96 ELEMENTS OF PHYSIOLOGY 



coats are not prolonged into them. It should be stated 

 here that although the extremely minute size of the capil- 

 lary tubes increases the friction and the pressure which the 

 heart must expend in sending the blood through them, yet 

 their resistance to the blood flow is lessened by their great 

 capacity. The united capacity of the capillaries is six hun- 

 dred times that of the blood arteries that supply them. 

 In the capillaries the blood flows slowly like a river which 

 flows into and out of a lake. 



158. Rate of Flow and Differences of Pressure. The 

 velocity of flow is 15 inches per second in the aorta, 3^ of 

 an inch per second in the capillaries, and about 7^ inches 

 per second in the inferior vena cava. When an artery is 

 cut, the blood is thrown out by jerks corresponding to the 

 pulse, and flows slowly between the jerks because the 

 arteries are kept over-full and the blood is pressed out by 

 their elastic walls. The pressure that results from the 

 arteries being always over-full, likewise explains why the 

 blood flows constantly from the arteries into the capillaries. 

 Hence, although the blood is thrown into the aorta at in- 

 tervals by the beat of the heart, it flows from the arteries 

 to the capillaries continuously, being only quickened in its 

 flow by each beat. In the capillaries, owing to the great 

 friction in the minute passages, there is strong resistance 

 to the flow of blood, and the great pressure in the arteries 

 is necessary to send the blood through the capillaries; 

 a great deal of this pressure is used up in doing so, hence 

 the pressure beyond the capillaries (that is to say, in 

 the veins) is much less than the pressure in the arteries. 

 Enough is left to force the blood part of the way through 

 the veins ; but it is not sufficient to carry the blood back to 

 the .heart, for it gives out entirely before the heart is 

 reached. We shall learn later how the contraction of the 

 muscles and their squeezing effect upon the veins passing 

 through or beneath them, aids the heart to move the blood ; 



