<P> The Frank--Starling law of the heart (also known as Starling's law and the Frank--Starling mechanism) represents the relationship between stroke volume and end diastolic volume . The law states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles, before contraction (the end diastolic volume), when all other factors remain constant . As a larger volume of blood flows into the ventricle, the blood stretches the cardiac muscle fibers, leading to an increase in the force of contraction . The Frank - Starling mechanism allows the cardiac output to be synchronized with the venous return, arterial blood supply and humoral length, without depending upon external regulation to make alterations . The physiological importance of the mechanism lies mainly in maintaining left and right ventricular output equality . </P> <P> The Frank - Starling mechanism occurs as the result of the length - tension relationship observed in striated muscle, including for example skeletal muscles, arthropod muscle and cardiac (heart) muscle . 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, as determined usually by the fixed distance between the attachment points of tendons to the bones (or the exoskeleton of arthropods) at either end of the muscle . In contrast, the relaxed sarcomere length of cardiac muscle cells, in a resting ventricle, is lower than the optimal length for contraction . There is no bone to fix sarcomere length in the heart (of any animal) so sarcomere length is very variable and depends directly upon blood filling and thereby expanding the heart chambers . 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 longer sarcomere lengths, this is the result of there being less overlap of the thin and thick filaments; for shorter 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 related to the sarcomere length at the time of muscle cell activation by calcium . The stretch on the individual fibers, caused by ventricular filling, determines the sarcomere length of the fibres . Therefore the force (pressure) generated by the cardiac muscle fibres is related to the end - diastolic volume of the left and right ventricles as determined by complexities of the force - sarcomere length relationship . </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 in the pulmonary circulation (were the right ventricle producing more output than the left) or the systemic circulation (were the left ventricle producing more output than the right). </P>

Congestive heart failure (chf) of the right ventricle quizlet