Phases of the Cardiac Cycle

The cardiac cycle is the cycle of events that occurs as the heart contracts. There are two phases of the cardiac cycle. In the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood into the arteries.

Cardiac Cycle: Diastole Phase

During the diastole phase, the atria and ventricles are relaxed. Blood flows into the right and left atria. The valves located between the atria and ventricles are open, allowing blood to flow through to the ventricles. Here is a summary of the events that occur during the diastole phase:

• Atrioventricular valves are open.
  
• The sinoatrial (SA) node, which starts cardiac conduction, contracts causing atrial contraction.
  
  
• The atria empty blood into the ventricles.
  
  
• Semilunar valves close preventing back flow into the atria.

Cardiac Cycle: Systole Phase

During the systole phase, the ventricles contract pumping blood into the arteries. The right ventricle sends blood to the lungs via the pulmonary artery. The left ventricle pumps blood to the aorta. Here is a summary of the events that occur during the systole phase:

• The ventricles contract.
  
  
• Atrioventricular valves close and semilunar valves open.
  
  
• Blood flows to either the pulmonary artery or aorta.

One cardiac cycle is completed when the heart fills with blood and the blood is then pumped from the heart. The audible sounds that can be heard from the heart are made by the closing of the heart valves. These sounds are referred to as the "lub-dupp" sounds. The "lub" sound is made by the contraction of the ventricles and the closing of the atrioventricular valves. The "dupp" sound is made by the semilunar valves closing.