维持血液在正常方向循环的结构是心脏的四个瓣膜。其中有两个瓣膜分别分隔左右心房与左右心室。另外两个瓣膜则分别分隔右心室和肺动脉，左心室和主动脉。
在一次正常心搏过程中，静脉血从全身通过腔静脉回流至右心房。右心房收缩，将血液通过三尖瓣泵送到右心室。然后，右心室收缩，血液通过肺动脉瓣，进入与肺脏相连的肺动脉。
于此同时，动脉血从肺脏通过肺静脉回流到心脏。肺静脉的血全部进入左心房后，左心房收缩，动脉血通过二尖瓣进入左心室。左心室收缩，血液通过主动脉瓣进入主动脉，然后分配到全身各个动脉。
心脏的血液供应是通过冠状动脉，它是从主动脉向外分支发出的。
During a normal heartbeat, 在一个心动周期中
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The structure that maintains the normal circulation of blood is the four valves of the heart. Among them, two valves separate the left and right atriums from the left and right ventricles, while the other two valves separate the left ventricle from the pulmonary artery, and the left ventricle from the aortic artery, respectively.
During a normal heartbeat, the peripheral blood circulates throught the vena cava into the right atrium. The right atrium contracts, pushing the blood into the right ventricle through the tricuspid valve. Then, the right ventricle contracts, pushing the blood into the lung arteries connected with the lung through the pulmonic artery valve.
