Frank Starling's curve, often referred to as the Frank-Starling mechanism or law of the heart, is a fundamental concept in cardiovascular physiology that explains how the heart's performance is related to how much it is stretched before it contracts.
Specifically, the left ventricular performance (Frank-Starling) curves relate preload, measured as left ventricular end-diastolic volume (EDV) or pressure, to cardiac performance, measured as ventricular stroke volume or cardiac output.
Understanding the Relationship
This curve visually represents the Frank-Starling Law, which states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles before contraction, when all other factors remain constant.
Think of it like stretching a rubber band: the more you stretch it (within limits), the more forcefully it snaps back. In the heart, the stretch is caused by the incoming blood during diastole (filling phase), which is the preload.
The curve typically shows:
- X-axis: Preload (commonly represented by left ventricular end-diastolic volume (EDV) or pressure). This is the amount of stretch on the heart muscle fibers just before contraction.
- Y-axis: Cardiac Performance (commonly represented by ventricular stroke volume or cardiac output). Stroke volume is the amount of blood pumped out with each beat; cardiac output is the total amount pumped per minute (Stroke Volume x Heart Rate).
As preload increases (more blood fills the ventricle), the heart muscle fibers are stretched more. This increased stretch leads to a more forceful contraction, resulting in a larger stroke volume and, subsequently, a higher cardiac output (assuming heart rate is constant).
Practical Significance
This intrinsic ability of the heart allows it to automatically adjust its output to match the venous return (the amount of blood flowing back to the heart). For example, during exercise, increased venous return stretches the ventricles more, and the Frank-Starling mechanism ensures that the heart pumps out the extra blood, preventing blood from backing up and meeting the body's increased demand.
However, there are limits. If the ventricle is stretched too much (as can happen in certain heart conditions), the force of contraction may no longer increase, and performance can even decline. The Frank-Starling curve beautifully illustrates this optimal range of stretch for peak performance.
