50 THE CIRCULATION OF THE BLOOD 



Rate of Flow. The velocity of the blood current is equal 

 to the volume flowing through a determined section in one 

 second divided by the cross section. The rate is determined by 

 the pressure, the friction in the vessels, and the cross section of 

 the vessels. 



The combined cross section of the capillaries is greater than 

 the combined cross section of the arteries or the veins, so the rate 

 of flow must be greater in the arteries and veins than in the cap- 

 illaries. The friction is greater in the smaller vessels than in the 

 larger which retards the flow. The pressure is greater in the 

 arteries than in the capillaries and veins. From these facts it is 

 evident that the velocity is greater in the arteries than in the 

 capillaries and veins, but increases in the veins as compared to 

 the capillaries. 



In the large arteries the rate is 200-400 m.m. per second, in the 

 capillaries, 6-8 mm. and in the large veins it is but little less than 

 in the arteries. 



Valves in the Veins. At frequent intervals in the course of 

 the veins are found small folds of membrane protruding into the 

 lumen of the vessels. The flow of the blood in the veins is more 

 sluggish than in the arteries, because, as we have seen, the pres- 

 sure lessens in the veins while gravity and friction tend to cause a 

 stoppage. These protruding folds of the mucous membrane or 

 valves found in the veins aid in the circulation by overcoming 

 gravity and preventing a backward flow of blood, by holding 

 the blood until a fresh impulse can impel it forward. They are 

 found in pairs and are most abundant in the veins of the 

 extremities where gravity impedes the onward flow of the 

 current. 



Capillary Importance. The capillaries are the smallest 

 blood-vessels and the most important as to function. Being of 

 only one thickness of epithelium and in direct touch with the 

 lymph flow, we can readily see that the food products brought by 

 the arterial blood can be exchanged here for waste brought bv 



