What is defined as the amount of blood that fills the left ventricle?

The amount of blood that fills the left ventricle is referred to as preload. Preload is a critical concept in understanding cardiac function, as it represents the volume of blood in the ventricles at the end of diastole, prior to contraction. This volume influences the stretch of the cardiac muscle fibers, which is essential for the heart's ability to generate an effective contraction.

Preload is important because it directly affects stroke volume; an increased preload generally leads to increased stroke volume based on the Frank-Starling mechanism, which demonstrates the relationship between the volume of blood returned to the heart and the amount of blood pumped out. Understanding preload helps in assessing heart performance and managing conditions such as heart failure, where preload may be altered.

In contrast, afterload refers to the resistance the heart must overcome to eject blood, contractility pertains to the strength of cardiac muscle contraction, and cardiac output is the total volume of blood pumped by the heart per minute. While all these terms are related to cardiac function, preload specifically addresses the volume of blood in the left ventricle just before it contracts.