056 Regulating Peripheral Resistance – Part 2

Here is the second part of Regulating Peripheral Resistance. Leslie explains two more ways on how it can be influenced and how it affects someone’s blood pressure and mean arterial pressure. Watch to learn more!

Have fun!

Transcript of Today’s Episode

Hello and welcome to yet another episode of Interactive-Biology TV where we’re making Biology fun! My name is Leslie Samuel and in this episode, Episode 56, I’m going to continue talking about, ‘Regulating Peripheral Resistance.’ This is part 2, and I think this is going to be the final part about this. So, let’s get directly into the content for today.

In the last episode, we emphasized, we re-emphasized the fact that mean arterial pressure is equal to cardiac output times peripheral resistance (M.A.P. = CO x PR). We’ve spoken about the fact that we are modifying peripheral resistance. We’re looking at the different ways in which peripheral resistance is influenced. In the last episode, you can go back to Episode 55, we spoke about vasoconstriction and we said that that is going to cause an increase in peripheral resistance. We spoke about vasodilation which is going to cause a decrease in peripheral resistance.

We’re going to talk about two other ways in which we can influence peripheral resistance. The first way that we are going to talk about today is called, blood viscosity. By viscosity what I mean is basically the thickness of the blood. This is very logical.

For example, a few weeks ago I was in Colombia and we remember we went to a restaurant and I ordered a mango milk shake. The milk shake was very, very, very thick. I was sucking on the straw trying to get it out and it was really hard to get that mango, I mean it was a very good tasting mango milkshake but, it was hard to get it in my mouth because of how thick it was.

This is the same thing. The thicker the blood is, the more resistance we’re going to have to blood flow. If we increase blood viscosity, we’re going to increase peripheral resistance significantly. By the viscosity, specifically, I am talking about the ratio of RBCs (red blood cells) to the blood plasma:

RBCs : plasma

By plasma, we’re basically talking about the fluid. If we have more red blood cells, or we increase the ratio of red blood cells to plasma, we are increasing the thickness of the blood. So, the overall message is, and let me just divide this in two, if we increase blood viscosity, that of course is going to result in an increase of peripheral resistance. On the other hand, if we (let’s use a different color), decrease blood viscosity, that is going to cause a decrease in peripheral resistance.

What is an example of a way we can increase blood viscosity? Well, for example if we are dehydrated. What that’s going to do is it’s going to reduce the amount fluid in the blood, so the plasma is going to be less. That is going to cause an increase ratio of red blood cells to the plasma, we’re going to have an increase in blood viscosity, and that’s going to cause an increase in peripheral resistance.

What can cause decrease in blood viscosity? For example, loss of blood volume due to anemia or if there’s hemorrhage, that’s another example (forgive my R’s… My students always make fun of me for my R’s). If there’s anemia or hemorrhage, that’s going to cause a decrease in blood viscosity causing a decrease in peripheral resistance. So, the first that we’re looking at today is by influencing blood viscosity.

The second way is by looking at the total blood vessel length. The message here is, the longer the blood vessels, the higher is the peripheral resistance. So, if you increase the blood vessel length, you are going to naturally increase peripheral resistance. That should also make sense. If something is much longer, you have a tube that’s very long, it’s going to be much harder to get the blood through. If my straw for my mango shake was extremely long, let’s say that straw was two-feet long, that would take a lot of work for me to get that great tasting mango shake into my mouth because it’s longer, increase in peripheral resistance.

Now, how would this translate to the human being? I’ll give you a very good example in America and other places also. If someone is overweight, what that’s going to do is that’s going to naturally increase blood vessel length. I’m going to give you some numbers right now that can be very disturbing. Or, it can’t be very disturbing depending on how you look at it. If you gain 2.2 pounds of weight of additional fat, that is going to add approximately, and this is very scary, 400 miles of blood vessels. That’s one kilogram of fat and approximately 650 kilometers of blood vessels. So, you can see, by gaining weight, you’re gaining more blood vessels. That’s basically increasing the blood vessel length, and that is going to increase peripheral resistance. And we know what increase in peripheral resistance will do to mean arterial pressure and to blood pressure because, we keep coming back to this:

M.A.P = CO x PR

More fat, longer blood vessels, increased peripheral resistance, and that is going to cause an increase in mean arterial pressure.

I guess, the take home message for today is, watch your weight.

That’s pretty much it for this episode. As usual, I want to invite you to check out the website at Interactive-Biology.com for more Biology videos and other resources. You can join the community over there, ask your questions in the forums, and just take part in everything that we have going there. That’s it for this video and I’ll see you in the next one.



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  • ooooh I always wondered why McDonalds milkshakes were impossible to drink!! I havent had one since I was about 11 cos they are too thick!! It also explains how in the cool hawiian bar here, with the really long straws for their crazy volcano themed cocktails… its reallly difficult to drink your cocktail… but I didnt stop going there of course!! I once again feel enlightened after watching your videos… and its given me an extra incentive to resist the icecream this summer too !!

  • ooooh I always wondered why McDonalds milkshakes were impossible to drink!! I havent had one since I was about 11 cos they are too thick!! It also explains how in the cool hawiian bar here, with the really long straws for their crazy volcano themed cocktails… its reallly difficult to drink your cocktail… but I didnt stop going there of course!! I once again feel enlightened after watching your videos… and its given me an extra incentive to resist the icecream this summer too !!

  • All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  • Ok, so if blood viscosity increses with dehydration, that will increase the PR and also the MAP. Now i remember learning that with dehydration, BP decreases because of hypovolemia… right??

  • @InteractiveBiology THANK YOU!!! for the great community service, your time, efforts and talent for teaching is much appreciated!!!

  • THANK YOU!!! for the great community service, your time, efforts and talent for teaching is much appreciated!!!

  • Id really appreciate it if you could make videos about pulmonary oedemas and cardiac arrhythmias I really need help understanding those concepts

    You seem to make concepts very clear and I really enjoy the way you explain things

  • Id really appreciate it if you could make videos about pulmonary oedemas and cardiac arrhythmias I really need help understanding those concepts

    You seem to make concepts very clear and I really enjoy the way you explain things

  • “it was hard to get it in my mouth because of how THICK!! it was…” haha

  • 7:45 “increase PR increase MAP”. if PR goes up for dehydration MAP goes up.

  • This video really helped with some of the exam questions. There was question saying that PR increases __when viscosity increases (I chose this) I thought of this video and got it. I also remembered how to calculate MAP ’cause of this video. Thanks a lot! I got myself 6 points because of this. I would probably have chosen different answers. I think I got around 80-82 for this exam. My teacher said it is the hardest exams ever. I really hope I got around 80-82

  • Since some of your video helped with exam 2, I am going to be watching your videos from now on 🙂 They are very helpful! We’re doing blood and Lymph system next.

  • This video did not load from the interactive website. Possibly because the URL was ht tp”v”…
    I removed the “v”, and was able to view it here on you tube. You rock. Thanks for the free schooling.

  • Hi Samuel!
    first, thank you for good explanations and videos
    I cannot open Peripheral resistance part 2, neither here or on youtube,

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