178 PHYSIOLOGY FOR DENTAL STl'DENTS. 



Just as the combined cross area of the small pipes leading from 

 the main pipe of the water system is greater by many times than 

 the area of the main pipe, so it has been estimated that the total 

 cross section of the capillaries of the body is 800 times larger 

 than that of the aorta. 



It has been estimated that the rate of blood flow in the aorta is 

 about 320 mm. per second. The average rate of flow in the capil- 

 laries must then be 800 times less than that in the aorta, or 0.4 

 mm. per second. As the length of a capillary has been estimated 

 to be about 0.5 mm., the blood takes about a second to pass 

 through them into the veins. This has been verified by micro- 

 scopic examination of the blood flow in the capillaries. 



The velocity of the blood must be altered whenever the size of 

 the vascular area is changed, and since during a cardiac cycle 

 exactly the same amount of blood is delivered into the right 

 auricle as the left ventricle forces out into the aorta, it follows 

 that the same amount must pass through the vascular area of the 

 body in the same time. In other words, the amount of blood 

 which flows in a given series of blood vessels in a given time is in- 

 dependent of the size of the blood vessels. 



The Return of the Blood to the Heart. We must now con- 

 sider the nature of the force which propels the blood, and study 

 what changes take place in the movement of the blood during its 

 passage through the vessels. 



The blood is expelled from the left ventricle with consider- 

 able force and at a high velocity. On its way through the body 

 much of the energy given out by the contraction of the heart is 

 used to overcome the resistance offered by the walls of the ves- 

 sels and the capillaries. In consequence of this, the velocity and 

 the pressure of the blood on the sides of the vessels are much re- 

 . duced. 



The blood is collected from the capillaries by the veins, and 

 since the volume of the veins is less than the volume of the capil- 

 laries its velocity is much increased. The relatively large calibre 

 of the veins, however, offers little resistance to the flow of blood 

 and the energy remaining from that imparted to the blood by the 

 heart has full power to make itself felt. Nevertheless, this is not 



