376 CIRCULATION AND RESPIRATION 



the pressure of the blood in the veins, while feeble, is 

 sufficient to force blood from the veins into the auricles. 

 The body has ways of helping the feeble blood pressure 

 in the veins. Many veins are so placed that they are 

 easily compressed by the body muscles that are near them. 

 Muscles in active use can thus hasten the flow of blood 

 in the veins. Veins of this sort have valves opening 

 toward the heart, but not away from the heart. Nature 

 takes care to have the blood go in the right direction. 



377. Capillaries. The capillaries are the smallest of 

 the blood vessels (cf. 32), with an average diameter of 

 about 2^0 of an inch. While the purpose of the blood's 

 circulation is to get food and oxygen to the cells, and to 

 remove waste materials from the cells, yet the walls of 

 arteries and veins are too thick to permit the passage of 

 blood materials. It is only in the capillaries that the 

 walls are thin enough. 



The capillaries extend to all parts of the body. They are in all 

 glands (cf. 358), for it is through the capillaries that glands get the 

 juices which they secrete; they are near the digestive organs, to take 

 the digested food into the blood (cf. 365) ; they are in the lungs, to 

 permit the blood to give off its waste carbon dioxide, and to take in 

 oxygen. 



Capillaries may be looked upon as the tiniest branches of the 

 arteries, carrying blood to the body; they are also the small beginnings 

 of the veins, in which the blood is gathered for transportation back to 

 the heart. We can compare the flow of blood through arteries, veins, 

 and capillaries with the flow of a stream of water through a bed of 

 sand. The channel that brings the water is like an artery; the channel 

 through which it flows after passing through the sand is like a vein; 

 the thousands of tiny channels, by which the water finds its way 

 through the sand, correspond to the capillaries. 



