1032 PHYSIOLOGY 



Hg. as the average pressure at the beginning of the aorta, and 500 mm. 

 per second as the velocity imparted to the blood thrown into the aorta, 

 we can calculate the work done by the human heart at each beat. 



QR = 60 X 0-100 m. X 13-6 = 81-6 grammetres, 

 or roughly 80 grammetres. On the other hand, the expression 



wV 2 60 X (0-5) 2 



= 2 - = 0-7 grammetres. 



20 2 X 9-8 



It is evident that this latter factor is negligible, and that for all practical 

 purposes we may regard the work of the heart as proportional to 

 the output multiplied by the average arterial blood pressure. Taking 

 the average pressure in the pulmonary artery at 20 mm. Hg., the work 

 of the right ventricle at each beat would amount to about 16 gram- 

 metres, a total for the two ventricles of about 100 grammetres per 

 beat, which is equivalent to about 10,000 kilogrammetres in twenty- 

 four hours for a man at rest. 



This work is done by a contraction of the muscle fibres surrounding 

 the cavities of the ventricles. It is important to remember that the 

 strain or tension which is thrown on these fibres and which resists their 

 contraction will not be simply determined by the blood pressure 

 which has to be overcome, but also by the size of the ventricle 

 cavities. Since the pressure in a fluid acts in all directions, the ten- 

 sion caused by any given pressure on the walls of a hollow vessel will 

 increase with the diameter of the vessel. Thus if we take a sphere 

 with a radius of 10 cm. filled with fluid at a pressure of 10 cm. Hg. 

 there will be a pressure on each square centimetre of the inner surface 

 of the sphere of 136 grm. The total distending force, i.e. the pressure 

 on the whole of the inner wall of the sphere, will be equal to this 

 pressure multiplied by the area, i.e. to 136 X 4?rr 2 = 136 X 4-rr 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-rr 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 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 enfeeble- 



