384 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ward according to the laws of fluid pressure, and constitutes a strong resist- 

 ance to the onward movement of the blood out of the heart itself. Friction 

 is everywhere present in the vessels, but is greatest in the very small ones 

 collectively. 



Power. Where the aorta springs from the heart, the rhythmic contrac- 

 tions of the left ventricle force open the arterial valve, and force intermittent 

 charges of blood into the arterial system, overcoming thus the opposing force 

 derived from friction. The wall of the arterial system is very elastic every- 

 where. Thus the high pressure in the arteries results from the interaction of 

 the power derived from the heart's beat and the resistance derived from fric- 

 tion. That the high pressure is continuous depends upon the capacity for 

 distentiou possessed by the elastic arterial wall. 



Balance of the Factors of the Arterial Pressure. In order to 

 study the causation of the arterial pressure, let us imagine that it has for some 

 reason sunk very low ; but that, at the moment of observation, a normally 

 beating heart is injecting a normal blood-charge into the aorta. The first 

 injection would find the resistance of friction present, and the elastic arterial 

 wall but little distended. For this injection some room would be made by 

 the displacement of blood into the capillaries. But it would be easier for the 

 arterial wall to yield than for the friction to be overcome, so the injected 

 blood would largely be stored within the arterial system and thus raise the 

 pressure. Succeeding injections would have similar results ; it would continue 

 to be easier for the injected blood to distend the arteries than to escape from 

 them ; and the arterial pressure would rise rapidly toward its normal height. 

 Presently, however, a limit would be reached ; a time would come when the 

 elastic wall, already well stretched, would have become tenser and stiffer and 

 would yield less readily before the entering blood ; and now a larger part 

 than before of each successive charge of blood would be accommodated 

 by the displacement of an equivalent quantity into the capillaries, and a smaller 

 part by the yielding of the arterial wall. Normal conditions of pressure 

 would be reached and maintained when the blood accommodated, during each 

 systole of the ventricle, by the yielding of the arterial wall should exactly 

 equal in amount the blood discharged from the arteries into the capillaries 

 during each ventricular diastole ; for then the quantity of blood parted with 

 by the arteries during both the systole and the diastole of the heart would 

 be exactly the same as that received during its systole alone. 



We see that, at each cardiac systole, the cardiac muscle does work in main- 

 taining the capillary flow against friction, and also does work upon the arte- 

 rial wall in expanding it. A portion of the manifest energy of the heart's 

 beat thus becomes potential in the stretched elastic fibres of the artery. The 

 moment that the work of expansion ceases, the stretched elastic fibres recoil ; 

 their potential energy, just received from the heart, becomes manifest, and 

 work is done in maintaining the capillary flow against friction during the 

 repose of the cardiac muscle. At the beginning of this repose the arterial 

 valves have been closed by the arterial recoil. When, at each cardiac systole, 



