As promised, here is a video focusing on peripheral resistance. Understand what it is and how it affects mean arterial pressure by watching along and listening as Leslie once again explains the concept with full clarity.
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 in this episode, Episode 55, I’m going to be talking about, ‘Regulating Peripheral Resistance,’ and this is going to be part 1. Initially, I was going to make one video about peripheral resistance, but, then I started going into it and I decided to split it up into at least two parts. So, this is going to be the first part. It might end up being two parts, it might end up being three parts. We’ll see how it all turns out.
We’ve been talking about the cardiovascular system or the circulatory system. We have spoken a lot about the heart and the blood vessels that lead from the heart and to the heart. We said that when the heart beats, and let’s say the ventricles contract, that sends the blood, if it’s the left ventricle, it sends the blood into the aorta which then sends the blood into the rest of the body. It’s going to the organs, and to the tissues. It’s taking oxygen and nutrients to the organs and the muscles, and so on. And, of course, it’s bringing waste away from the muscles and organs also.
What we’re going to do today, we’ve been talking about cardiac output, we’ve been talking about the mean arterial pressure and in the last episode, we focused on mean arterial pressure and we said that:
M.A.P. (mean arterial pressure) = CO x PR
This is one of the formulas that we use for calculating mean arterial pressure. Just as a reminder, the other one is:
M.A.P. = Diastolic pressure + 1/3 (systolic pressure – diastolic pressure)
You can go back to Episode 54 for more of an explanation on this two. We’re not going to focus on this guy right here. We are focusing on this indirectly. Why? Because today, we’re going to talk about peripheral resistance. We already defined what peripheral resistance is. Peripheral resistance is basically opposition to blood flow. Of course, you have the heart that’s beating and sending the blood through these blood vessels. But, of course, it’s not a frictionless environment. There’s going to be friction between the blood and the walls of the blood vessels that is going to cause resistance. If something is trying to get through a tube, there is resistance. This is exactly what we have here, the blood is trying to get through many tubes all throughout the body. Of course, that is going to encounter resistance. Just to give you an idea, if you were to take all of the blood vessels out of your body and just make it in one long line, it would be long enough to wrap around the globe twice. So, we have a significant amount of blood vessels going through the body. That is what peripheral resistance is.
What we are going to do is we are going to talk about how we can regulate peripheral resistance. So, I said, we have the heart and that sends the blood to the rest of the body. There are a number of different types of vessels that we can encounter. We have, of course, the aorta, and we’ve looked at this. From the aorta, it’s going to go to the arteries. From the arteries, it’s going to go to arterioles, and from the arterioles, it’s going to go to the capillaries, and then, from the capillaries, (let’s use a different color here), that’s going to take us to the venules, to the veins, and then, via the vena cava, and then, that is going to go back to the heart. That cycle continues.
When it comes to peripheral resistance, the place that we’re going to focus on will be the arterioles. The aorta and the arteries are relatively thick. Yes, they are flexible but, we don’t have much in terms of changing the diameter of these guys. The arterioles has a smooth muscle layer, a significant smooth muscle layer, that we can constrict or we can dilate. If we constrict it, the diameter is going to be smaller. If that’s the case, that is going to increase peripheral resistance. If we expand it, if we relax the muscles, we dilate the muscles, the arterioles are going to expand, and, of course that is going to decrease peripheral resistance. It’s really simple. It is harder to get something through a very narrow tube than it is to get something through a thicker tube. So, by constricting the arterioles, that is going to increase the peripheral resistance; by dilating the arterioles, that is going to decrease peripheral resistance. Just to give you a visual here. Here we have an artery. We’re delivering blood to a specific tissue whatever that is. So, we have some tissue cells here. From the arteries, it goes via arterioles and then, we have the capillaries going to the venule and then, to the vein, and as I said before, then back to the vena cava and back to the heart. The magic is happening right here in the arterioles. If we have some constriction, that is going to increase peripheral resistance. If we have dilation, that’s going to decrease peripheral resistance.
Now that we know that, let’s talk about some specific instances and some specific ways in which we can have these kinds of effect.
We’re going to look at two hormones. We’re going to look at epinephrine which is another name for adrenaline, and you’ve heard a lot about adrenaline. And, we’re also going to look at norepinephrine. So, these are the two that we’re going to look at today.
First, we’re going to focus on epinephrine. The interesting thing about these things, I’m going through one example of one pathway for each but, epinephrine, for example, in certain instances, it can be a vasoconstrictor. In other instances, it can be a vasodilator. What we’re going to do, the typical one that you hear about is the vasoconstriction. I’m going to talk a little bit about how this causes vasodilatation.
Epinephrine binds to what we call, beta-2 receptors and these are on the smooth muscles of the arterioles and then, what that’s going to do is it’s going to activate a G-protein which then is going to activate an enzyme called adenylate cyclase. What this enzyme is going to do, it’s going to take ATP, adenosine triphosphate, which is the energy currency of the body and convert that into a different form. That’s going to be a cyclic AMP. What that’s going to do is it’s going to activate calcium pumps in the cisternae of the smooth muscle cells. That is going to cause the calcium to be pumped back into the cisternae which is going to decrease calcium level. So, we’re going to get calcium decrease. If you remember from one of the earlier episodes, calcium in the muscle causes muscle contraction. If we’re decreasing the calcium, we’re not going to get contraction which would cause constriction. We’re going to get vasodilation. When we have vasodilation, of course, what that’s going to do is it’s going to decrease peripheral resistance because we have a wider tube, blood can flow much easier. So, that’s going to cause a decrease in peripheral resistance. This is one example of one way in which epinephrine can affect peripheral resistance.
Let’s talk about norepinephrine. (Let’s come over here). Norepinephrine is going to activate a different type of receptor. We’re going to call those alpha-receptors. That is going to activate a G-protein, once again. However, in this case, the G-protein is not going to activate adenylate cyclase. It’s going to activate phosphatidylinositol and then, that is going to activate the molecule IP3. That is going to do the exact opposite. That is going to cause calcium release from the cisternae. Of course, now, we have calcium release that is going to cause contraction which in this case would be vasoconstriction. Of course, then, the effect of that is going to be to increase peripheral resistance.
We’ve been looking at the formula: M.A.P. (mean arterial pressure) = CO x PR. In these situations, we are affecting PR, which is peripheral resistance and if you increase peripheral resistance, which would be the effect of norepinephrine in this specific situation, that is going to cause an increase in the mean arterial pressure. If you decrease peripheral resistance as in this situation, that is going to cause a decrease in mean arterial pressure.
So, by influencing vasodilation or vasoconstriction, what we’re doing is we are making the diameter of the blood vessels smaller or larger. That is going to influence peripheral resistance. If it’s smaller, we have more resistance. It’s harder to get stuff through. If it’s larger, it’s going to be easier to get stuff through because there’s less resistance to flow.
There’s one more thing that I want to add to this. Here we have an example of atherosclerosis and that is where we have plaque buildup in the arteries. So, here you can see the plaque, and that is building up in the arteries. What that’s going to do is, it’s going to cause for us to have a narrowed arteries. So, the diameter is going to be significantly smaller. This can happen as a result of having too much cholesterol in our diets. That’s just an example but, that is going to cause a smaller opening. Of course, if you have a smaller opening, what that’s going to do is, it’s going to increase peripheral resistance. If you increase in peripheral resistance, and we know that:
M.A.P. = CO x PR,
You increase this over here, that is going to increase the mean arterial pressure. So, you’re basically increasing the blood pressure when there is plaque in the arteries. You’re doing that by increasing peripheral resistance.
Take home message is: Watch your diet, exercise, live a healthy lifestyle, so that this doesn’t happen, so that this doesn’t go up and so that your blood pressure, your mean arterial pressure does not go up. That’s the health nugget for this lesson.
That’s pretty much it for this episode. As usual, you can visit the website at Interactive-Biology.com for more Biology videos and other resources that we’re adding over there. This is Leslie Samuel. Thank you for watching and I’ll see you on the next one.