374 



THE BLOOD AND ITS CIRCULATION 



man where the arteries break up into capillaries, and these in turn 

 form veins. 



Structure of the Arteries. — A distinct difference in structure 



, exists between the arteries and the veins in the human body. The 



arteries, because of the greater strain received from the blood which 



is pumped from the heart, have thicker muscular walls, and in 



addition are very elastic. 



/ Cause of the Pulse. — The pulse, which can easily be detected by press- 

 Ang the large artery in the wrist or the small one in front of and above the 

 external ear, is caused by the gushing of blood through the arteries after 

 each pulsation of the heart. 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 and 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 impor- 

 tance, for the force of the blood as it gushes through the arteries becomes 

 very much less when it reaches the smaller vessels. This gushing move- 

 ment is quite lost when the capillaries are reached, first, because there is 



so much more space for the 

 blood to flU, and secondly, 

 because there is considerable 

 friction caused by the very 

 tiny diameter of the capil- 

 laries. 



-' Capillaries. — The cap- 

 illaries 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. 



Diagram of capillary circulation. Note that 

 the artery breaks up into smaller vessels, 

 which unite again to form a vein. The plasma 

 passes through the walls of the capillaries to 

 nourish the body cells ; some of the lymph 

 then enters the lymph vessels and the rest 

 returns to the capillaries. 



