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Often, one of the first things we get checked for at the clinic is our blood pressure. What do the values reflect? When we measure using the mean arterial pressure formula, what is actually being measured? Watch and listen to learn more.
Hello and welcome to another episode of Interactive-Biology TV where we’re making Biology fun! My name is Leslie Samuel. In this episode, Episode 54, I’m going to be talking about Blood Pressure and Mean Arterial Pressure.
When you go to the doctor’s office, one of the first things they do is take your blood pressure. And, you know what? After today, you’re going to know exactly what they’re doing and what it means if you don’t already know. So, let’s get right into the topic for today.
Here, we have the heart. The heart plays a very important role in the cardiovascular or circulatory system. It’s pumping the blood throughout the body. The blood carries oxygen. It also carries nutrients to the muscles and other organs in order for you to live and carry out all your activities.
Here is the heart. Let’s take the heart and put it inside the human body. It serves its purpose of pumping the blood through these arteries to the rest of the body. Of course, the blood is coming back via these veins to the heart.
That process goes over and over. It’s also sending the blood to the lungs so that it can get the oxygen that it needs. It then sends that to the body and so on. We’ve kind of spoken about that in previous episodes.
Understanding Blood Pressure
We want to talk about blood pressure. First, I’m going to define blood pressure and I’m going to do it simply by writing here.
We have the blood here and the blood vessels. As the heart is beating and it’s sending that blood out to the body, it’s going via these blood vessels. Because it’s being pumped, that is going to exert pressure on the blood vessels. We’re going to call this pressure hydrostatic pressure. The reason we call it hydrostatic pressure is because blood is a fluid. When fluids exert pressure on something, that is called, hydrostatic pressure.
The blood is being pumped. It’s going through these blood vessels. It’s hitting against the walls of the blood vessels, the inner lining of the blood vessels. That is exerting pressure on those blood vessels. This is what we mean when we say blood pressure.
Blood Pressure Readings – Systolic and Diastolic Pressures
When the doctor or nurse is taking your blood pressure, they are checking to see how much pressure is exerted by the blood on the blood vessels. That is a very important measure when it comes to the health of your body.
When the blood leaves the heart, as we’ve shown before, the blood then goes into the aorta. The aorta is this vessel that’s leaving from the heart, and then, that goes down here. This is called the descending aorta and it goes via these other blood vessels to the rest of the body.
It would make sense to understand that you’re going to feel more pressure as you get closer to the heart. If you are right by the heart, you’re going to feel more pressure than if you are all the way down here in the toes, right? That is because this is where the heart is beating, and the farther away you go from there, the lower the pressure is going to be.
Let’s look at how this works. What I’m going to do is I’m going to draw a little graph here. Here, we have the y-axis and then, here, we are going to draw the x-axis.
Let’s chart the blood pressure as the blood flows. Blood is going to leave the heart via the aorta and goes to the smaller vessels called arteries. From there, it’s going to pass through even smaller arterioles and then, to the capillaries.
That is where it actually crosses over from being in the arteries section, arteries, and arterioles, to where it’s going into the veins. But, before it goes to the veins, it’s going to go via the venules and then, the veins. When it’s in the venules and the veins, it’s because it’s going back to the heart eventually via the vena cava.
We’re going away from the heart via the aorta and then, we go to the arteries. A very easy way of remembering this is the ‘a’ in ‘arteries,’ ‘arterioles,’ and ‘aorta’ is going ‘away’ from the heart. When it reaches the tissues and the organs, it’s going to have an exchange in the capillaries where it then goes into the venules, to the veins, and then, back via the vena cava to the heart.
Let’s look at the pressure on the y-axis. I’m going to give some values of 20, 40, 60, 80, 100, 120, and 140. On the y-axis, we’re dealing with pressure in millimeters of mercury (mm Hg).
When the heart contracts, we have ventricular contraction and that’s when we’re going to get the greatest amount of pressure because the ventricles are larger than the atria. When they contract, sending the blood to the rest of the body, that’s going to give you the systolic pressure, which is going to be the greatest pressure.
We’re going to see here, (let me draw this in… Let’s go with red), the ventricles are contracting so the pressure is going to increase significantly, and then, as the atria contract, you’re going to get that little bump there. That process continues as the heart beats. We’re in the arteries.
As we go away from the arteries and into the arterioles, what you’re going to see, we’re getting away from the heart a little bit, the pressure is going to start kind of going down, going down. As you go away, you’re going to see smaller fluctuations. You’re not getting as great of an effect.
As we reach into the capillaries, it’s kind of dying down even more and the pressure is going to continue going down and down until, on the way back to the heart, there’s hardly any pressure remaining. I mean, in comparison to up here, where we had pressures of up to 120 mm Hg, or sometimes even more in this situation, in comparison, the farther away we get from the heart and as the blood is being pushed back to the heart, we don’t get these fluctuations in pressure and there’s significantly less pressure as the blood is going back to the heart.
When the doctor takes your pressure and the doctor says, “You are in excellent health. Your blood pressure is great,” what are the numbers that you usually hear? The numbers that you usually hear are 120 over 80 (120/80). What that refers to, of course at the top, we have the systolic pressure and here we have the diastolic pressure – systolic pressure and diastolic pressure.
Systolic is during systole, contraction so, that’s the higher point. Diastolic is during relaxation where we have a lower point. If you have that 120/80, you are a happy camper. All is well with the world, at least where your blood pressure is concerned. That is what we want to have.
Calculating Mean Arterial Pressure
Now, let’s talk a little bit about ‘mean arterial pressure.’ (M.A.P. for short). Mean arterial pressure, when you hear the word mean, you always think average. The mean arterial pressure is basically the average pressure in the arteries. We’re not looking at the fluctuations. We are looking at the average.
Let’s take the average and plot a second line. It would look something like this.– a straight line here and as it goes down, it’s going to look a little like this until where we have a straight line here. It follows that straight line. That gives us the average pressure in the arteries.
Two Ways to Calculate Mean Arterial Pressure (Mean Arterial Pressure Formulas)
1. Calculating Mean Arterial Pressure using Cardiac Output (CO)
There’s a formula that we use to calculate mean arterial pressure. Mean arterial pressure, M.A.P., is going to be equal to CO times PR.
M.A.P. = CO x PR
Now, two of these you know already: M.A.P., mean arterial pressure; and CO, you should know that that is cardiac output. PR is one that we haven’t covered. PR is ‘peripheral resistance.’ As I said, M.A.P. = CO x PR.
We’re going to go more into peripheral resistance in the next episode so, I’m not going to deal too much with this. The main thing that you want to know is the two factors that are going to influence mean arterial pressure are: cardiac output and peripheral resistance.
Peripheral resistance is basically, we’re going to define that as, opposition to blood flow. The blood is flowing but, of course, there’s going to be some resistance, there’s going to be friction between the blood and the walls of the blood vessels, and so forth.
2. Calculating Mean Arterial Pressure from Measured Systolic and Diastolic Blood Pressure Values
There’s another way of calculating mean arterial pressure. Mean arterial pressure is also equal to diastolic blood pressure, so, that’s during diastole, relaxation, plus 1/3 times systolic blood pressure minus diastolic blood pressure.
M.A.P. = diastolic BP + 1/3 (systolic BP – diastolic BP)
Once again, if you’re looking at that graph where we had something kind of looking like this, in the beginning, and if this is 80 and this is 120, the mean arterial pressure is going to be equal to 80 plus 1/3 of 120 minus 80, is 40, and that’s going to be equal to 80 plus let’s see, 1/3 of 40, let’s go with 13.3, I’m just going to leave that at 13, so, mean arterial pressure would be equal to 93 mm Hg.
M.A.P. = 80 + 1/3 (40) = 80 + 13 = 93
That’s Mean Arterial Pressure.
Two ways to calculate it (Mean Arterial Pressure): (1) Cardiac output times peripheral resistance. We’ve dealt with cardiac output, that’s the amount of blood pumped by the heart every minute and we’re multiplying that by peripheral resistance which is the opposition to blood flow (in Episode 55, we’re going to go a little more into peripheral resistance).
Or, (2) we can take the diastolic blood pressure plus one-third of the difference between the systolic and diastolic blood pressures which in this case would be 80 plus a third of 40 which would be approximately 13, which gives us 93 mm Hg:
M.A.P. = 80 + 1/3 (40) = 80 + 13 = 93 mm Hg
That’s pretty much it for this episode. Of course, if you want to check out some more Biology videos and other resources like quizzes and the community that we have at Interactive-Biology, you can check out the website at Interactive-Biology.com.
That’s all for now, and I’ll see you on the next one.