924 PHYSIOLOGY 



will be proportional to the velocity of the flow between these two points. The velocity, 

 on the other hand, will vary directly as the difference of pressures, and inversely as the 

 resistance between the two points. These relations may be expressed by the formula 



In the vascular system, while the circulation is maintained, the largest 

 difference of pressure exists between the arteries on the one side and the 

 small veins on the other, a great fall occurring between the arteries and 

 the capillaries themselves. This distribution of pressure points to the 

 chief resistance in the vascular system as being situated in the arterioles. 

 The resistance presented by these vessels is due to the fact that they are 

 maintained in a state of tonic contraction by the agency of the central 

 nervous system. The total .bed of the 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 arterioles 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 resistance 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 determining 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 under- 

 stood 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 peri- 

 pheral resistance by partial closure of the end of the tube will not affect 

 tln intermittent character of the flow, but will merely serve to dimmish 

 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 



