What three factors regulate stroke volume to ensure that the left...

Question

What three factors regulate stroke volume to ensure that the left and right ventricles pump equal volumes of blood?

Accepted Answer

Welcome back, everyone. Let's look at our next question. It says, what does afterload refer to in relation to the stroke volume index? A the degree of stretching in ventricular muscle cells during ventricular diastole B, the amount of force produced during a contraction at a given preload. See the pressure against which the heart must work to eject blood during Sicily or d the amount of blood in the ventricle after contraction. So first let's recall what stroke volume index is and this is a measure of the heart's output. Specifically, it's the volume of blood pumped by the heart with each beat divided by the body's surface area and units of milliliters per square meter. So we're talking about how much blood the heart can manage to pump out with each beat. And looking at specifically the concept of afterload in terms of that ability to pump out blood and how much it can. And the answer here is choice c the pressure against which the heart must work to eject blood during systole. So we're talking about the ejection of blood, how much blood is coming out and there's a pressure opposing that, that we refer to as the afterload and of course, the amount of pressure opposing that ejection is going to affect how much gets pumped out. So let me give a quick diagram here in this case, just to make things look a little simpler as we think about this, this is just one side, the left side of the heart. Here's the left atrium up top of the diagram with the left ventricle, down below the A V valve between them is closed at this point. And on the side, next to the left atrium, because on the left side is the aorta. Now we're going to think about in this drawing right at this moment, the semi lunar valve is open, but we're going to start by thinking about at the very beginning of Sicily. So the ventricle has begun contracting that has forced the A V valve closed due to pressure on the blood within that ventricle. That pressure is being forced up against the A V valve, pushing it closed and it's also being forced against the semi lunar valve. Now opposing it is the pressure of arterial blood which would be pushing down essentially just by the force of gravity. So after load, that amount of force that needs to be overcome. For heart to be pumped out of the blood is mainly going to be dependent on arterial pressure. The pressure of the blood in the artery leading away from the heart into which this blood is going to be pushed out. Of course, as this ventricular contraction continues, the pressure will build and build until it is great enough to open up the semi Lunar valve and push blood out into the aorta and be pumped out into the body. So we'll change that on the diagram. So the semi lunar valve is opened and blood is flowing up and out of the heart. But again, there was that level of pressure that was there that had to be opposed by the strength of the ventricular contraction in order to eject blood during systole. So let's look at our other answer choices to understand why they are not correct choice. A the degree of stretching in ventricular muscle cells during ventricular diastole. So at this point, we're not talking about the ventricle contracting, we're talking about as it's relaxed and it's filling. So at this point, the ventricle is filling with blood from the atrium and as it fills with blood, the ventricle is being stretched. This is the preload, not the afterload. It's essentially the filling pressure of the ventricle and is determined by the volume of blood that fills it. So that's not our correct answer. The wrong load. Choice B says the amount of force produced during the contraction at a given preload. So you've got that pressure of blood within the ventricle and the ventricle has to contract and exert pressure on that blood to force it out. Well, that would be the contractility. And that is essentially the strength of the contraction of the heart muscles during systole. So connected to the content to the concept of afterload. In that this is going to determine how well the blood is going to be pumped against that afterload, determined by the strength of that contraction, but it's not the afterload itself. So choice B not correct. And then finally choice d the amount of blood in the ventricle after contraction. So this would occur after the afterload has taken effect. So the blood has been pumped out at the end of the contraction, ventricular pressure drops. And now we see that afterload come into play in that the blood begins flowing back down the aorta back towards the ventricles, shutting the sent of my lunar valve. But you still have some blood remaining in the ventricle at that point. And that is called end stroke volume. So it is not the afterload. Although the afterload affects it because that's the pressure that will be causing the blood to flow back into the ventricle and blood to be remaining there after the contraction, this is the lowest volume ever in the heart during its whole cycle. And it is determined by afterload and contractility, determined by afterload because that will determine how much blood flows back into the heart and contractility. How strong that contraction was to push ventricular blood out, that will help determine how much gets left behind after the contraction. But again, it's not after load. So not a correct answer. So what does afterload refer to to see the pressure against which the heart muscle must work to eject blood during Sicily. See you in the next video.

Struggling with Anatomy & Physiology?

What three factors regulate stroke volume to ensure that the left and right ventricles pump equal volumes of blood?

Watch next

Cardiac Cycle practice set