Cardiac Cycle

The cardiac cycle is the sequence of events that occurs when the heart beats. 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 to the arteries. One cardiac cycle is completed when the heart fills with blood and the blood is pumped out of the heart. The events of the cardiac cycle described below trace the path of the blood as it enters the heart, is pumped to the lungs, travels back to the heart and is pumped out to the rest of the body. It is important to note that the events that occur in the first and second diastole phases actually happen at the same time. The same is also true for the events of the first and second systole phases.
Cardiac Cycle: 1st Diastole Phase
During the diastole phase, the atria and ventricles are relaxed and the atrioventricular valves are open. De-oxygenated blood from the superior and inferior vena cavae flows into the right atrium. The open atrioventricular valves allow blood to pass through to the ventricles. The SA node contracts triggering the atria to contract. The right atrium empties its contents into the right ventricle. The tricuspid valve prevents the blood from flowing back into the right atrium.
Cardiac Cycle: 1st Systole Phase
During the systole phase, the right ventricle receives impulses from the Purkinje fibers and contracts. The atrioventricular valves close and the semilunar valves open. The de-oxygenated blood is pumped into the pulmonary artery. The pulmonary valve prevents the blood from flowing back into the right ventricle.
The pulmonary artery carries the blood to the lungs. There the blood picks up oxygen and is returned to the left atrium of the heart by the pulmonary veins.
Cardiac Cycle: 2nd Diastole PhaseIn the next diastole period, the semilunar valves close and the atrioventricular valves open. Blood from the pulmonary veins fills the left atrium. (Blood from the vena cava is also filling the right atrium.) The SA node contracts again triggering the atria to contract. The left atrium empties its contents into the left ventricle. The mitral valve prevents the oxygenated blood from flowing back into the left atrium.
Cardiac Cycle: 2nd Systole Phase
During the following systole phase, the atrioventricular valves close and the semilunar valves open. The left ventricle receives impulses from the Purkinje fibers and contracts. Oxygenated blood is pumped into the aorta. The aortic valve prevents the oxygenated blood from flowing back into the left ventricle.
The aorta branches out to provide oxygenated blood to all parts of the body. The oxygen depleted blood is returned to the heart via the vena cavae.

1. 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 PhaseDuring 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 PhaseDuring 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.