# The And, from our equation, we see

The amount of blood your heart pumps is constantly changing to meet the needs of your body. In this lesson, you will learn how the heart maintains an adequate cardiac output, heart rate and stroke volume.

## Circulatory System

There’s a lot riding on the proper amount of blood being pumped by your heart with each heartbeat. A good, strong heart allows the proper amount of blood to flow through your body, which carries gases to your lungs and body tissues where they can be exchanged. It also delivers nutrients to the trillions of cells in your body and carries away cell waste.

In this lesson, we will look at just how we calculate the amount of blood being pumped by your heart.

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## Cardiac Output

Cardiac output (CO) is the term used to show the amount of blood pumped per minute by each ventricle. When your body’s at rest, your heart beats about 75 times per minute.

Each time it pumps, it pushes out about 75 milliliters of blood, which is about a third of a cup – it’s about the amount that you could hold in your cupped hand. When you multiply the number of heartbeats per minute times the amount of blood being pumped during each heartbeat, you get the cardiac output.

If we do the math using the examples above, we see that 75 heartbeats per minute times 75 milliliters of blood pumped during each heartbeat equals the average cardiac output of about 5.6 liters of blood pumped through your heart each minute. That’s a lot of blood, and if you consider that large bottles of soda often come in 2 liter containers, that means that your heart pumps the contents of more than 2 and a half of these soda bottles every minute.It’s also interesting to consider that the total amount of blood in your body is usually between 5 and 6 liters; this means that your heart pumps the entire blood supply every minute.

We’ve said it before, but it’s true to say it again: the heart is quite a workaholic!

## Heart Rate and Stroke Volume

What we have from the example just described is a formula for figuring out the amount of blood pumped per minute by each ventricle, what we call the cardiac output. So, let’s look at this formula a little bit closer. Cardiac output equals the number of heartbeats per minute times the volume of blood pumped out of the ventricles with each heartbeat. When we look at this equation in cardiovascular physiology, we use the terms heart rate (HR) to describe the number of heartbeats per minute and stroke volume (SV) to describe the volume of blood pumped by the ventricles with each heartbeat. Therefore, our equation looks like this: CO = HR x SV.

We know that if you increase the amount of blood being pumped by your heart every minute, what you’re really doing is increasing your cardiac output. And, from our equation, we see that this means you either increased your heart rate or your stroke volume or both. Think of it this way: if you want to use buckets to fill up a kiddie pool for a child who wants to play in the water and that child is growing impatient, you’ll want to fill the pool faster to increase the amount of water getting into the pool.In other words, you want to increase your water output. You can do this in a few ways: you can either move faster, filling more buckets per minute, which is an increased rate, or you could increase the size of the bucket, which is an increased volume.

Or, you could do both! The same goes for the heart. If your body demands more blood, due to something such as exercise, your heart will beat faster and beat stronger.

## Clinical Importance

Knowing the cardiac output of the heart can reveal many things about how well your heart is working and if it’s an efficient pump or not. For example, doctors can monitor your heart rate, usually by taking your pulse, to get an idea of your general cardiovascular health. Doctors can also monitor the stroke volume to gather clues about the strength of your heart. So, our equation, CO = HR x SV, is important in both the study of the heart and in clinical practice.

## Lesson Summary

Let’s review. Cardiac output (CO) is the term used to show the amount of blood pumped per minute by each ventricle. Cardiac output is determined by multiplying the number of heartbeats per minute times the volume of blood pumped by the ventricles with each heartbeat.

Therefore, we can use the equation CO = HR x SV. This equation tells us that the cardiac output equals the heart rate (HR), which is the number of heartbeats per minute, times the stroke volume (SV), which is the volume of blood pumped by the ventricles with each heartbeat. If your body needs more blood, then your heart will increase the cardiac output. This can be done in a few ways – either your heart rate will increase, your stroke volume will increase, or both.

## Learning Outcomes

Once you have studied this lesson, you’ll be able to:

• Define heart rate, stroke volume and cardiac output
• Write the equation for cardiac output
• Explain the importance of heart rate and stroke volume
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