142 STUDIES IN PHYSIOLOGY 



so small, however, that there is a great deal of friction, and 

 the blood is therefore obliged to move more slowly. These 

 two characteristics of structure explain the absence of a 

 pulse in the capillaries. When the blood passes into the 

 veins, its current is also slow and without any pulse. 



3. THE CIRCULATION OF THE BLOOD 



Having completed our survey of the structure and action 

 of the heart and the blood vessels, we are ready to study 

 the blood system as a whole and to learn how the blood gets 

 to, through, and from every organ of the body. 



The Two So-called Systems of Circulation. In our study of 

 the heart we always referred to the right and left hearts as 

 though they were entirely apart from each other. Let us 

 keep this distinction in mind in considering the circulation 

 of the blood, for there are likewise two distinct systems of 

 blood vessels. One system carries the blood to, through, 

 and from the lungs; it is for this reason called the pulmo- 

 nary circulation. The other system is known as the sys- 

 temic, which supplies blood to every other portion of the body, 

 that is, to the general system. 



The Pulmonary Circulation. In order to understand the 

 pulmonary circulation we need only review the facts we 

 have already learned. The blood is driven by the contraction 

 of the right ventricle past the semilunar valves into the pul- 

 monary artery, which soon branches, giving off an artery to 

 each lung. After passing through the finer pulmonary arter- 

 ies and capillaries, the blood is finally collected by the -four 

 pulmonary veins, which empty into the left auricle, whence 

 the blood comes into the left ventricle (see Fig. 55). 



The Systemic Arteries. From the left ventricle the blood 

 stream is forced into the aorta. This great supply pipe of 

 the body first arches over toward the left, like a shepherd's 

 crook ; it then passes posteriorly through the dorsal part of 

 the chest cavity and, piercing the diaphragm, it enters the 

 abdomen. Three regions of the aorta, therefore, can be 



