CIRCULATION 



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lar tubes, side by side, each having two chambers (auricle and ventricle). 

 As the large arteries pass away from the heart, the diameter of each 

 individual artery becomes smaller. At the very end of their course, 

 these arteries are so small as to be almost microscopic in size. They are 

 very numerous. There are so many that if they were placed together, 

 side by side, their united diameter would be much greater than the 

 diameter of the large artery (aorta) which passes blood from the left 

 side of the heart. This fact is of very great importance, for the force 

 of the blood as it gushes through the arteries becomes very much less 

 when it reaches the smaller vessels. This gushing movement is quite lost 

 when the capillaries are reached. First, because there is so much more 

 space for the blood to fill; secondly, there is considerable friction caused 

 by the very tiny diameter of the capillaries. 



Capillary NetworH 



Capillary network, showing change from arterial to venous blood. 



Capillaries. — The capillaries form a network of minute tubes 

 everywhere in the body, but especially near the surface and in the 

 lungs. It is through their walls that the food and oxygen pass to 

 the tissues, and carbon dioxide is given up to the plasma. They 

 form the connection that completes the system of circulation of 

 blood in the body. 



Function and Structure of the Veins. — If the arteries are pipes 

 which supply fluid food to the tissues, then the veins may be 

 likened to drain pipes which carry away waste material from the 

 tissues. Extremely numerous in the extremities and in the muscles 

 and among other tissues of the body, they, like the branches of a 



