Greetings! So today we're gonna talk about the heart as a pump. we want to consider how it generates pressure and how by generating pressure it's able to move blood in a unidirectional manner. It distributes blood to the lung and it can also distribute blood to the systemic circulation. To do this we'll first consider the structure and the function of the heart chambers and their associated valves. Then secondly, we want to talk about what's called systole and diastole. Systole is the time when the heart is contracting, generating pressure, and ejectiing the blood. Diastole is when the heart muscle is relaxing, the valves open, and the heart fills with blood. The third, we want to explain the cardiac cycle, One systole and one diastole. are one cardiac cycle. That's one heartbeat. And, we want talk about the associated heart sounds. Those are those sounds that you hear from the stethoscope, the lub-dub. [SOUND] And then fourth, we want to talk about what's known as the pressure volume loops, stroke volume, and cardiac output. These are properties by which we tell how well the heart is functioning. They give us some assessment tools for seeing how well the heart is performing. So let's get started. The first thing is, as you recall the last time we met, We said that the heart has a right side and a left side. Both sides, the right and the left, have two chambers. The upper chamber is called atrium and the lower chamber ventricle. We have a right atrium and a right ventricle and we have a left atrium, which is shown here, and the left ventricle. Between the atria and the ventricles, at the base of the heart, there is the cardiac skeleton. Within this cardiac skeleton, there are valves. There are four valves. There are two atrio-ventricular valves, one on the right, and one on the left. Then we have two other valves, one called the pulmonary valve, pulmonic valve. This pulmonic valve allows blood to be move from the right ventricle to the lung, through the pulmonary artery. And the other valve is called the aortic valve. The aortic valve allows blood to move from the left ventricle out to the aorta, a large blood vessel which distributes blood to the rest of the body. The things to remember then is that we have these two chambers. The two chambers are act in a concerted manner. We have blood moving in a unidirectional manner from one side, the right side, to the left side. The amount of blood that moves from the right side to the lung is the same amount of blood that returns from the lung into the left ventricle. These two chambers on the right and left sides of the heart are in series. The amount of blood ejected from the right has to equal the amount which is ejected from the left. The other thing that we should keep in mind as that on the right side, the walls of the heart are fairly thin relative to the left side. This is because on the right side of the heart, we are not generating as high pressures. THis is because the organ system that's downstream,the lung, does not have a high resistance to flow. This is a low pressure system. So the right atrium generates about ten millimeters of mercury when it contracts. The right ventricle geerates 25 millimeters of mercury when it contracts. The left atrium generates 15 millimeters of mercury when it contracts. The left ventricle generates 120 millimeters of mercury when it contracts. Therefore, the left ventricle generates the most pressure within the entire system. It has the thickest walls. It is the chamber with the thickest walls. One other point to remember, is that the blood is moving from the ventricles out through the valves. The valves are located at the base of the heart. So when the heart is contracting, the blood moves in this direction. First, we have blood entering into the right chamber, into the right atrium. It then moves into the right ventricle, out through the pulmonic artery and into the lung. It goes out through the pulmonary valve, or pulmonic valve. Then it returns from the lung to the left atrium. When it enters into the left atrium it moves through the AV valves, that's the atrio-ventricular valve, and into the left ventricle. Then from the left ventricle, out through the aortic valve, into the aorta and out to the rest of the body. So unidirectional movement of blood through the system. The valves are interposed between the chambers to prevent back flow. This is what we see on this next slide. We have the four cardiac valves. They're to prevent back flow. We want to always have movement of blood going from atria to ventricle and from the right side of the heart going to the left side of the heart. The pulmonary and aortic valves prevent back flow from the arteries when the ventricles are relaxing and they have low pressure. The valves in general open and close due to changes in pressure across the valves. That's true for all of the valves. When there's a pressure difference between the two chambers, say for instance, the atrium has a higher pressure than the ventricle, then the AV valve opens. When the ventricle has a higher pressure than the atrium, then the AV valve closes. Okay, the same is true for the aortic valve and the pulmonic valve. The AV valves are the ones which experience the highest pressure. These are the valves which are closed when the ventricles are contracting. These valves have an unusual structure attached to them. The are chordae tendineae. That's shown here on this diagram. They look like strings in this particular diagram. But they are tendons which attach those valves to the papillary muscles of the ventricles. This is to prevent the valves from everting and allowing the blood to flow backwards from the ventricle into the atria as the ventricles are generating pressure. So unidirectional flow through the valves. The valves close and open depending upon a change in pressure across the valves. All right so let's consider our cardiac cycle. So as I said previously, the cardiac cycle consists of two portions. One is called systole. This occurs when the heart is filled with blood and it starts to contract to eject blood. This occurs because of the increase in pressure within the ventricle. The second portion of the cycle is called diastole. Diastole is when the heart relaxes. The muscle relaxes and now the heart fill again with blood. Diastole is actually longer in time than systole in a given cardiac cycle. One cardiac cycle has one systole and one diastole. Let's start the cardiac cycle where the ventricles are relaxed. The AV valves are open. The ventricles are filling passively. This is passive filling because the pressure that's in the atria is higher than the pressure in the ventricles. Blood is passively flowing into the ventricle. The ventricles fill. We now have an electrical event occurr. That is the P-wave. The P-wave simply is a depolarization of the atria. When that occurs, the atria contract. As the atrium contracts, the pressure in the atrium increases. That expels a last little bit of about 20 to 30% of the blood volume into the ventricle. The ventricle now has a higher pressure than the atrium and the valve closes. The atrio-ventricular valves close. So the AV valve closes. At this point in time, we have a second electrical event. That's called the QRS complex. The QRS, as you recall, the QRS complex, is when there is a depolarization of the ventricles and following depolarization of the ventricles, we'll have contraction of the ventricles. As the ventricles contract, they will increase pressure. Note all the valves are closed. So the blood that's within those chambers is isolated. As the chamber volume gets smaller, pressure increases on the liquid or the blood that's within the chamber. So isovolumic contractions simply means all valves are closed and the pressure is increasing. Pressure eventually increases to where the aortic valve and obviously the pulmonic valves open. Once that occurs, then the ventricles continue to contract. Pressure continues to increase and we have ejection of blood. Now as the the blood leaves the ventricle, the pressure within the ventricles drops. As it drops, then it becomes less than the pressure that's in the aorta or less than the pressure that's in the pulmonary artery. And at this point, the valves close. Then again, all valves are closed. but the pressure continues to fall. It is isovolumic because all the valves are closed. The volume that's within the heart chamber is isolated. On the electrocardiogram, we have the T wave occur. The T wave indicates relaxation, depolarization of the cardiac myocytes. Following depolarization, we have relaxation of the cardiac myocyte. And as they relax, the volume of the ventricle increases. Pressure within the ventricle will decrease and eventually the pressure within the ventricles is less than the pressure in the atria. The AV valves open. So during the time of isovolumic relaxation, all the valves are closed. The ventricle is relaxing, and the pressure is falling. There is a very complicated way of looking at the ECG changes, changes in the pressure within the left ventricle, and changes in volume within the left ventricle as well as the sounds that you hear. That's what's diagrammed on this particular chart. This chart is called the Wigger's diagram. So if you understand Wigger's diagram then you understand completely the cardiac cycle. Let's just go through it fairly rapidly. Note that this diagram depicts only the left ventricle. I'll go through it very quickly. Then do come back and look at this diagram because, as I say, if you study this and understand this then you will understand everything that's happening in the heart during a cardiac cycle. Notice at the bottom we have the electrocardiogram. At the top, we have ventricular pressure. That is the pressure which is in the left ventricle. That's the blue line. We will follow this blue line, which is pressure, over time. So this is time. The first thing that we notice is that we have a P-wave. Following the P-wave, the passive filling occurring in the ventricle increases. Then there is this last little push. A last little pressure increase. That causes closure of the AV valves. Once the AV valve close, all the chambers of the heart are isolated. And at this point, we have the QRS complex occur. The QRS complex occurs and then the haert starts to contract. Contraction increases pressure in an isovolumic condition. The pressure rises. That's what's shown here. Pressure increases until it exceeds the pressure in the aorta. At that point the aortic valve opens. As the aortic valve opens, we have ejection of blood. The blood leaves the heart and enters into the aorta. Pressure continues to rise simply because there is still contraction of the ventricle. Eventually we have a T wave. When the T wave occurs that causes the ventricles to relax. At that point, pressure falls within the ventricle because enough blood has left. The aortic valve closes. A T wave occurs. We have relaxation of the muscle. As we have relaxation of the ventricles, then the pressure falls. This is an isovolumic relaxation. That's what's shown here. The end of isovolumic relaxation, the pressure now is less than what's in the atria. The AV valve opens and we again start to fill. So if you notice, the period from isovolumic contraction to isovolumic relaxation, is called systole. We have isovolumic contraction and ejection in systole. Then when we start the isovolumic relaxation and eventually, filling, that's diastole. That's what's shown at the bottom of the plot. The other thing that's added to this plot are the heart sounds. The first of the heart sounds is what's shown here. The first heart sound occurs after you close the AV valves. This is called the lub, of the lub-dub. Lub-dub. The lub is when you close all of the AV valves. All of the valves are shut. That isolates the ventricles. By closing the AV valve, the blood which is moving into the ventricle, then sloshes back up against the valve. [SOUND]. What is heard is that slamming up against the valve. That's what causes the first sound. So the first heart sound is closure of the AV valve, and the beginning of systole. The second heart sound occurs when there is closure of the aortic valve. As you close the aortic valve, again, the blood can slosh back up against the valve. And you hear this "dub" sloshing against the valve. And that's the second sound. The first sound's lub, second sound is dub. So, you have lub-dub, lub-dub. That's one heartbeat or one cardiac cycle. So for every lub-dub, that's one cardiac cycle. In a person at rest, you have 60 beats per minute, Sixty of these cardiac cycles occurring every minute. If you start running and your heart rate increases to 180, then you have 180 of these cardiac cycles occurring per minute. There's another way of looking at the changes that occur within the ventricles. This is called the pressure-volume loops. We diagram the pressure on the Y axis versus the volume on the X axis. And again, we're only looking at the left ventricle. Let's only consider the black circle or the black loop, which is the A, B, C, D loop. So if we start at A, the left ventricle has 50 millilitres volume of blood in it. And as you go from A to B then the volume increases to 90 millilitres. So A, B is filling, passive filling, of the ventricle. At B, now the ventricle has enough pressure that closes the A-V valve. Once the A-V valve closes we now have the end of diastole, the end of filling, At that point, there isovolumic contraction occurs. So isovolumic contraction is from B to C. We're increasing pressure, but there's no change in volume within the ventricle. At C, the aortic valve opens because we've exceeded the pressure that's in the aorta. The aortic valve opens. We continue to have pressure building up within the ventricle because there's still contraction of the ventricle. That causes ejection of the blood, which is a loss of volume within the left ventricle. And we proceed from C to D, this is ejection of the blood. When we reach D, then the ventricle has less pressure than in the aorta and the aortic valve closes. Now we have isovolumic condition where all of the volume that's within the left ventricle is constant. The ventricle relaxes. So between D to A, we have isovolumic relaxation and pressure falls but the volume does not change within the chamber. A couple things to notice. So at B, B is called the end diastolic volume. The end diastolic volume is the amount of volume present within the heart as it starts systole. At D, this D is called the end systolic volume. The end systolic volume is the amount that's left within the heart after ejection of blood. So notice, the heart's never empty. There's not a vacuum within the heart. There's blood within the heart. Under this condition, there's 50 millilitres present at end systolic volume. The difference between end diastolic volume, that is the filled volume, and end systolic volume, the volume after ejection, is called stroke volume. That's the amount of blood that's been ejected from the heart. The end diastolic volume minus end systolic volume is equal to stroke volume, In this case, it would be 90 millilitres minus 50 millilitres, or 40 millilitres would be the stroke volume. We are interested in the end-diastolic volume because it tells us how much volume is present within the heart, how big the heart is. We're gonna return to end-diastolic volume and stroke volume again in the later lectures. But today, we'll say that the end-diastolic volume in each ventricle is normally about 125 milliliters. During one systole, the volume of blood that's ejected, or the stroke volume is usually about 70 millilitres. Since not all of the blood is ejected from the ventricle, only part of the blood that's been filled into the ventricle is ejected, the volume that's remaining is that end-systolic volume. This is usually 55 millilitres. That's what's shown here. So we have the SV = EDV- ESV. Now the cardiac output is the amount of blood that the heart is delivering to provide enough oxygen for all the tissues. And cardiac output has to vary depending upon the demands of the tissues of the body. The cardiac output is the volume which is pumped per minute. It depends on both the heart rate and on the stroke volume. The cardiac output is equal to stroke volume times heart rate. We said that the heart rate was typically about 72 beats per minute. The stroke volume is about 70 beats, 70 millimeters. That means the cardiac output is about five liters per minute. If you recall in our athlete, we said that the athlete could have a very low heart rate. This resting heart rate could be something like 35 or 50 beats per minute. But his cardiac output is about the same. His cardiac output is around 5 L/min. So how is he able to cope with having a cardiac output of 5 L/min and a heart rate that's only lets say 50 beats per minute? The way that he copes is that cardiac output equals stroke volume times heart rate. If the heart rate is 50, and the cardiac output is 5000 milliliters per minute or 5 liters per minute, Then the stroke volume equals cardiac output divided by heart rate. In this particular case, then, we'd have 5,000 divided by 50. Or 100. So the stroke volume is equal to 100 ml. We said that a normal, untrained person has a stroke volume of about 70 milliliters. In the trained athlete, who has a lower heart rate, he has a larger stroke volume. What happens with training is that the heart becomes stronger, the walls of the heart become thicker and the chambers can become a little larger. The heart grows and becomes stronger because it has to meet the demands of whatever the athletic event requires. The althlete compensates by having a larger stroke of volume for a lower heart rate. This is an advantage. When they start to exercise they are at 50 beats per minute. They end up at 200 beats per minute, which is the maximum heart rate that the fellow can have. The pace is limited because if the heart rate is too fast, it can not fill adequately. If they start with a 50 beats per minute and a large stroke volume, then they can reach 200 beats per minute which is a large functional range. Their ability to vary their heart rate and his cardiac output is expanded more then say I an capable. If I start with 80 beats per minute in a much smaller stroke volume. And again, I end up with 200 beats per minute as my maximum heart rate. But I have a shorter range in which to increase my cardiac output. All right, so, what are the key concepts then? The heart consists of two separate pumps. Blood moves in a unidirectional manner from the right heart to the lung for gas exchange, then returns to the left heart from which it is pumped to the systemic circulation for delivery to tissues. Secondly, each beat of the heart that's our cardiac cycle starts with an electrical activaton of the atria and then of the ventricle. The sequence is the same on the right side of the heart as it is on the left heart. And thirdly, ventricular contraction and ejection occurs during systole. The beginning of systole coincides with the first heart sound and ends with the second heart sound. The first heart sound occurs after closing of the AV valves. The heart is full. The second heart sound occurs after closing the aortic and pulmonic valves. The heart now is starting to relax. Pressure in the ventricles are lower than what's downstream. Fourth, the ventricles relax and fill during diastole. Diastole begins with that second heart sound and ends with the first heart sound. Diastole is when the muscle is relaxing, pressure is falling and we have passive filling with blood. Next we have the volume of blood that's ejected with each beat is called the stroke volume. The sum of the stroke volume ejected in one minute is our cardiac output. Okay, so the next time you come in we're going to consider the cardiovascular system in more detail so I'll see you then.
