Unformatted text preview: towards heart closing semilunar valves. The ventricles are totally closed off again in this relaxation phase. Atrial filling The atria have been filling with blood. When the pressure exerted by the blood on the atrial side ofthe AV valves exceeds that ofthe ventricles below, the AV valves are forced open and the ventricular filling phase begins again. (Both atria contract at the same time and both ventricles contract at the same time) Therefore, blood flow is controlled by pressure changes: 1. Pressure changes reflect the contractions and relaxations ofthe heart. 2. Blood moves from higher to lower pressure (pressure gradient) 3. Pressure changes cause the valves to open and close which keeps blood flowing in the forward direction. 9r...
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- Spring '11
- ventricles, ventricles contract