Cardiac Cycle

Phases The cardiac cycle can be broken down into two phases: Systole: contracting phase Diastole: resting phase

Steps of the Systole Phase

Step 1 Blood enters the right atrium of the heart through the superior and inferior vena cava The tricuspid valve is closed and blood collects in the right atrium

Step 2 The tricuspid valve opens and the blood flows from the right atrium to the right ventricle Once the blood from the right atrium has moved to the right ventricle, the tricuspid valve closes to prevent blood from flowing backwards into the right atrium

Step 3 The right ventricle contracts causing the blood to flow through the pulmonary semilunar valve into the pulmonary trunk

Step 4 The blood divides into the left and right pulmonary arteries which bring the blood to the capillaries in the respective lung While at the lungs, the blood gives up carbon dioxide and picks up a fresh supply of oxygen ** Note: The pulmonary artery carries deoxygenated blood.

Step 5 The blood flows from the capillaries through increasingly larger blood vessels until it reaches the left and right pulmonary veins. **Note: The pulmonary veins carry oxygenated blood.

Step 6 The pulmonary veins meet and pour the oxygenated blood into the left atrium.

Step 7 Once the left atrium is full, blood flows through the AV valve into the left ventricle. Once the left ventricle is full the AV valve closes so blood cannot flow back into the left atrium.

Step 8 The left ventricle then contracts and blood is pumped through the aortic semilunar valve into the aorta, from which the blood will be sent around the body.

Diastole Phase While the atria fill with blood the heart rests before contracting again.

Timing Both atria fill at the same time Both ventricles fill at the same time Both ventricles eject blood at the same time when the heart contracts Contraction begins at the apex and travels upward to ensure all the blood is expelled from the heart