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Regulation of Heart Rate and Stroke Volume

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  • 0:05 Cardiac Output
  • 0:40 Regulation of Heart Rate
  • 1:56 Regulation of Stroke…
  • 2:54 Frank-Starling Law of…
  • 4:21 Afterload
  • 5:27 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

The amount of blood being pumped through your body changes constantly due to the demands of daily life. In this lesson, you will learn how your body adjusts the heart rate and stroke volume to increase or decrease blood flow.

Cardiac Output

The volume of blood your heart pumps per minute, known as your cardiac output, varies with the demands you put on your body. We previously learned that the cardiac output (CO) is calculated by multiplying the number of heartbeats per minute (which is your heart rate, or HR) times the volume of blood pumped out of the heart with each heartbeat (which is your stroke volume, or SV). So:

cardiac output (CO) = heart rate (HR) x stroke volume (SV)

The cardiac output of your heart is a constantly changing amount that adjusts to any physical or emotional demands you put on your body. In this lesson, we will look at some of the factors that influence your heart rate and stroke volume.

Regulation of Heart Rate

The SA node, or pacemaker, is responsible for the beating of your heart without outside influences
Pacemaker

Let's start by taking a look at some of the factors that influence your heart rate. We previously learned that the heart can beat on its own without any influence from outside stimulation thanks to the automatic pace-setter of your heart, known as the SA node. This is often nicknamed the 'pacemaker' of the heart.

Normally, though, the heart rate is changed for short periods by the autonomic nervous system. We remember that the autonomic nervous system is the division of the nervous system that's under involuntary control. We also know that the autonomic nervous system has two divisions: the sympathetic division (or fight-or-flight division) and the parasympathetic division.

If you exercise or experience some type of emotional stress, the nerves of the sympathetic division stimulate the SA node to fire more frequently, increasing your heart rate. You've probably noticed how your heart beats faster during times of physical or emotional stress. When the stress goes away, the heart rate readjusts back to a normal resting heart rate. This happens thanks to the parasympathetic division of the autonomic nervous system.

Therefore, we see that the sympathetic nervous system increases heart rate, which increases cardiac output. We also see that the parasympathetic nervous system decreases heart rate, which decreases cardiac output.

Regulation of Stroke Volume and Preload

Now that we know what influences the heart rate, let's look at some of the factors that influence the amount of blood pumped during each heartbeat, which is your stroke volume. We previously learned that during the resting phase of the heart, known as diastole, the ventricles of the heart are passively filling with blood. Then at the end of diastole, the atria contract, filling the ventricles even more.

Like a rubber band, heart ventricles contract harder when blood volume stretches them
Preload

The volume of blood in the ventricles at the end of diastole is referred to as the end-diastolic volume. An increase in end-diastolic volume then results in more stretching of the ventricles because there's more blood there. When the ventricle is stretched more, it contracts harder, just like a rubber band - if you stretch it really far, it's going to contract harder. A good way to think about end-diastolic volume is to think about it as the amount of blood 'loaded' into the ventricles prior to contraction. Thus, it is often referred to as preload.

Frank-Starling Law of the Heart

Two scientists named Frank and Starling noticed that the increase in end-diastolic volume, or preload, results in an increased stroke volume and more blood being pumped out of the heart with each heartbeat. This became a popular law in cardiac physiology and is known as the Frank-Starling law of the heart.

Frank and Starling were looking at the amount of blood in the heart before the ventricles contract. It's interesting to note that during the contraction not all of the blood leaves the ventricles. That's right - a healthy heart does not pump every last drop of blood out of the ventricles when they contract no matter how strongly they contract.

This amount of blood remaining in the ventricles after contraction is known as the end-systolic volume. You can recall this term by remembering that systole is the term used to describe contraction of the ventricles. The end-systolic volume can vary due to how forcefully the ventricles contract. If they contract strongly, more blood will be ejected and the end-systolic volume will be lower - just like forcefully exhaling air from your lungs leaves less air in your lungs than exhaling normally. Here we see that the end-systolic volume is closely related to stroke volume. If the ventricles contract hard, they push more blood out of the heart with each heartbeat, leaving less blood in the ventricle. Therefore, we see that a decreased end-systolic volume indicates an increase in stroke volume.

Afterload

Another important influence of end-systolic volume, and therefore stroke volume, is how much pressure exists in the arteries leaving the heart. If there's a lot of pressure in the arteries leaving the heart, the heart will have a harder time pumping blood. This arterial pressure that represents the amount of resistance the ventricle has to overcome to eject blood is called the afterload.

Afterload refers to how much arterial pressure the heart must overcome to pump blood
end-systolic volume

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