Pressure-volume loops

Pressure- volume loops are graphs, where the pressure inside the left ventricle is on the y axis and the volume of the left ventricle is on the x axis. Each loop represents one cardiac cycle, including both ventricular systole and diastole, or more simply, one heartbeat.

Alright, let’s start at the lower right hand corner. This is the end-diastolic point, and it’s the point in the cardiac cycle when diastole is over. The mitral valve, or the “communicating door” between the left atrium and left ventricle has just closed. And as it closed, it made a loud, long sound, kind of like a “lup”. That’s known as the first heart sound, or S1. So, at this point, the left ventricle is filled with the maximum volume of blood, known as end- diastolic volume, which is normally about 120 milliliters. You can imagine the ventricle as a relaxed sack of muscle that’s full of blood, so pressure is low. After that, the left ventricle contracts, and that marks the beginning of systole. This makes the pressure shoot up, but since both mitral and aortic valves are closed, blood can neither enter nor leave the ventricle, the volume doesn’t change. This phase is called isovolumetric contraction and it lasts for about 0.05 seconds. Eventually the pressure inside the left ventricle reaches approximately 75 mmHg, becoming just higher than the pressure inside the aorta, forcing the aortic valve to pop open. This pressure reflects the pressure in the aorta right at the beginning of ejection phase, or the lowest that the blood pressure in the aorta is going to be and is known as the diastolic blood pressure, or DBP for short.

Okay, now, once the ejection phase starts, it lasts for about 0.25 seconds. That’s when blood is ejected out of the left ventricle and into the aorta, decreasing left ventricular volume. The left ventricle continues to contract, so ventricular pressure keeps rises further. Meanwhile, blood is rushing through the aorta, so its pressure increases, as well. In fact, during this phase, left ventricular and aortic pressures are essentially equal. This goes on, until they reach a peak of about 120 mmHg, known as systolic blood pressure, or SBP for short, before finally starting to come down again. The difference between systolic and diastolic blood pressures, in this case 120 - 75 equals 45 mmHg and is called pulse pressure, because it reflects the throbbing pulsation felt in an artery during systole. Eventually, aortic pressure exceeds the pressure within the left ventricle, so the aortic valve snaps shut, making a short, sharp sound, kind of like a “dub”. And this dub is called the second heart sound, or S2. So, this marks the end of the systolic period, also known as end-systolic point. At this point, the volume left inside the left ventricle, known as end- systolic volume, is normally about 50 milliliters. If we calculate the difference between end-diastolic volume and end-systolic volume, we’ll get the stroke volume, which is the volume of blood the left ventricle ejects or simply, squeezes away, on every heartbeat, or stroke, during this ejection period. In this case, 120 minus 50 is a stroke volume of 70 milliliters. Stroke volume is a useful measurement, but it can vary based on the size of a person’s heart. So another helpful measurement is the ejection fraction, which is the stroke volume divided by the end-diastolic volume, Ejection fraction = Stroke Volume / End- Diastolic Volume. In this example, it’s 70/120, or about 58%. In other words, 58% of the blood volume in the left ventricle gets pumped out during each heartbeat.

Once the aortic valve has closed, the ventricular muscle starts relaxing, so pressure within the ventricle falls. However, all valves are closed, so the volume remains constant. This is the isovolumetric relaxation phase, and it lasts for about 0.15 seconds. Eventually, the pressure drops below left atrial pressure, and that allows the mitral valve to open.