960 PHYSIOLOGY 



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 



-V* = 60 x (0-5)' = grammetre , 

 2g 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 grammetres, 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. 



During muscular work this figure would be largely increased. Not only 

 does QR become much larger, but the velocity factor is no longer negligible, 

 since the work done in imparting velocity to the blood increases as the 

 cube of the output per minute. If we take, as an example, a maximum 

 effort on the part of an athlete, we may assume an output per beat of 180 c.c. 

 and a pulse rate of 180 per minute (an output per minute of 324 litres) 

 and an average arterial pressure of 120 mm. Hg. 



Then 



QR = 180 X '120 X 13-6 = 294 grammetres. 



To determine the velocity of output, we assume that 180 c.c. of blood 

 are thrown out into the aorta during f of J second, the time of outflow 

 being about f of each cardiac cycle. This gives a velocity of 2-3 metres 

 per second, assuming a cross section of 625 mm. 2 at the root of the aorta. 

 Therefore 



wV* 180 x (2-3) 2 

 2 3 ' 2X9-8 = 



The total work of both sides of the heart will be : 



294 -f- 5 -f" 59 -f- 5 = 363 grammetres per beat, or 65 kilogrammetres per 

 Left HL'hi minute. 



This rate of work could probably not be maintained for more than a few 

 minutes. 



Tins work is done by a contraction of the muscle fibres surrounding the cavil irs 

 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 be determined not only 

 by the bkrod 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 tension caused by any 

 given pressure on the walls of a hollow vessel will increase with the diameter of the 

 Vessel Tims 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, t. e. the pressure on the whole of 

 the inner wall of the sphere, will be equal to this pressure multiplied by the area, 



