24 



THE HUMAN MECHANISM 



thence it passes through the lungs and again to the left 

 auricle and ventricle, thus completing the " circulation." 

 The term " circulation," strictly speaking, is applied to the 

 entire circuit which the blood must traverse before it returns 

 again to the point from which it started; it is often con- 

 venient, however, to use it to denote the course from the 

 right ventricle to the left auricle, or from the left ventricle 

 to the right auricle; in this case we speak of the former as 



the pulmonary and of 

 the latter as the sys- 

 temic, or aortic, circu- 

 lation. In this sense 

 there may be said to 

 be a " double " circu- 

 lation. 



The veins have 

 thinner walls than the 

 corresponding arter- 

 ies, and those of the 

 systemic circulation 

 contain purplish or 

 even bluish blood, 

 while the arteries of 

 the same circulation 

 contain bright-scarlet 

 blood. The bright 

 color of the arterial blood is due to the fact that it contains 

 more oxygen. The change from purple to scarlet occurs in 

 the lungs, and the reverse change in the organs supplied 

 by branches of the aorta. Consequently the blood of the 

 pulmonary arteries is blue, or venous, in color and that of the 

 pulmonary vein scarlet, or arterial. 



16. The course and branches of the pulmonary artery and 

 vein. Soon after leaving the right ventricle the pulmonary 

 artery divides into two branches, one going to each lung. 



FIG. 16. A network of capillaries, with the 

 artery a and vein v (highly magnified) 



