<P> The Frank - Starling mechanism occurs as the result of the length - tension relationship observed in skeletal muscles . As a muscle fiber is stretched, active tension is created by altering the overlap of thick and thin filaments . The greatest isometric active tension is developed when a muscle is at its optimal length . In most relaxed skeletal muscle fibers, passive elastic properties maintain the muscle fibers length near optimal . In contrast, the normal point of cardiac muscle cells, in a resting individual, is lower than the optimal length for contraction . In the human heart, maximal force is generated with an initial sarcomere length of 2.2 micrometers, a length which is rarely exceeded in a normal heart . Initial lengths larger or smaller than this optimal value will decrease the force the muscle can achieve . For larger sarcomere lengths, this is the result of less overlap of the thin and thick filaments; for smaller sarcomere lengths, the cause is the decreased sensitivity for calcium by the myofilaments . An increase in filling of the ventricle increases the load experienced by each cardiac muscle fiber, stretching the fibers toward their optimal length . </P> <P> The stretching of the muscle fibers augments cardiac muscle contraction by increasing the calcium sensitivity of the myofibrils, causing a greater number of actin - myosin cross-bridges to form within the muscle fibers . Specifically, the sensitivity of troponin for binding Ca increases and there is an increased release of Ca from the sarcoplasmic reticulum . In addition, there is a decrease in the spacing between thick and thin filaments, when a cardiac muscle fiber is stretched, allowing an increased number of cross-bridges to form . The force that any single cardiac muscle fiber generates is proportional to the initial sarcomere length, and the stretch on the individual fibers is related to the end - diastolic volume of the left and right ventricles . </P> <P> Due to the intrinsic property of myocardium that is responsible for the Frank - Starling mechanism, the heart can automatically accommodate an increase in venous return, at any heart rate . The mechanism is of functional importance because it serves to adapt left ventricular output to right ventricular output . If this mechanism did not exist and the right and left cardiac outputs were not equivalent, blood would accumulate throughout circulation . </P> <P> Premature ventricular contraction causes early emptying of the left ventricle (LV) into the aorta . Since the next ventricular contraction occurs at its regular time, the filling time for the LV increases, causing an increased LV end - diastolic volume . Due to the Frank--Starling mechanism, the next ventricular contraction is more forceful, leading to the ejection of the larger than normal volume of blood, and bringing the LV end - systolic volume back to baseline . </P>

The starling law of the heart describes a proportional relationship between which of the two terms