May 23, 2011

053 Cardiac Output

Learn more about calculating cardiac output and how changing stroke volume and heart rate can increase or decrease its value.

Have fun!

Transcript of Today’s Episode

Hello and welcome to another episode of Interactive-Biology TV where we’re making Biology fun! My name is Leslie Samuel, and in this episode, Episode 53, I’m going to talk about the ‘Cardiac Output.’ I’m first going to talk about what it is and then, I’m going to talk about how to calculate it, and some of the things that are going to influence cardiac output. So, let’s get right into it.

Here, we’re looking at a picture of the heart. A diagram of the heart during systole, so it’s during contraction, ventricular contraction, and what’s happening here, is as the ventricles contract, the left ventricle is sending blood to the aorta which then goes to the rest of the body, the right ventricle of course, is sending the blood to the lungs so that, it can get oxygenated, come back to the heart and then, be sent to the rest of the body.

Now, we’re going to talk about cardiac output today so, let me write that here, ‘Cardiac Output.’ It kind of is exactly what it sounds like. Cardiac output is talking about the volume of blood that’s being ejected from the ventricles every minute. So, we’re talking about the amount of blood that’s being sent from the left ventricle to the rest of the body via the aorta, and also we’re talking about the amount of blood that’s being sent from the right ventricle to the lungs. You expect those to be the same or else that can lead to some other problems. That we’re not going to get into in this episode.

Right now, we are just going to try to look at how to calculate cardiac output. It’s a pretty simple formula and it involves two things that we’ve already looked at in previous episodes. Cardiac output is equal to SV, which is stroke volume, times HR, which is heart rate:

CO = SV x HR

Now, stroke volume tells you the amount of blood that’s ejected with each beat. So, we’re talking about milliliters of blood per beat, and then, with heart rate, we’re talking about how many times the heart is beating in one minute. So, we calculate that as beats per minute. So, stroke volume is the amount of blood ejected from the ventricles, let’s say the left ventricle in each beat, with each beat. And, the heart rate tells us how many times the heart is beating, how many times the ventricles are contracting in one minute. If we, multiply these out of course, if you apply some simple algebra, we’re going to be canceling out the beats, so, cardiac output is calculated in milliliters per minute – some pretty straightforward algebra there.

Now, let’s plug in some values. The average adult male has a stroke volume of approximately 70 millliters per beat and an average heart rate of 75 beats per minute. Now, if you want to calculate cardiac output, the amount of blood ejected from the ventricle each minute, cardiac output would be equal to 70 milliliters per beat times 75 beats per minute and that’s going to give us a value of 5,250 milliliters per minute. Of course, you can make that into 5.25 L per minute. So, this is the average adult male at rest, 5.25 L/min cardiac output. Now keep in mind that the average adult male has about 5L of blood in their body. So, every minute the heart is re-circulating pretty much all of the blood if you’re just at rest. All right, so the heart is doing a significant amount of work. It’s sending the blood to the muscles, that organs that need to get oxygen and the nutrients that come via the blood.

We know how to calculate cardiac output. Now, if you want to change cardiac output or if you want to influence cardiac output, it should be obvious that we can do that in three ways by influencing stroke volume, by influencing heart rate, or by influencing both of them. So, if you make a change, to the stroke volume or the heart rate, that of course, in turn is going to make a change to cardiac output.

Now, we’ve also looked at factors that are going to influence volume and heart rate. We’ve spoken about those factors in Episodes, I think it’s 46 and Episode 50. Episode 46 talks about how we can influence heart rate. Episode 50 talks about influencing stroke volume. You can always revisit those and see how that works.

When you’re exercising, you’re going to have a little bit of a different situation. You know when you’re starting to exercise and you’re doing some aerobic activity, your heart rate is going to increase, your cardiac output is also going to increase because you’re increasing stroke volume and your heart rate so, that’s going to give you some different values.

Let’s take an example where we’re trying to calculate cardiac output again. As usual, cardiac output is equal to stroke volume. We’re going to take that and multiply that by heart rate. Same exact situation.

CO = SV x HR

But, okay, I’m showing ladies here exercising but, we’re going to stick with the example for the calculations, we’re going to stick with males, and we’re going to see during moderate exercise, the stroke volume might increase to a point of let’s say about 100 mL/beat, and let’s say the heart rate increases before it was 75 and let’s say we have, let’s go with a 100 because that’s easy to calculate, a hundred beats per minute (100 beats/min). So, we have more of an intense situation, it’s not a very stressful situation but, you’re doing some exercise. So, that’s going to influence cardiac output of course. Cardiac output is going to be equal to:

CO = 100 mL/beat x 100 beats/minute

Giving us a cardiac output of 10 L/min. So, by doing a little bit of exercise, we’ve almost doubled the cardiac output. In other words, your blood is being pumped throughout your body twice in that minute with moderate exercise.

As you exercise and you train, hopefully we’re doing this on a regular basis, hopefully, like three times a week, **hint, hint, hint** but, as you’re doing this more regularly, there are going to be some adaptations that happen. One of the main adaptations is that your stroke volume increase, (I’m just going to put an up arrow here showing that your stroke volume increases). In other words, everytime that your heart pumps, it pumps more blood than the average untrained individual.

What that can do in turn is that can actually cause a lower resting heart rate. All right, so, a trained person on average has a lower heart rate than an untrained person. However, they’re still able to get the same cardiac output. In other words, your heart is becoming more efficient at pumping the blood. Everytime it contracts the ventricles, it’s sending a significant amount more than the untrained person and you can achieve the same effect if not even a better effect with less work.

So, the take home message there I guess you could say is, do like this ladies, get into the gym and start doing some exercise. That’s always good and it’s going to pay off in the long run.

That’s not what I intended to teach but, hey it’s something that’s good for us to learn. That’s pretty much it for this episode. As usual, you can visit the website at for more Biology resources and a bunch of other resources that we’re adding there. We’re adding quizzes and we have just added a brand new Biology community where you can ask questions, get answers, give input. Join the community and make the community stronger and much more fun.

Thank you for watching this episode and I’ll see you on the next one.

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