Concept of the Week: The Frank-Starling Law of the Heart

The Frank-Starling Law of the Heart:

Named for physiologists Otto Frank and Ernest Starling, this property of the heart concerns its intrinsic ability to regulate stroke volume (output) in response to changes in venous return (input). There is a directly proportional relationship between end diastolic volume (EDV) and stroke volume (SV), within limits. That is, if venous return increases, end diastolic volume will increase, causing an increase in stroke volume. Essentially, the Frank-Starling Law states that, within defined limits, the heart will pump whatever volume of blood it receives! Simple, right?

The tricky bit is in understanding how this all works. As EDV increases, the ventricles become more full, and the cardiac muscle cells in the walls of the ventricle become increasingly stretched. Any muscle cell has an optimal length for contraction, but in a normal resting heart, the muscle fibers are shorter than their optimal length. Thus, as EDV increases, the cardiac muscle cells get stretched closer and closer to their optimal length for contraction. This results in more forceful contractions and the ejection of a greater stroke volume.

What's really neat about this law is that it regulates each ventricle independently of the other and adjusts strength of contraction on a beat-by-beat basis. The end result is that each ventricle pumps out the same volume that it receives, preventing blood from pooling in either the pulmonary or systemic circuit! How cool is that?