THE CIRCULATION OF THE BLOOD 



187 



other, like the corresponding chambers on the left side, and 



there is a valve preventing the back-flow of blood from the 



ventricle to the auricle. The 



right ventricle pumps blood into 



a large artery that carries blood 



toward the capillaries of the 



lungs {pulmonary artery). 



217. The double circulation. 

 The blood stream may be 

 traced from any point back 

 again to the start only by pass- 

 ing through the two sides of 

 the heart and through the pul- 

 monary, or lung, circulation and 

 the general body, or systemic, 

 circulation. Thus, beginning, 

 for example, in the capillaries 

 of the hand, the blood flows 

 into the veins and is gathered 

 into larger and larger vessels, 

 reaching the right auricle ; from 

 this it goes to the right ven- 

 tricle, and when the latter con- 

 tracts, it is forced into the 

 pulmonary artery, which di- 

 vides into smaller and smaller 

 branches, the smallest being 

 the capillaries that lie under 

 the lining of the air sacs of the 

 lungs. As the blood flows on, 

 it is gathered into larger veins 

 that unite to form the pulmo- 

 nary vein, which empties into the left auricle. From the left 

 auricle the blood goes to the left ventricle, and from this it 

 is pumped into the aorta. An artery branching from the main 



Fig. 70. The " double circulation " of 

 the blood 



The arrows indicate the direction of blood 

 flow. The shaded portion represents blood 

 lacking in oxygen. From the right heart 

 (shaded) the blood passes to the lungs, 

 from which it returns to the receiving 

 chamber of the left heart with its carbon 

 dioxid replaced by oxygen. From the 

 pumping chamber of the left heart the 

 blood passes to all parts of the system, 

 or body, and returns to the receiving 

 chamber of the right heart with its oxygen 

 replaced by carbon dioxid 



