THE CIRCULATION 279 



of no small use in adapting blood-pressure, etc., to the ever- varying 

 organic needs. (See p. 300 et seq.) 



The ventricles force into their respective outlet-tubes (against a pressure 

 of about 150 millimeters of mercury in the aorta and of about 50 milli- 

 meters in the pulmonary artery) not far from 120 grams of blood at every 

 contraction (Cowl), although some have claimed that the amount of 

 blood is less. 150 millimeters of mercury exert about the same pressure 

 as would 1.92 meters of blood, (taking the specific gravity of blood as 

 1055 and that of mercury as 13.5), so that the work done by the left 

 ventricle at each systole equals that required to raise 60 grams of blood 

 1.92 meters against gravity, or 0.1152 kilogram-meters of work. With 

 a pulse-rate of 75, there are 108,000 beats in twenty-four hours, and the 

 left ventricle would do 12,441.6 kilogram-meters of work daily. If we 

 estimate that done by the right ventricle as one-third that by the left and 

 add it in, we have 16,588.8 kilogram-meters of work as that done by 

 the ventricles daily. On the amount of energy then, used up by his 

 heart's ventricles alone in one day, a man of average weight, say 70 kilos, 

 could climb 237 meters up a mountain; it is equivalent to 36.7 kilocalories 

 of energy. During severe muscular exertion this output by the heart 

 may be as much as doubled (Zuntz). This increase is accomplished 

 not only by a marked increase in the pulse-rate but through a large ex- 

 pansion of the heart's chambers by the mechanism which governs the 

 tonus of the heart. Most of this large expenditure of energy by the heart 

 is used in overcoming the friction of the blood-stream against the arterial 

 walls. 



The ventricular systole which forces 120 cubic centimeters or less of 

 blood into the aorta already under an internal tension of about 150 

 millimeters of mercury, occurs rather suddenly (in 0.3 second) and the 

 aortic blood is of necessity quickly displaced. This displacement 

 occurs not only in the aorta but also in the pulmonary vein 75 times per 

 minute, and with such a frequency even in rigid tubes there would be 

 something approaching a continuous flow owing to the considerable 

 friction. But the arteries are quite the opposite of rigid, for elasticity 

 combined with strength is their most marked mechanical characteristic. 

 (See the experiments with the circulation-schema described in the 

 Appendix.) 



THE RECOIL OF THE ARTERIAL WALLS, mentioned as the second most 

 important cause of the circulation, depends immediately on this high 

 elasticity,, and must not be confused with the muscular vaso-motor con- 

 traction of the medial coat. The muscle-fibers present in the arterial 

 wall increase somewhat the latter's elasticity, but this elasticity is mostly 

 passive and the recoil is probably only the back-action of force put into 

 the arterial wall by the systolic energy of the ventricles. The arteries 

 being already full, the accession of blood at each beat in part pushes 

 forward the blood already present and in part distends the arteries 

 (this distention being the "pulse"). Compared with that of the aorta, 

 the diameter of the single capillaries is insignificant, although the com- 



