148 



Life and Health 



open and through it the blood pours till the ventricle is 

 full. Instantly this begins, in its turn, to contract. The 

 tricuspid valve at once closes and blocks the way backward. 

 The blood is now forced through the open semilunar valves 



into the pulmonary 

 artery. 



The pulmonary 

 artery, bringing ve- 

 nous blood, by its 

 alternate expansion 

 and recoil draws the 

 blood along until it 

 reaches the pulmo- 

 nary capillaries. These 

 tiny tubes surround 

 the air spaces of the 

 lungs, and here an 

 exchange takes 

 place. The dark, 

 venous blood here 

 gives up its waste in 

 the form of carbon 

 dioxide and water, 

 and in return takes 

 up a large amount of 

 oxygen and becomes 

 bright red. Thus the 

 blood brought to the lungs by the pulmonary arteries leaves 

 the lungs entirely different in character and appearance. 

 This is often called the pulmonary, or lesser, circulation. 

 226. The Systemic, or Greater, Circulation. The four pul- 

 monary veins bring back bright scarlet blood and pour it into 

 the left auricle of the heart, whence it goes past the mitral 



FIG. 76. Showing the Carotid Artery and 

 Jugular Vein on the Right Side, with Some 

 of their Main Branches. 

 (Some branches of the cervical plexus, and the 

 hypoglossal nerve are also shown.) 



