x CIRCULATION OF THE BLOOD 133 



of the lymph around the cells. The capillaries unite to 

 form the small veins. The blood then continues to flow 

 on in larger and larger veins until it returns to the right 

 auricle. Then it goes to the lungs as before and back 

 to the left heart. In this way it keeps on circulating 

 day and night, from birth till death. A study of figures 

 60 and 70 will show the course of the blood through 

 the larger vessels. 



Let us trace the blood from the right auricle to the 

 stomach and back again : right auricle, tricuspid valve, 

 right ventricle, semilunar valve, pulmonary arteries, 

 capillaries of the lungs, pulmonary veins, left auricle, 

 mitral valve, left ventricle, semilunar valve, aorta, arch 

 of aorta, thoracic aorta, abdominal aorta, gastric artery, 

 capillaries of the stomach, gastric vein, portal vein, 

 capillaries of liver, hepatic vein, ascending vena cava, 

 right auricle. In the same way blood may be traced to 

 any part of the body. 



99. Rate of Blood Flow. The blood does not flow 

 at a uniform rate in all parts of its course. We know 

 that water in a stream flows most rapidly where the 

 bed is narrowest, and for the same reason the blood 

 flow is most rapid in the aorta and diminishes in rate 

 throughout the arteries. In the capillaries it moves 

 very slowly. For, although the capillaries are very 

 small, they are so numerous that their combined area 

 is many times that of the aorta. In the veins the blood 

 flows faster again and increases in rate toward the heart. 

 Figure 72 indicates the combined area of the blood 

 vessels in different parts of the system. The rate of 





