THE CIRCULATION 



of the bicuspid valve, closing the way back to the left au- 

 ricle. The pressure of the ventricle opens the semilunar 

 valve in the mouth of the great aorta, which is the large 

 artery carrying the blood from the left ventricle. The 

 aorta takes the blood to every part of the body except the 

 lungs. It gives off smaller arteries, and the division is 

 repeated until arteries are supplied to every organ and tis- 

 sue. In the tissues the arteries 

 empty into smaller tubes called 

 capillaries. The aorta, with its 

 branches, becomes distended with 

 blood, and as more and more is 

 forced into it by the left ventricle 

 at each heart beat, the distention is 

 kept up, and some of the blood 

 already in the aorta is forced along 

 its branches, and the same pressure 

 forces it through the capillaries and 

 into the veins (Plates V and VIII). 



150. The blood flows slowly 

 through the capillaries and per- 

 forms its function of exchanging 

 substances needed for those used 

 up. It next goes into the veins on 

 the return journey to the heart 

 where it enters the right auricle 

 again, which was our starting place 

 in this description (Fig. 89). 



151. Review of Circulation. - 

 The blood comes from the tissues 

 through the veins and enters the 

 right auricle, goes through the tri- 

 cuspid valve into the ventricle, then 



through the semilunar valve it enters the pulmonary artery. 

 Traversing the capillaries of the lungs, it goes by the pulmo- 



FlG. 89. Diagram illustrating 

 the Circulation. 



i, right auricle; 2, left auricle; 3, 

 right ventricle; 4, left ventricle ; 

 5, vena cava superior ; 6, vena 

 cava inferior ; 7, pulmonary 

 arteries; 8, lungs; 9, pulmonary 

 veins; 10, aorta ; n, alimentary 

 canal ; 12, liver ; 13, hepatic 

 artery ; 14, portal vein ; 15, 

 hepatic vein. Follow the arrows 

 and see whether you come 

 around to the starting point again. 



