Frank-Starling law of the heart
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The Frank-Starling law of the heart (also known as Starling's law or the Frank-Starling mechanism) states that the greater the volume of blood entering the heart during diastole (end-diastolic volume), the greater the volume of blood ejected during systolic contraction (stroke volume).
This means that as the heart fills with more blood than usual, the force of the muscular contractions will increase; this is a result of an increase of the load experienced by each muscle fibre due to the extraneous blood entering the heart. This stretching of the muscle fibres increases the affinity of troponin C for Calcium, causing a greater number of cross-bridges to form within the muscle fibres; this increases the contractile force of the cardiac muscle. The force that any single muscle fiber generates is proportional to the initial sarcomere length (known as preload), and the stretch on the individual fibers is related to the end-diastolic volume of the ventricle. In the human heart, maximal force is generated with an initial sarcomere length of 2.2 micrometers, a length which is rarely exceeded in the 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.
This can be seen most dramatically in the case of premature ventricular contraction. The premature ventricular contraction causes early emptying of the left ventricle (LV) into the aorta. Since the next ventricular contraction will come at its regular time, the filling time for the LV increases, causing an increased LV end-diastolic volume. Because of the Frank-Starling law, the next ventricular contraction will be more forceful, causing the ejection of the larger than normal volume of blood, and bringing the LV end-systolic volume back to baseline.
For example, during vasoconstriction the end diastolic volume will decrease due to an increase in TPR; since this decreases venous return, cardiac output will also decrease according to the Frank-Starling graph. A simplified way of thinking about this relationship is that "the heart will pump what it receives."[citation needed]
The above is true of healthy myocardium. In the failing heart, the more the myocardium is dilated, the weaker it can pump, as it then reverts to Laplace's law.
[edit] History
The law is named after the two physiologists, Otto Frank and Ernest Starling who first described it.
Long before the development of the sliding filament hypothesis and our understanding that active tension depends on the sarcomere's length, in 1914 Ernest Starling hypothesized that "the mechanical energy set free in the passage from the resting to the active state is a function of the length of the fiber." Therefore, the initial length of myocardial fibers determines the work done during the cardiac cycle.
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