52 THE VITAL PROCESSES 



which explains the double structure of his heart. The 

 two divisions are known as the pulmonary and the systemic 

 circulations. By the former the blood passes from the right 

 ventricle through the lungs, and is then returned to the 

 left auricle ; by the latter it passes from the left ventricle 

 through all parts of the body, returning to the right auricle. 



The general plan of the circulation is indicated in Fig. 

 23. All the blood flows continuously through both circu- 

 lations and passes the various parts in the following order : 

 right auricle, tricuspid valve, right ventricle, right semi- 

 lunar valve, pulmonary artery and its branches, capillaries 

 of the lungs, pulmonary veins, left auricle, mitral valve, 

 left ventricle, left semilunar valve, aorta and its branches, 

 systemic capillaries, the smaller veins, superior and inferior 

 venae cavas, and then again into the right auricle. 



In the pulmonary capillaries the blood gives up carbon 



dioxide and receives oxygen, changing from a dark red 



to a bright red color. In the systemic capillaries it gives 



up oxygen, receives carbon dioxide and other impurities, 



and changes back to a dark red color. 



In addition to the two main divisions of the circulation, 

 special circuits are found in various places. Such a circuit 

 in the liver is called the portal circulation, and another in 

 the kidneys is termed the renal circulation. To some extent 

 the blood supply to the walls of the heart is also outside of 

 the general movement ; it is called the coronary circulation. 



Blood Pressure and Velocity. The blood, in obedience to physical 

 laws, passes continuously through the blood vessels, moving always 

 from a place of greater to one of less pressure. Through the contrac- 

 tion of the ventricles, a relatively high pressure is maintained in the 

 arteries nearest the heart. 1 This pressure diminishes rapidly in the 



iThe pressure maintained by the left ventricle has been estimated to be nearly 

 three and one half pounds to the square inch a pressure sufficient to sustain a 

 column of water eight feet high. The pressure maintained by the right ventricle is 



