992 PHYSIOLOGY 



stream in the region of the arterioles, while greater than that of 

 the arteries, is considerably less than that of the rich meshwork 

 of capillaries, while the difference between the diameters of arte- 

 rioles and capillaries is not very great. On this account the 

 velocity of the blood in the arterioles is very much greater than that 

 obtaining in the capillaries, and since friction and therefore the resist- 

 ance varies as the square of the velocity, the resistance to the flow of 

 blood through the arterioles must be much greater than that presented 

 by the capillaries. The large part taken by the arterioles in deter- 

 mining the difference of pressure between the arteries and veins is 

 shown by the fact that this difference can be diminished to one half 

 by any means which causes a dilatation of the arterioles, as, for example, 

 destruction of the vasomotor centre. 



THE CONVERSION OF AN INTERMITTENT INTO A 



CONSTANT FLOW 



Not only is the blood pressure in the veins much lower than in the 

 arteries, but the flow of blood has been converted on its passage through 

 the peripheral resistance from a pulsatory into a continuous flow. 

 This change is connected with the distensible elastic nature of the 

 arterial walls. 



Since this is a purely mechanical question it will be more easily 

 understood by a simple illustration. The heart may be regarded as 

 a pump, forcing a certain amount of blood (in man about 60 c.c.) into 

 the circulation at each stroke. If a pump be connected with a rigid 

 tube, every time that a certain amount is forced into the beginning of 

 the tube an exactly equal quantity will be forced out at the other 

 end. Increasing the peripheral resistance by partial closure of the 

 end of the tube will not affect the intermittent character of the flow, 

 but will merely serve to diminish the quantity thrown in, as well as the 

 quantity which escapes at the other end of the tube, supposing that 

 the work done by the pump is equal in both cases. If instead of a 

 rigid tube we employ an elastic tube and the end be left open so that 

 no resistance is offered to the outflow of the fluid, the effect will be 

 the same as when we used the rigid tube ; the outflow will correspond 

 exactly to the inflow and will be just as intermittent. But now, if 

 the end of the elastic tube be clamped so as to increase the resistance 

 to the outflow, there will be a marked difference from the results 

 obtained when the rigid tube was partially obstructed. Each stroke 

 of the pump forces a certain amount of fluid into the tube. Owing 

 to the peripheral resistance this cannot all escape at once, and so part 

 of the force of the pump is spent in distending the walls of the tube, 

 and part of the fluid that was forced in remains in the tube. The 

 distended elastic tube tends to empty itself and forces out the fluid 



