The Frank-Starling Curve and Ejection Fraction | Visual AI Learning Guide

The curve on the screen is the Frank-Starling relationship. The horizontal axis is filling pressure, the vertical axis is stroke volume. Start at a low filling pressure and drag it upward: stroke volume rises steeply at first, then flattens into a plateau. That plateau is the limit of preload reserve. Now change contractility. When you raise contractility, the whole curve shifts upward, so the same filling pressure produces a larger stroke volume. When you lower contractility, the curve shifts downward and the operating point drops. Watch the ejection fraction readout as you do this: it is stroke volume divided by end-diastolic volume. Lowering contractility lowers stroke volume while filling stays the same, so the ratio falls. That is why a failing ventricle shows a reduced ejection fraction.

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The Frank-Starling relationship states that within physiological limits, the more the ventricle is filled during diastole, the more forcefully it contracts during systole, so stroke volume rises with preload. The mechanism is at the level of the sarcomere: stretching the cardiac muscle fiber optimizes the overlap between actin and myosin and increases the sensitivity of the contractile proteins to calcium, so each contraction generates more force. The relationship is plotted as a curve with filling pressure or end-diastolic volume on the horizontal axis and stroke volume on the vertical axis. The curve rises steeply at low filling, then flattens into a plateau, and at very high filling it can fall — the descending limb. Ejection fraction is a separate index of pump performance: \(EF = \frac{SV}{EDV} \times 100\%\), where \(SV\) is stroke volume and \(EDV\) is end-diastolic volume. A normal ejection fraction is roughly 55–70%. Because ejection fraction is a ratio, it falls when contractility falls even if filling is unchanged, and it can appear normal when both stroke volume and end-diastolic volume fall together. In the simulation, you control filling pressure and contractility and observe how stroke volume and ejection fraction respond.