Med-Surg - Cardiovascular System, part 2: Heart Conduction System, Cardiac Function, Blood Pressure

by Cathy Parkes September 01, 2021 Updated: September 05, 2021

Full Transcript

Hi, I'm Cathy with Level Up RN. In this video, we are going to continue our anatomy and physiology review of the cardiovascular system. If you have our medical-surgical nursing second-edition cards, be sure to pull those out and follow along with me. So in this video, we are going to talk about the heart conduction system, cardiac function, and blood pressure. And at the end of the video, I'm going to give you guys a quiz to test your understanding of some of the key concepts that I'll be covering in this video. So definitely stay tuned for that.

Let's first talk about the steps of the heart conduction system using an illustration from our medical-surgical nursing flashcard deck. So we have our SA note here, our sinus node, which will initiate an electrical impulse which stimulates the atria to depolarize and contract. The impulse then travels to the AV node, where there is a delay to allow the blood in the atria to empty into the ventricles. Then, from the AV node, the impulse travels to the bundle of His, then the left and right bundle branches, and then through the Purkinje fibers. And ultimately, this causes depolarization of the myocardium, which triggers contraction of the ventricles. So understanding this path, the conduction system is going to be important for you to know when we start talking about dysrhythmias, which will definitely be covering later on in this video playlist. Now let's talk about some important terms and concepts that relate to cardiac function. Within the cardiac cycle, we have diastole and systole. So during diastole, this is where the ventricles and the atria are relaxed and they are filling with blood. Then, during systole, the ventricles and atria contract and eject that blood. So within the cardiac cycle, about two-thirds of the time is spent in diastole, and about one-third of the time is spent with systole. Cardiac output is the volume of blood in liters that is ejected from the left ventricle every minute. And you would calculate cardiac output by taking the patient's heart rate and multiplying that times the stroke volume. So the heart rate is the number of times that the ventricles contract within a minute. So for an adult, this usually varies between 60 and 100 beats per minute. The stroke volume is the volume of blood in liters that is ejected from the ventricle with each heartbeat. So you take that heart rate times the stroke volume, and that would give you cardiac output. And cardiac output for an adult usually varies between 4 and 8 liters per minute. Another important term to know is something called left ventricular ejection fraction, or LVEF. This is the percentage of blood that leaves the left ventricle each time it contracts. And in a healthy adult, this will vary between 55 and 70 percent. And when we talk about heart failure in another video in this playlist, you will see that most of these patients will have a reduced LVEF that we would determine through an echocardiogram. Then we have preload, which is another important thing to understand. This is the volume of blood in the ventricles at the end of diastole, so right before contraction. And this determines the amount of stretch that we are placing on those myocardial fibers in the heart. This differs from afterload. So afterload is the peripheral resistance that the left ventricle must overcome in order to push that blood into systemic circulation. So when we are caring for a heart-failure patient, we are going to administer medications often that will reduce preload and/or afterload. So we're trying to reduce the amount of stretch on that heart and the amount of workload that heart needs to do by reducing preload. And we're also trying to reduce afterload to make it easier for the heart to push that blood into systemic circulation.

The last topic I want to cover in this video is blood pressure, which is comprised of systolic blood pressure and diastolic blood pressure. So systolic blood pressure is the amount of pressure that is exerted against those arterial walls during left ventricular contraction, as opposed to diastolic blood pressure, which is the amount of pressure exerted against those arterial walls during left ventricular relaxation, or diastole. Blood pressure in the body is regulated by the autonomic nervous system. So the autonomous nervous system will regulate blood pressure based on input it receives from different receptors. So this includes baroreceptors, chemoreceptors, and stretch receptors as well in the blood vessels near the heart.

All right. Time for a quiz. I have three questions for you. First question. How do you calculate cardiac output? You calculate cardiac output by taking the heart rate times the stroke volume. Question number two. What do you call the volume of blood in the ventricles at the end of diastole? If you said preload, you are correct. Third question. The percentage of blood leaving the left ventricle each time it contracts is called what? If you said left ventricular ejection fraction, you're right. I hope this review has been helpful. If so, be sure to like this video. Leave me a comment. And if you haven't already done so, be sure to subscribe. In my next video, we will talk about some A&P concepts as it relates to the hematologic system. So definitely stay tuned for that.


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