THE MECHANISM OF THE HEART PUMP 



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t. e. to 136 X 4t7rr* = 136 X 4?r X 100. If by a contraction of the walls the radius 

 be reduced to 5 cm., the total pressure on the internal surface will be reduced to 

 136 X 4?r X 25, i. e. will be one quarter of the previous amount. Moreover in the 

 case of the heart, with increasing distension the wall becomes thinner and the number 

 of muscle fibres in a given area fewer, so that the larger the heart the more strongly 

 will each fibre have to contract in order to produce a given tension in the contained 

 fluid. At the beginning of systole the distended heart must therefore contract 

 more strongly than at the end of the systole, in order to raise the blood it contains to 

 a pressure sufficient to overcome that in the aorta. 



It is evident that an unrestricted diastolic filling of the heart is not of 

 unqualified advantage to this organ. If during diastole the heart be too 

 forcibly distended, as may easily occur after opening the pericardium, or 

 in cases of enfeeblement of the heart's action by chloroform poisoning or 

 otherwise, the muscle fibres of the heart may be quite unable to contract 

 against the distending force represented by a pressure in the heart equal 

 to that in the aorta. Under such conditions we may have sudden heart 

 failure, which can be relieved only by diminishing the diastolic distension, 

 as, e. g. by letting blood from the veins opening into the heart. 



