150 OUR BODIES AND HOW WE LIVE 



blood. The blood is driven into the pulmonary artery past 

 the semilunar valves. 



The pulmonary artery carries the blood to the lungs. 

 The dark, impure blood is driven along smaller and smaller 

 vessels until it reaches the capillaries of the lungs. Here 

 it is, as it were, spread out to be purified. Exposed to the 

 oxygen of the air, the blood gives up carbon dioxide and 

 other impurities and loses its purple color. It takes up a 

 great deal of the oxygen of the air in exchange, and in 

 a purified state and of a bright scarlet color, it comes 

 back to the heart by the four pulmonary veins which pour 

 it into the left auricle. 



209. The Systemic, or Greater, Circulation. From the 

 left auricle the blood is forced past the mitral valve into 

 the left ventricle. As soon as the left ventricle is full it 

 begins to contract. The mitral valve at once closes and 

 blocks up the passage into the left auricle, and the blood 

 has no other way open but past the semilunar valves l into 

 the aorta. 



The aorta and its branches, as we already know, dis- 

 tribute the blood through every tissue of the body. From 

 the tissues it is again returned by the veins to the right 

 auricle of the heart, and thus the round of circulation is 

 continually kept up. 



Experiment 43. To illustrate some of the phenomena of circula- 

 tion. Attach a piece of rubber tube about six or eight feet long to 

 the delivery end of a common rubber bulb syringe. 



To represent in a very crude way the resistance made by the capil- 

 laries to the flow of blood, slip the large end of a common glass medi- 

 cine dropper into the outer end of the rubber tube. This dropper 

 has one end tapered to a fine point. 



1 The entrance to the aorta is guarded by three semilunar valves similar 

 to those at the entrance of the pulmonary artery. 



