preload
Think of stretching a rubber band before you let it snap: the more you stretch it, the harder it flings. Preload is how much the ventricle is stretched by the blood that fills it just before it contracts.
More precisely, preload reflects the volume and pressure in the ventricle at the end of filling, which stretches the muscle fibers to their starting length. Within limits, a more stretched (more loaded) ventricle contracts more forcefully and ejects more blood — the basis of the Frank–Starling mechanism.
Preload rises with anything that returns more blood to the heart: lying down, a fluid infusion, or fluid retention in heart failure. It falls with dehydration, major bleeding, or drugs such as nitrates and diuretics that pool blood in the veins or remove it.
Preload is a clinically useful idea, but it is hard to measure directly. In practice doctors estimate it from surrogates such as central venous pressure or the size of the ventricle on an echocardiogram, knowing these are approximations.
Giving intravenous fluids to a dehydrated patient raises venous return and preload, which can lift a low stroke volume back toward normal.
Restoring preload with fluids can improve output in a depleted patient.
Preload is closely tied to end-diastolic volume — the amount of blood in the ventricle right before systole. The two are often used almost interchangeably in teaching, though preload technically refers to the resulting fiber stretch.