The continuity equation illustrates the conservation of energy. In the case of blood flow within the (normal) heart, volumetric flow (cardiac output) at serial sites should be equal. The image shows flow velocities across the left ventricular outflow tract (LVOT) and through the aortic valve. Because these structures are in a closed system and in series, flow through one should equal flow through the other. Mathematically, we calculate the flow (cardiac output) through the LVOT and aortic valve by measuring their respective cross-sectional areas (CSA) and calculating the velocity-time integral (VTI, distance) blood travels during each heart beat. Area x distance = stroke volume. Multiply that stroke volume by the heart rate, and you have cardiac output.
In other words: CSAaorta x VTIaorta = CSALVOT x VTILVOT
By plugging in known values, we can solve for the variable in question. As always, drop me a comment below with questions! 🙂