cardiac cycle summary

phase

1. systole
2. diastole

Systole

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. 

• 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.

Diastole

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. 

• 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. 

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