Transcription
[Music] I've run dr. Mike here. In this video, we're talking about blood pressure, but more specifically, we're talking about one part of the blood pressure equation which is really important, something called systemic vascular resistance, also known as total peripheral resistance. I know, it sounds like a mouthful and doesn't seem to make sense from the beginning, but let's have a quick look.
When we look at blood pressure, blood pressure is the amount of force exerted on the walls of the blood vessel when the blood is moving through. So, for example, this is the left-hand side of the heart, which delivers blood to the whole body. As you can see, the heart, the brain, the GI tract, the renal, the muscle, the skin, and multiple other aspects. Now, in order for the blood to get from this left ventricle or chamber, it needs to travel through these blood vessels to get there, and it does so when that left ventricle contracts. Now, when it contracts, it squeezes blood, which obviously pushes blood. And as that blood pushes through, it's gonna put pressure on the walls of these vessels, and that simply is blood pressure.
Now, we know here from the left ventricle to the aorta that the blood pressure is going to be around about 120 millimeters of mercury under its strongest contraction. Okay, that's pressure. Now, this blood pressure, we can gather from an equation which involves cardiac output, which is the amount of blood we eject every minute, and multiply that by what we're talking about today, the systemic vascular resistance.
Now let's have a look what systemic vascular resistance actually is. When this blood moves through from the aorta, the large elastic vessels, and then goes to all the areas where it branches off to different tissues of the body, these smaller arteries we term arterioles. These are the arterioles here. And something I've told you about in the past is that arterioles have huge amounts of smooth muscle in the inside layer of their walls. So that means all of these arterioles have smooth muscle around them. All right.
Now, what does that mean? Well, it means that muscle can contract. And if you've got muscle lining a tube and it contracts, it narrows the hollow inside of that tube, which means less blood can move through. Now, an interesting thing is when we talk about pipes and fluid movement, especially in the bloodstream, if I have a pipe of a particular length and then I have the size of that pipe in regards to having the diameter of that pipe, you would think that we have now have the amount of blood that can go through. But in actual fact, it is to the power of four. So that means 1 over 2 times 2 times 2 times 2. That actually means 1/16. That means instead of having the amount of blood when you have the diameter of the vessel, you decrease the amount to 1/16. Only 1/16 of the amount of blood gets through. So that means when I constrict any of these particular arterioles, I significantly alter how much blood goes past going to the various tissues of the body to feed it.
Right. Now let's think of an actual scenario that happens in our body. You are walking in the park late at night, you hear a rustle in the bush, and then all of a sudden a big dog jumps out, starts barking at you. What happens? You get scared. You activate that fight or flight system, that we turn the sympathetic nervous system. Now, when you activate the sympathetic nervous system and you get scared, you can feel your heart beat harder, stronger, faster. But also, if you looked into a mirror, you look white. Now, why do you look white when you get scared? Because your sympathetic nervous system, which is the home of adrenaline, gets released into your body. And this adrenaline goes to the arterioles of the skin, which has lots of muscle, and it tells it to constrict. The blood vessels feeding your skin can constrict.
Now, 5% of all the blood that exits your left ventricle, 5% goes to the skin. Which means when they constrict, that blood doesn't go to your skin, so you turn pale and backs up into the system. Another thing that happens is your GI tract, your gastrointestinal tract. When you're scared, you don't need to use that. So the muscles there constrict as well in the arterioles. And 25% of your cardiac output, all blood that comes out of your left ventricle every minute, goes to your GI tract. So now 25 plus 5% is 30% of your blood volume gets shunted back into this systemic system. Now, again, when you put your thumb on the end of a hose, this is what you're effectively doing here. The pressure backs up in, increases. So it backs up, backs up, backs up, increases. And what have we now effectively done? Simply by changing the diameter of these tubes, we've increased blood pressure. So systemic vascular resistance, also known as total peripheral resistance, if you decrease the diameter of it, you're going to increase the resistance. You increase the resistance, you increase blood pressure.
Now, this is diameter. But you can also change systemic vascular resistance by changing the length of the tube, the length of the blood vessel. Okay. Now think about this. You have a 10-meter long garden hose at home, and you turn it on. You see at the other end water squirting out at a particular pressure. Now, if you were to keep that tap turned on at the same pressure, yet you lengthen that tube to a kilometer, what do you think the pressure would be on the other end of that tube? Would it be coming out at the same pressure as the 10-meter tube? No, it wouldn't. The pressure would be decreased because the longer a blood vessel is, the more the blood gets stopped by it as it moves through. Right. So there's more resistance. Therefore, what you get is a problem. That means in order to think about this, if I had, instead of a 10-kilometer long blood vessel, I had a 1-million kilometer long blood vessel, it means that at the other end of that vessel, the pressure is going to be lower with my heart just pumping normally. So my heart would need to respond by pumping harder just to get the same pressure on the other end, just to get these tissues fed.
When's a clinical example that this is going to happen? In individuals who are obese. The larger your body mass, the longer your blood vessels. For every kilogram of fat that you put on your body, you need to generate at least a thousand kilometers of blood vessels. Which means the heart, in response, needs to pump harder just to get the same amount of pressure getting to your tissues. That means there's more strain on your heart over time. That means increased likelihood of cardiovascular disease. All right. So that's length. But you can't change length in the short term. You can in them, you can change diameter in the short term through the sympathetic nervous system. Can't change length in the short term. That's over time, increasing body size.
And viscosity is the last way you can alter systemic vascular resistance. Viscosity is how many things are in your blood, how thick is your blood. Now, how do you change this? Well, you can change this with the number of red blood cells. Okay. So some people can have polycythemia, which is too many red blood cells. That makes your blood thicker, which means if the blood's thicker, it's harder to move through and it increases systemic vascular resistance. Or you can have anemia, and it's thinner, and it's gonna affect systemic vascular resistance. Or you could blood dope with erythropoietin, EPO, as some cyclists have done in the past, and that will also make your blood more viscous and again increase systemic vascular resistance.
All right. So, and when it comes to blood pressure, your blood vessels themselves, diameter, like the viscosity, have a very important role in altering that blood pressure. Now, another way that you can alter the diameter of your blood vessel. So I said short-term constriction. In the long term, you can see your kidneys require 20% of your cardiac output. Really important. If that drops for any particular reason, your kidneys actually respond by releasing a hormone called renin, which then stimulates the release of another hormone called angiotensin II. And that tells the skin to constrict, or at least tells the blood vessels to the skin to constrict, and it backs the blood up, increasing pressure again, and saying, "Hey, skin doesn't need it, I need it," and it backs up. That's the renin-angiotensin-aldosterone system, and I'll do an entire video just on that.