184 ESSENTIALS OF PHYSIOLOGY. 



60 c.c.). The peripheral resistance is due to friction between the flow- 

 ing blood and the walls of the vessels, the amount of friction varying 

 inversely with the bore of the vessels and directly with the velocity of 

 the blood. The peripheral resistance caused by this friction is very 

 large in the arterioles, which are numerous and of small bore, and in 

 which the blood flows rapidly. In the capillaries the blood flows so 

 slowly that, although their calibre is very minute, the resistance is 

 much less than in the arterioles ; and in the large arteries it is com- 

 paratively slight. 



With each beat of the heart an additional quantity of blood enters 

 the arterial system, and, if there were no peripheral resistance, an equal 

 quantity would instantly escape through the arterioles into the capillaries. 

 But the resistance offered by the arterioles is so great that when the 

 blood is forced during systole into the already distended arterial 

 system, there is not an immediate escape of a corresponding amount 

 from the arterioles into the capillaries. Most of the force of the heart 

 is expended in further distending the arteries in order to accommodate 

 this additional blood sent into them, and the pressure within them 

 rises. In the interval between two heart beats the distended arteries 

 shrink by virtue of their elasticity, thereby forcing blood through the 

 arterioles, and the pressure falls slightly. The result is that the flow 

 of blood along the capillaries and veins takes place both during arid 

 between the heart beats as a steady stream, whereas blood enters the 

 aorta only during the heart beat, and the flow along the arteries 

 is jerky. 



When the amount of blood entering the arterial system during 

 systole is equal to that leaving it, partly during systole and partly 

 during diastole, the mean arterial pressure remains steady. Any 

 increase or decrease in the amount of blood entering the aorta at each 

 beat will tend to raise or lower the arterial pressure. Similarly, dilata- 

 tion of the arterioles will diminish the resistance to the escape of blood 

 into the capillaries and the arterial pressure will fall, whereas constric- 

 tion of the arterioles will produce a rise of pressure. 



Although the driving force of the heart pump is sufficient to propel 

 the blood round the body and back to the heart, its action is normally 

 assisted (1) by the respiratory movements, which will be considered 

 later (page 283), and (2) by skeletal muscular movements. Every 

 muscular movement tends to squeeze blood along the veins towards 

 the heart, any reflux being prevented by the valves with which these 

 veins are provided. 



Mean Systemic Pressure. When the heart ceases to beat the 

 arterial pressure falls, the venous pressure rises, and finally there is 



