MATERIAL WITHIN THE BODY 



183 



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 au- 

 ricle. From this it goes to the 

 right ventricle ; and when the 

 latter contracts, the blood is 

 forced into the pulmonary 

 artery. The lung arteries di- 

 vide into smaller and smaller 

 branches, the smallest being 

 the capillaries that lie under 

 the lining of the air sacs in the 

 lungs. As the blood flows on, 

 it is gathered into larger and 

 larger veins that unite to form 

 the pulmonary 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 artery carries 

 blood into the arm, and as 

 the arteries divide, becoming 

 smaller and smaller, we at last 

 reach the capillaries in the 

 hand, from which we started. 

 This "double circulation" 

 makes possible a rapid ex- 

 change of carbon dioxid for 

 oxygen. In the human body all 

 the blood passes through the 

 heart (and therefore through 



the capillaries of the lungs) once in from twenty- three to thirty 

 seconds. The exchange of gases in the air sacs of the lungs has 

 already been described (see Fig. 85). 



Fig. 96. 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 car- 

 bon 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 oxy- 

 gen replaced by carbon dioxid 



