right.

It's 220 beats per minute minus your age.

So if you are 15 years old, your maximum heart rate typically would be 205 beats per minute.

So Laura's wondering then, why does she feel butterflies before a race? Well, that's that fight or flight response that I mentioned, and it kicks in just before exercise starts to help prepare the body for exercise.

So adrenaline gets released.

That triggers what we call the anticipatory rise in heart rate, and it gets more blood flow to the working muscles.

So our heart rate increases from perhaps 60 to 80 beats per minute up to a maximum of 220 minus your age, depending on the intensity of exercise.

So the harder the exercise, the higher your heart rate goes.

If I just went out for a walk, might settle at just, you know, 100, 110 beats per minute.

But if I turn that into a jog, or into a run, or into a run uphill, it would keep going up.

And this is a nice graph then that shows if I went for a 12 minute steady jog, my resting heart rate at 70 beats per minute increases just before I start exercising.

So I started exercising at four minutes on this graph.

Goes up steeply.

Settles at about 155 beats per minute, so we call that a plateau, where there's a steady state heart rate where we're providing enough oxygen for our body to keep working at the intensity that it is, and then exercise is finished at 16 minutes, and then instead of the heart rate dropping straight back down to its resting level, we've got that oxygen debt to repay.

So it takes a little while to recover back to resting levels.

So what Alex is wondering is what would happen to your heart rate if you were playing a game of football then? Perhaps pause the video now, have a go at drawing a graph, and did yours look anything like this? Probably not because it's completely random, isn't it? If they went for a sprint, the heart rate would go up.

If they were resting, their heart rate would go back down.

If they jogged for a period of time, it might stay up.

So it's quite a spiky graph of heart rate response to exercise, and it would depend on what position you were playing as well, wouldn't it? Goalkeepers would stay much lower than this, whereas midfielders perhaps would be higher for more of the match.

What happens to your heart rate if you run as hard as you can to exhaustion then? Well, it would look a little bit like this.

So that heart rate, you still have the anticipatory rise, and then it would just keep climbing and keep climbing and keep climbing to that point just over 200 beats per minute.

And what happens there? That's right.

That's the point of fatigue, where that blood lactate has accumulated to such a level that it forces us to stop, and then we've got that recovery, and recovery takes much longer if we've taken ourselves to exhaustion.

So Laura is wondering, why does our stroke volume increase during exercise? Well, our heart pumps more forcefully, doesn't it, when we start to exercise.

You can feel it thumping in your chest.

Consequently, more blood is ejected from the ventricles per beat, and that means your stroke volume has increased to further increase the supply of blood containing oxygen and nutrients to the working muscle.

So your heart rate's up, and your stroke volume's up.

So let's connect that in with cardiac output then.

And we know, don't we, that cardiac output is that volume of blood pumped out of the heart per minute, and we calculate it as heart rate multiplied by stroke volume.

And we know that, at rest, the cardiac output is about seven, sorry, five litres per minute.

Well, that's based on perhaps 70 beats per minute times 70 millilitres of stroke volume.

But actually, during exercise, your heart rate goes up to perhaps, well, we've said it, haven't we, up to 220 minus your age.

So let's say 200 beats per minute, and it's more forceful, pumping out more per beat, so maybe 100 millilitres per beat.

So that would give us our 20 litres.

But in fact, for elite athletes, it can go up even higher than that because their stroke volume is huge.

So it can go up as high as 40 litres per minute.

Amazing, isn't it? The human body.

So let's do another quick check.

Which of the following graphs is incorrect and why? So is it that A, the cardiac output is a response to four sets of 400 metre sprints? Is it B, the cardiac output response to a football game? Or is it C, the cardiac output response to a maximal exercise? Have a think.

That's right.

It's B.

That's showing steady state exercise, whereas a football match would be going up and down, random patterns depending on whether they're sprinting, jogging, or perhaps just walking around the pitch.

But there's another problem with that Graph B.

Can you see it? That's right.

It's suggesting that cardiac output at rest is zero.

And of course, we know that we've got a basal metabolic rate, a basal metabolism, so blood is always circulating around the body.

So your cardiac output at rest is five litres per minute, not zero.

And then Laura's gonna delve deeper with us into this vascular shunt mechanism.

What was that again, and how does it help exercise response? Well, have you ever been told not to swim after you've eaten? The reason you have been told that is because when we exercise, blood is redistributed to the working muscles to ensure maximum flow to where it's needed most, and that involves vascular constriction, so the arteries that are carrying blood to your digestive system will close off.

They'll squeeze tighter to let less blood flow there, and vasodilation, so opening up, dilation of those arteries that supply blood to the working muscles and to the surface of the skin, so that more blood can go there.

And the problem if you've just eaten is your brain's sending weird messages, going "Digest food.

"No, exercise and swim.

"No, digest food.

"No, exercise and swim." And so that's why it's ideal not to swim just after you've eaten, because there's a danger that you might perhaps get a stitch, which, of course, is cramp of that diaphragm muscle.

Okay, let's do a quick check.

Which of the following happens when we start to exercise? Is it A, vasoconstriction reduces blood flow to the digestive system? Is it B, vasodilation decreases blood flow to the working muscles? Or is it C, vasodilation increases blood flow to the working muscles? That's right.

So A and C are correct.

Vasoconstriction, so reduced blood flow to the digestive system, and vasodilation, to increase blood flow to your working muscles.

Okay, so the second task I'd like you to do in this lesson is split into two parts.

So I want you to describe the four main cardiovascular responses to exercise, and then I want you to explain their combined benefit on a sports performer.

Pause the video now whilst you do that, and come back to me when you're ready.

Okay, let's compare your answers with mine.

So these four main cardiovascular responses.

So we've got increase in heart rate from resting value of 60 to 80 beats per minute up to a maximum of 220 minus your age.

Stroke volume increases, and this enables more blood to be ejected from the heart per beat.

Cardiac output, which is the product of heart rate multiplied by stroke volume, also increases significantly from five litres per minute at rest up to 20 to 40 litres per minute during intensive exercise.

And then fourthly, that vascular shunt mechanism kicks in, and that causes that redistribution of blood by vasoconstricting the artery supply and the digestive system, and vasodilating, to increase the blood supply to the working muscles and also to the surface of the skin.

Second part of this, what's their combined benefit? Well, you might have said that increase in heart rate and stroke volume results in a bigger increase in cardiac output.

Meanwhile, redirecting more blood to the working muscles and less to the digestive system further increases that blood supply to our working muscles during exercise.

Here's the important bit.

As a consequence of that, more oxygen and nutrients are made available to the performer, and therefore, they can keep exercising harder and for longer, and also they're able to remove more carbon dioxide and those fatiguing byproducts of lactic acid from the muscles, and that helps stop performance from being impaired.

Okay, into the third part of today's lesson then.

So let's link this in with the respiratory system.

So when we start to exercise, there's these immediate changes that happen to the respiratory system too.

Lucas is wondering, can you name and describe any of these changes? Pause the video now.

Have a quick go.

And as I'm sure you're expecting, a quick summary of them.

So we have an increase in respiratory rate, an increase in tidal volume, an increase in minute ventilation, and an increase in gaseous exchange.

Let's delve a bit deeper into those, shall we? So Laura's wondering, what happens to your breathing rate then during exercise, and why does that happen? Well, exercise triggers a need for more oxygen, doesn't it? And if we need more oxygen at our working muscles and more carbon dioxide removed, then that oxygen comes from the air that we breathe in.

Consequently, we breathe more frequently, so our breathing rate increases, and at rest, it might be 16 to 20 breaths per minute.

But during exercise, it goes up to perhaps 40 to 60 breaths per minute during maximal endurance events, like a marathon run.

What about tidal volume then? Can you remember what that's all about? Well, we also breathe deeper during exercise, so our tidal volume increases from about 500 millilitres to enable a greater exchange of oxygen and carbon dioxide in those alveoli.

And it achieves this by lifting the ribcage up and out more, and hence creating a bigger chest cavity, so more air can rush into the lungs, and also expiration becomes active, so we forcibly pull our rib cage down and in using our abdominals muscles and our intercostals.

Have a quick reminder here.

That spirometer trace showing those different measures, and our tidal volume is that amount of air breathed in or out per breath, and it's able to increase during exercise 'cause it eats into that inspiratory reserve volume and the expiratory reserve volume to create a bigger wave on that diagram.

So quick check.

Which of the following is correct? Is it A, tidal volume increases during exercise? Is it B, inspiratory reserve volume decreases during exercise? Is it C, expiratory reserve volume decreases during exercise? Or is it D, residual volume decreases during exercise? That's right.

The incorrect answer is residual volume decreases 'cause we know, don't we, that residual volume stays the same all of the time, and that stops our lungs from collapsing.

Okay, so Laura's wondering, what happens to your minute ventilation during exercise, and why does that happen? Well, the minute ventilation is that volume of gas that is inhaled or exhaled from the lungs per minute.

It's the product of breathing rate or breathing frequency multiplied by tidal volume.

So if those two go up, it's also gonna go up, isn't it? And in the same way that cardiac output goes up significantly, minute ventilation goes up significantly to enable more gaseous exchange to happen.

So links nicely to this next question.

What is gaseous exchange, and what happens to it during exercise? Well, hopefully, you remember that it's that exchange of gases where they move from higher concentration to lower concentration via diffusion until there's a steady pressure of those different gases on both sides of, in this case, the alveoli wall, but it might be the capillary wall at the muscles.

So it's that movement of gases between the alveoli and the bloodstream, and then between the bloodstream and the muscle site, and they move from high to low pressure, and the oxygen diffuses into the bloodstream, and then it's unloaded at the muscle site to enable aerobic respiration to happen.

Meanwhile, we've got carbon dioxide travelling in the opposite direction to get out of the muscles into the bloodstream, and then out of the bloodstream into the alveoli, and then exhaled.

So quick check.

True or false? Gaseous exchange is the movement of oxygen and carbon dioxide from areas of high concentration to areas of lower concentration.

That's right.

That's true, isn't it? So gases can only move from high concentration or pressure to an area of lower concentration or pressure, and they do that until there's a point of equilibrium.

So equal quantities on both sides of that wall, and the body is only able to accept more oxygen if supplies in the bloodstream are depleted, or if you've trained and you've built up more haemoglobin, and we'll learn more about that in the long-term effects of exercise lesson.

So that brings us to our last practise task of the lesson.

Can you describe four main respiratory responses to exercise, and then secondly, can you explain how rates of gaseous exchange will adjust in response to a gentle walk, a jog, and then a maximal effort to exhaustion? An example of that might be the multi-stage fitness test.

So you run between the beeps, and the gap between the beeps get shorter, so you have to run faster, and you keep going as long as you can.

Okay, so pause the video if you haven't already, and have a go at that task, and then come back to me when you are ready, and let's see what I've come up with.

Okay, so breathing rate increases, and by breathing more times per minute, we get more air in and more air out of the lungs.

Tidal volume increases, and that increased depth of breathing results in a greater quantity of air moving into and out of the lungs per breath as well.

And then minute ventilation increases because that's the product of those two multiplied together, which means there's a much greater volume of air entering the lungs per minute, which means that gaseous exchange can happen faster during exercise, which means there's a bigger difference of that concentration or percentage or pressure of oxygen in the alveoli compared with the bloodstream, so it diffuses faster, and becomes more available faster at the muscles.

Meanwhile, carbon dioxide is doing the same on the way out of the body.

What about the second part then? So how do those rates of gaseous exchange adjust in different exercise? Well, if I go for a walk, there'll be a small increase in gaseous exchange because the muscles will require a little bit more oxygen, and they'll produce a little bit more carbon dioxide walking versus resting.

If I turn that into a jog, there's increased aerobic respiration taking place, so significantly more oxygen is needed.

Diffusion of oxygen in happens faster, and diffusion of carbon dioxide happens out faster during exercise.

And then if we turn that into a maximal effort, that creates the highest ventilation rates and the biggest gaseous exchange.

But unfortunately, we also have that buildup of lactic acid due to the inability to provide enough extra oxygen for the intensive exercise that's happening.

And as a consequence of that, fatigue will eventually happen, which prevents us from carrying on.

That gives us just enough time to summarise today's lesson.

So just before we start exercising, our body responds to that increased demand placed on it, and it does that by the circulatory system will increase heart rate, stroke volume, and cardiac output, but also more blood is redistributed to the working muscles.

Secondly, the respiratory system responds by increasing the breathing rate, the tidal volume, and the minute ventilation.

Those two things combined mean that we can provide more oxygen to our working muscles to enable them to work harder for longer.

But let's remember that if we work at a higher intensity, we will also build up lactic acid in our muscles, and it's that lactic acid that causes fatigue.

I hope you've enjoyed today's lesson, and I'll see you next time.