THE FLOW OF BLOOD IN THE CAPILLARIES 219 



the way back to the heart. The wavelike movement is reflected, therefore, at 

 the capillaries, etc., and under normal circumstances the pulse cannot be 

 perceived in the veins. 



Supposing now, as we have done, that the pressure is everywhere the same 

 to begin with, then if the regular contractions of the heart were repeated 

 rapidly, enough, there would be an accumulation of blood in the arteries, for 

 at each systole more blood would be thrown into the arteries than could be 

 pressed through into the veins in the same time. At every diastole of the 

 heart the total quantity of blood in the veins would be still further reduced, 

 because more blood would pass from them into the heart than could come 

 into them through the capillaries from the arteries. Thus the quantity of 

 blood in the arteries would go on increasing, and the quantity in the veins 

 would go on decreasing until the difference in pressure between the two would 

 become so great that from one systole to another just as much blood was 

 pressed through the capillaries as was being discharged by the heart into the 

 arteries. Once this degree of difference in pressure between the two divisions 

 of the vascular system had been reached, if the heart activity continued the 

 same, the difference would become constant i. e., the pressure in the arteries 

 would be permanently greater than in the veins. 



It is because of this constant difference in pressure between arteries and 

 veins that the movement of the blood from the former to the latter takes 

 place in a steady stream. For this reason also the blood continues to flow 

 from arteries to veins for some time after the heart stops beating. Any sort 

 of influence which changes either the resistance in the vessels or the energy 

 of the heart, disturbs this stationary condition and a new equilibrium is 

 established at a different level of arterial pressure. Every variation in pres- 

 sure is in turn followed by a change of one kind or another in the character 

 of the blood flow. 



6. THE FLOW OF BLOOD IN THE CAPILLARIES 



The capillaries are unquestionably the most important part of the vascular 

 system. The purpose of the circulation, which consists in supplying com- 

 bustible materials and oxygen to the organs, and in relieving them of decom- 

 position products, is accomplished in the capillaries. In them the blood is 

 separated from the lymph by only a thin wall, consisting of a single layer of 

 cells, through which the exchange of diffusible substances is readily carried 

 on. The arteries and veins are only tubes conveying the blood to and from 

 the capillaries : the latter constitute the real clearing-house of the vascular 

 system. 



Since oxygen is consumed in large quantities in the tissues, it is evidently 

 of great importance that the blood should not flow too slowly through the 

 capillaries. The high pressure which prevails in the arteries is necessary in 

 order to keep the blood flowing through the capillaries with sufficient speed. 



Whenever the pressure in the aorta falls, the pressure in the capillaries 

 also falls. If an artery becomes constricted, the lateral pressure in this artery 

 central to the place of constriction increases, but at the same time the pressure 

 and velocity peripheral to that place, that is, in the capillaries, decreases. 



