The cardiac cycle consists of the hearts ventricles and atria contracting and relaxing to continuously pump blood around the body. The alternation of the chambers contracting and relaxing causes changes in volume and pressure allowing the blood to flow into and out of the heart.
Diastole and Systole
The relaxation and contraction of the heart can be described by the following two terms:
Diastole – relaxation of the heart muscle.
Relaxation of the heart muscle causes blood to flow into the the chambers and blood pressure to decrease.
Systole – contraction of the heart muscle.
Contraction of the heart muscle pushes blood out of the chambers and increases blood pressure.
Cardiac Cycle
The sequence of contraction and relaxation that makes up the cardiac cycle can be split into 3 main steps:
1 – Ventricular and Atrial Diastole
Both the atria and ventricles are relaxed. The pressure in both atria is lower than the pressure in the vena cava and pulmonary vein.
This allows blood from the vena cava to flow into the right atrium and blood from the pulmonary vein to flow into the left atrium.
The blood flows into the atria at a low pressure, but as the atria begin to fill the pressure on the tricuspid and bicuspid valves slightly increases. This causes the valves to begin opening and blood starts to flow passively into the right and left ventricles.
NOTE: Passively means the blood is flowing into the ventricles without any contraction from the atria.
2 – Atrial Systole
The atria contract, decreasing their volume and increasing their pressure – forcing the remaining blood into the ventricles. The ventricles are still relaxed.
As the ventricles fill their pressure increases slightly.
3 – Ventricular Systole
The ventricles contract (and the atria begin to relax), decreasing their volume and increasing their pressure.
The pressure of the ventricles becomes greater than the pressure in the atria, the aorta and the pulmonary artery. This pressure forces the tricuspid and bicuspid valves shut (preventing blood flow back into the atria) and the semilunar valves open.
The contraction of the muscles and difference in pressure forces blood from the right ventricle into the pulmonary artery and around the lungs. Blood from from the left ventricle is ejected into the aorta and around the body.
The steps repeat.
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