The Cardiac Cycle

The Cardiac Cycle

The cardiac cycle is the sequence of events that occur when the heart

beats. There are two phases of this cycle:

* Diastole - Ventricles are relaxed.

* Systole - Ventricles contract.

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 cava 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.

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.

In 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.

During the 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

cava.

Atrial systole Ventricular systole Ventricular Diastole