THE BLOOD AND CIRCULATION 321 



and 164. Thus we see that the flow of blood from auricle 

 to ventricle, and from ventricle to artery, can take place 

 in one direction only. In case of diseased valves there may 

 be a backward flow or regurgitation. 



How the heart works.- -The auricles are constantly fill- 

 ing with blood from the great veins. Both auricles contract 

 at the same time, the auriculo-ventricular valves open and 

 the blood is driven into the ventricles. The ventricles con- 



FIG. 164. Right cavities of the heart; auriculo-ventricular valves closed, 

 arterial valves open. (After Dalton.) 



tract at once and send the blood out into the great arteries. 



The heart beat is the alternate contraction (systole) and 

 relaxation or expansion (diastole) of the walls of the heart. 

 In man this occurs about seventy-five times per minute 

 under normal conditions. A child's pulse is more rapid. 



Systemic circulation. We have seen that the aorta 

 carries food and oxygen to the tissues. It passes anteriorly 

 from the left ventricle, then arches and passes posteriorly 



