140 



ADVANCED PHYSIOLOGY 



of the ventricle; Figs. 74 and 75. When the blood in the 

 pulmonary artery starts to run back into the empty ventricle, 

 it fills these little cups, causing them to swell until the three 

 stretch completely across the lumen of the artery, thus 

 wholly blocking the passage and preventing any backward 

 flow of blood. But when the ventricle contracts, blood is 

 pushed against the cups from below until it finally flattens 

 them against the walls of the artery so that blood can pass 

 them easily. These cup-like flaps remain flattened against 

 the walls of the pulmonary vessel as long as the contraction 

 Df the ventricle forces the blood onward. 



After the ventricle has contracted as much as it can and 

 has squeezed out practically all the blood it contained, the 



muscles in its walls relax, leav- 

 ing the cavity free to fill 

 once more. The blood which 

 has just been forced into the 

 artery starts to flow back but 

 immediately fills the semilunar 

 valves, which then block its 

 backward passage. This action 

 can be better understood, if 

 compared to the behavior of 

 an umbrella in a wind. When 

 pointing into the wind it offers 

 little or no resistance to the 

 currents of air, which would 

 tend to close it; but if turned 

 the other way, it is immediately 

 opened and filled, thus blocking 

 the passage of the wind. The blood cannot flow back into 

 the ventricle but as the ventricle relaxes, the tricuspid 

 valves fall down loosely into the ventricle again, thus allow- 

 ing more blood to enter it from the auricle and reinstating 

 the condition with which we started. 



FIG. 76. DIAGRAM 



Showing the veins entering and the 

 arteries leaving the left side of the 

 heart. Broken line marks the aorta. 



