 Let's connect what we're talking about in this lecture to the clinical concept of blood pressure. And many of you have taken blood pressures before, blood pressure, we'll just singularly talk about blood pressure. We're going to connect the clinical relevance of blood pressure, which is we measure systolic pressure over diastolic, right? Stolic. And, you know, we expect 120 over 80 is our, what, average blood pressure, although there's a significant amount of debate about this and a lot of evidence that perhaps our average values should be lower, like your blood pressure should be lower than this. Blood pressure is measured in millimeters of mercury and that's, we'll just accept that those are the units that we often will deal with when we talk about blood pressure and lots of other pressures that we're going to deal with. Okay, so systolic pressure is an estimate of the amount of pressure in the aorta measuring what? Ventricular systole. Remember, the aorta is the giant artery that's exiting the left ventricle of the heart and heading to the body. So it's a big artery, biggest artery in your body. And since it's so close to the left ventricle, which is the strongest chamber of your heart, it's going to be potentially experiencing a lot of pressure. Systolic pressure is the amount of pressure, the estimate of the amount of pressure in the aorta during ventricular systole when the ventricles are contracting. Diastolic pressure is the amount of pressure in the aorta during ventricular diastole. So you might be thinking, systole, your brain is thinking, wait a minute. In the ventricle during ventricular diastole, the pressure almost goes down to zero. But notice that in the aorta, the pressure isn't at zero. It's at 80. Why? It's because the aorta is super elastic. And when that huge push of pressure during ventricular systole pushes a giant blob into the aorta, the aorta expands in an elastic manner. And the pressure upon the expansion is 120 millimeters of mercury. And then the heart relaxes. When the heart relaxes, the ventricles go to about zero pressure. But the aorta then goes back to its original diameter. It elastically springs back to the original diameter and that provides more pressure to push the blood down the line. So that's why your diastolic pressure doesn't drop down to zero. Let's talk about how you take someone's blood pressure. Blood pressure is measured. I'm leaving my, whatever that is, web address up there so that you can see it. And the pressure is taken with a sphigmomanometer. What? How awesome is that word? And you actually cut off blood flow through the brachial artery. So you'll notice here in the arm, you put the cuff around the arm, not the forearm, the arm, and then you pump up the pressure in this cuff and you use your little column thing here to tell you how much pressure you have. But your goal is to cut off blood supply through the brachial artery. If you listen to the brachial artery, you can hear a pulse through that. Your goal is to eliminate the pulse, put the pressure up so high through the cuff that you cut off the blood supply. Now think about this for a second. How high is the pressure going to be if you cut off blood supply? It just has to be higher than the pressure of the heart pushing the blood out. Does that work for you? If the heart were pushing harder, if there was more pressure being applied to the blood by the heart, then you'd have to pump the cuff up even more to close off the blood vessel. Does that make sense? So now you pump up your cuff, you close off the blood vessel. Look, there it is. It's totally closed off. The brachial artery pinched off. Don't leave it like that. Then you slowly release the pressure. And your goal is to listen with your stethoscope for the first sounds you hear. Interestingly, the minute you hear sounds, that's because blood is coming back through. Now, the blood is actually turbulent, which means it's like flowing like the rapids in a river. And so you let the blood start to come through and it's going to be all turbulent and crazy. And that turbulence is what you're hearing. They're called carotcoff sounds. And that's what you hear. The second you hear that sound, you look at the pressure and you know that that's your systolic pressure. The heart is applying more pressure on what your cuff can prevent from coming through. The blood is coming through now. You mark that as systolic pressure. You keep listening. And when this carotcoff sounds go away and you can't hear any more sounds, that's your diastolic pressure. That means that the artery is fully open and now the blood is just flowing through normally. And that's your estimation of diastolic pressure in the aorta during ventricular diastole. Did you follow that? Of course you did. I hope that in lab we take blood pressure. There's one concept dealing with blood pressure that's clinically significant and that's mean arterial pressure and I want to talk about that next.