PULMONAEY CIRCULATION 195 



to a minimum in adult life, rising slowly as we enter extreme old 

 age. The following table gives the average rates: 



At birth 140 beats per minute. 



Infancy 120 



Childhood 100 



Youth 90 



Adults 75-70 



Old age 70 



Extreme age 75-80 



Systems of CiECULATioisr. 



From what has been said it follows that the blood which 

 passes through any given chamber of the heart must ulti- 

 mately return to that same chamber. Owing, however, 

 to the division of the heart into two parts, which beat in 

 unison, and the branching of the blood vessels, the actual 

 course of the blood requires some further explanation. 

 For convenience, the blood vessels that are concerned in 

 the transfer of the blood from the right to the left side of 

 the heart, and that have their branches in the lungs, are 

 spoken of collectively as the pulmonary system. Those 

 concerned with the transfer of blood from the left side of 

 the heart to the right side, and whose branches take it 

 all over the body, are spoken of as the systemic system. 



Pulmonary circulation. — AYhen the blood enters the 

 right auricle it is partly exhausted of its oxygen and is 

 dark and purplish in color, owing to the abundance of car- 

 bon dioxide which it contains. In this condition it enters 

 the right ventricle, and when that organ contracts it is 

 forced into the pulmonary artery which in turn breaks up 

 into two branches leading to the right and left lung, and 

 these into smaller branches within the lungs. These pul- 



