049 What Stroke Volume is and How to Calculate It
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 before I get into this episode, I just want to send a shout out to Richard Morris and his friend, Nathan. Richard sent me an email a few days ago letting me know that they appreciate the videos and that they have watched every single video that I have published here at the Interactive Biology TV. And that is just awesome. Richard and Nathan, I hope you have gotten a tremendous amount of value and, I hope that you continue to get a tremendous amount of value from the videos that you are watching here at Interactive-Biology TV.
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Inside this episode, Episode 49, I’m going to be talking about what is stroke volume and how to calculate it. So, let’s just get right into it.
When I’m talking about stroke volume, I’m talking about the amount of blood that is pumped by one ventricle during each heart beat. So, we know that the heart beats and, we know that blood comes into the heart, and when the ventricle contracts, it pushes that blood out to the rest of the body and to the lungs.
We’ve looked at this in detail, in Episode 44. If you need to review that, you can go back to Episode 44 to check out those details.
Here, we’re looking at the heart. Right now, we’re just going to look at the left ventricle. What is happening is blood is coming into the left atrium and then into the left ventricle. When it reaches in the left ventricle, after a short period of time, the ventricles contract and that pushes blood, as you can see here, pushes the blood out into the aorta and that then goes to the rest of the body.
That’s a brief overview but, once again, you can go back to Episode 44 to check out more details about that. There are a few definitions that I want you to know.
Definition number one: systole. Systole is the contraction of the heart. Then we have diastole and that’s the relaxation of the heart. So, systole is contraction, diastole is relaxation then, we have the End Diastolic Volume or the EDV which is the amount of blood in the ventricle right before ventricular contraction.
So, right before the ventricle contracts, the amount of blood that we have in the ventricle, we call that the End Diastolic Value which makes sense. It’s right at the end of diastole so, that’s the End Diastolic Volume. Then, of course we have the End Systolic Volume which is the amount of blood left in the ventricle right after the ventricular contraction. So, when the ventricles contract, and it pushes the blood out to the lungs and out to the rest of the body, the amount of blood we have left over, that is the end systolic volume which, once again, makes sense because its at the end of systole.
With those definitions, let’s look at the graph. All right. So, what we have here is a graph. On the x-axis we have time and on the y-axis we have volume in milliliters so, we’re looking for the amount of blood in the ventricle. For this example, we’re going to talk about the left ventricle which is the one that pushes the blood into the aorta to go to the rest of the body.
Let’s say, ventricular contraction has just finished. When that contraction is finished, the ventricles start to get filled with blood again. After that contraction, another cycle starts where blood flows into the ventricle. Here we have blood getting into the ventricle.
At a certain point, we’re going to have a more rapid filling, and when it reaches this point, and we’re not going to talk about too many details where this part is concerned. We’re going to talk more about that in a later episode. But, let’s say this is the point where the ventricle contracts.
What’s going to happen here is, when the ventricle contracts, that’s going to push the blood out of the ventricle and it’s going to go to the rest of the body. Here we have a certain amount of blood and here we have a certain amount of blood. The amount of blood that we have in here, right before the contraction, we’re going to call that the EDV. Once the contraction is over, we’re going to call that the ESV.
The way we calculate the stroke value is SV is equal to EDV minus ESD: SV= EDV – ESD.
If the end diastolic volume is 120 mL of blood and the end systolic volume is around, let’s say, it’s 50 mL. The stroke value is going to be 120ml minus 150 mL and that is going to be equal to 70 mL. I just realized here that I wrote 5. I meant to put 50 so, I’m just going to put that 0 here. So 120 minus 50 = 70. Seventy would be my stroke volume. Again, ventricle fills with blood, the ventricle contracts.
At a certain point, the ventricle contracts that pushes the blood out of the ventricle and to the rest of the body. At the end of the contraction, we have an end systolic volume. If the end systolic volume is 50 mL and the end diastolic volume is 120 mL, the stroke volume will be 120 minus 50 and that is going to be equal to seventy.
That’s pretty much it for this episode. Of course, if you want to see more, you can go to Interactive-Biology.com where we have more Biology videos and many other resources. We’re adding to the resources on a regular basis to try to help make Biology fun. That’s it for this video and I’ll see you on the next one.