5O THE CIRCULATION OF THE BLOOD 



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 capillaries. 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 mm. 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 pressure lessens in the veins while gravity 

 and friction tend to cause a stoppage. These protruding folds 

 of the endothelial 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 endothelium and in direct com- 

 munication with the lymph flow, we can readily see that the 

 food products brought by the arterial blood can be ex- 

 changed here for waste brought by the lymph. The flow in 

 the capillaries is constant, as we have already sfeen. We 

 can understand the importance of this when we take into 

 consideration the rapidity with which the tissues use oxy- 

 gen, the necessity of a constant supply, and the importance 

 of removing the carbon dioxide poisons. 



Innervation of Vessels. The blood-vessels are controlled 



