(ii) by altering the contraciility (now a days, vary frequently called the motropic state, following A V Hill's nomenclature, it may also be called,

contraction changes from time to time in the same heart. There are two fundamental mechanisms by which the force of contraction of the myocardium can be changed (i) by altering the initial length of the heart muscle (the 'end diabolic fiber length' EDFL), this is expressed by Frank Starling's [also called, simply. Starling's) law of heart. (ii) by altering the contraciility (now a days, vary frequently called the motropic state, following A V Hill's nomenclature, it may also be called, 'active state' of heart) These two terms, viz , the Starling's law and the contractility should not be confused. They are different (although not totally divorced from each other) entities. (1) Starling's law of heart in 1895 Otto Frank of Germany discovered (in the frog) that greater the volume of blood in diastole of heart, greater is the force of contraction of the heart In the 1910s, The great British physiologist Ernest Starling and his colleagues (Patterson and Piper) showed that this occurs also in subhuman mammals (Much later, in the 3960s, Sonnenblick, Braunwald and Ross worked extensively on this law and brought new facts lo light] Since the 1910s, This law is known as Frank Starling's law, and states Wilhin physiological limit, greater the end diastolic fiber length of the myocardium (i e greater the end diaslolic volume), greater is the force of contractian Figure 522, represents the law graphically The graph is also called the length tension interrelationship graph, because, it depicts the inter relationship between the initial length of the myocardium (_ the length at the beginning of the contraction = EDFL) and the force (= tension) developed by the muscle. Fig. 52.2. To illustrate the Starling's law of heart muscle. Normally, the cardiac muscla operates rather lower down in the ascending limb, whereas the skeletal muscles operate near the peak of the curve (N.B. The skeletal muscles also obey the Starling's law) Note, (i) that the length tension relationship curve (fig 522) has two limbs, an ascending and a descending limb The peak (sometimes called the "hump" by The clinicians) of the curve represents the physiological limit. Further note, (n) greater the cardiac inflow (- venous reiurn to the heart) = greater the EDFL (= end diastohc fiber length) Also note that (ill) when the physiological limit of sueiching (preload) (= peak or the hump) is exceeded, the law operates m its descending limb and now, greater initial length leads to fall of the contractile force Finally, (iv) more the initial length = more the fiber is stretched Concept of preload. Theter, 'preload' is very commonly used in eonneclion with myocardial (as well as skeletal muscular) contraction In connection with the heart, preload means the end diastolic volume of heart, ie. the load which operates before the beginning (i e. prior ta -pre) of contraction (that is why it is called pre load) .: Greater preload - greater stretching. The afterload Consider the left ventricle. When it contracts, it ejects blood into the aorta However, the hydrostatic pressure of the blood in the aorta opposes the ejection and thus ads as a load against the ventricular shortening. This load (= the hydrostatic pressure of aortic blood], begins lo operate, after the onsei of the ventricular contraction and hence called after load It follows, if the aflerload is sufficiently high, the force of cardiac contraction wilt fail to overcome the aflerload and the ventricle will contract isomelncally (That is, the ventricular contraction will fail to open the aortic valve) Note that, the Frank-Sterling's law can also be slated as follows within physiological limits, greater the preload (stretching), greater is the force of contraction developed. Importance of Frank-Starling's law.1. This law ensures that output per stroke of one ventricle = the same in the other ventricle. Thus it c&a readily be seen that is a Irfe saving device Suppose, the right ventncular output per stroke is slightly grealerthan that of the left. This will cause accumulation of blond in the left venTricle. Bui because to the presence of the Frank-Starling's law, as the blood accumulates, the Frank-Starling's law begins to operate more complete evacuation of the left ventricle the two ventricles now will have the same output again Lack of presence of this law, then, would have produced a catastrophe (accumulation of blood in the. pulmonary circulation pulmonary edema) .2. Frank-Starling's law is a life saving device in cardiac failure Suppose, there is left ventricular failure (LVF) due to insufficiency of contraction of the left ventricle accumulation of blood within. The left ventricle land lack of blood supply to the vital organs like brain (which can kill The palient) accumulation of blood in the ventricle leads to opeiation of the Frank Starling's law nreaer cardiac.