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.
The lower right hand corner is the end-diastolic point, and it’s the point in the cardiac cycle when diastole is over. Αt this point, the mitral valve is closed and the left ventricle is filled with the maximum volume of blood, known as end-diastolic volume. After that, the left ventricle contracts, and systole begins. This makes the pressure shoot up, but since both mitral and aortic valves are closed, the left ventricular volume doesn’t change. This phase is isovolumetric contraction. Eventually the pressure inside the left ventricle exceeds aortic pressure, forcing the aortic valve to pop open, and that starts the ejection phase. Blood leaves the left ventricle and goes into the aorta, decreasing left ventricular volume. The left ventricle continues to contract, so ventricular pressure keeps rises further, but then falls slightly and finally the aortic valve shuts when aortic pressure exceeds the left ventricular pressure. That point, called the end-systolic point, marks the end of systole. At this point, left ventricular pressure is called end-systolic pressure, and left ventricular volume is called end-systolic volume. The difference between end-diastolic volume and end-systolic volume, is the stroke volume. After that, the left ventricular muscle starts relaxing, so left ventricular pressure falls. However, all valves are closed, so the volume remains constant. This phase is isovolumetric relaxation. Eventually, the pressure drops below left atrial pressure, and that allows the mitral valve to open and blood to flow from the left atrium flows into the left ventricle. As the left ventricle fills with blood, its volume rises back to its end-diastolic volume, and the pressure increases only slightly. This relaxation phase continues until the mitral valve closes, letting the loop start all over again.
All this happens during one heartbeat. With every heartbeat, or stroke, the heart is doing work. And that’s called “stroke work” and it’s proportional to the area inside the loop. In other words, the bigger the loop and the more the area inside of it, the more stroke work our heart does.
Sometimes, you’ll see this simplified to look more like a box. The vertical dimension of the box is the distance from the x-axis to end-systolic pressure, and the horizontal dimension is the stroke volume. And we can draw it over the pressure volume loop - so that it looks like this. In terms of stroke work, it would be the area inside the box. The box isn’t exactly the same as the loop, but it’s a good approximation.