Stroke volume is not a fixed property of the heart; it is the result of three determinants acting together. Preload is the degree of ventricular filling just before contraction, measured clinically as end-diastolic volume or approximated by end-diastolic pressure. Afterload is the resistance the ventricle must overcome to open the aortic valve and eject blood, dominated by systemic vascular resistance and arterial pressure. Contractility is the intrinsic strength of the myocardium at any given fiber length — its inotropic state — and is independent of how much the ventricle is filled or what pressure it faces. The diagram shows a single ventricle with these three inputs: filling from the veins sets preload, the arterial pressure the ventricle ejects against sets afterload, and the intrinsic state of the muscle sets contractility. Increasing preload raises stroke volume through the Frank-Starling relationship; increasing afterload lowers it; increasing contractility raises it. These three determinants are the vocabulary used for the rest of the course, because every cause of heart failure and every compensation acts by changing one or more of them.
Learn MotionWhy the Heart Fails and How the Body Compensates
Cardiac Output and the Determinants of Pump Performance
Three Levers That Set Stroke Volume
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Look at the ventricle in the center of the diagram. Three arrows feed into it. The arrow from the veins represents preload, the filling of the ventricle before it contracts. The arrow from the aorta represents afterload, the pressure the ventricle must push against to eject. The arrow inside the muscle wall represents contractility, the intrinsic strength of the muscle itself. Follow each arrow: more filling stretches the muscle fibers and increases the force of contraction, so stroke volume rises. More afterload means the ventricle must generate more pressure before the valve opens, so less blood leaves and stroke volume falls. More contractility means the same filling ejects a larger fraction, so stroke volume rises. These three levers are what every later mechanism in this course will act on.
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