2O2 ESSENTIALS OF PHYSIOLOGY. 



The total work of the heart, therefore, is in this instance about 

 98 grm. metres per beat. 



The heart expels blood not only against the peripheral resistance, 

 but with a certain velocity. The work (W) done in imparting this 



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velocity to the blood is measured by the formula W = i^ , where 



M = mass of blood expelled, V = its velocity, g = the force of gravity ; 

 it amounts to approximately 1 per cent, of the total work of the heart 

 and is practically negligible. 



We may now consider the conditions which determine the output 

 and the work of the heart. 



The Output of the Heart. It has already been pointed out that, 

 when a skeletal muscle contracts, the contractile stress developed in it is 

 proportional to the initial length of the muscle fibre, i.e. its length just 

 before it begins to contract, and that if the muscle is stretched by means 

 of a weight it contracts more forcibly. The heart muscle behaves in 

 exactly the same way, the only difference being that, in the case of the 

 heart, the initial length of its muscle fibres depends upon the stretching 

 of the fibres produced by the contained blood. Hence the greater the 

 amount of blood present in the heart at the beginning of systole, the 

 greater the initial length of the fibres, and the greater the force with 

 which they contract. The result is that within wide limits the amount 

 of blood expelled from the ventricles at each beat is determined solely 

 by the amount entering the heart during diastole. This amount is 

 increased (1) by deeper respiratory movements, whereby more blood 

 is sent into the heart with each inspiration ; (2) by muscular move- 

 ment, which drives blood along the veins towards the heart; and 

 (3) by any increase in the total volume of the circulating blood, such 

 as is produced by the injection of saline solution into a vein. During 

 exercise the more forcible respiration and the active muscular move- 

 ment lead to a much larger output of blood than during rest. There 

 is a certain optimum filling of the heart and initial length of the 

 muscle fibres at which the mechanical efficiency of the muscle during 

 contraction is at its best; excessive filling of the heart may dilate 

 it to such an extent during diastole as to diminish its efficiency, and 

 its output falls. 



The output of the heart is not affected, except for a moment or 

 two, by alterations in the arterial blood pressure unless these are 

 extreme. The first effect of a rise of blood pressure is that for a few 

 beats the ventricle empties itself less completely, its volume during 

 diastole being thus increased. This distension of the ventricle during 

 diastole increases the length of the fibres, causing them to contract 



