THE CARDIAC CYCLE

1)Blood enters the atria. It cannot yet pass into the ventricles because the bicuspid and tricuspid valves are closed
2)The walls of the atria contract. This raises the pressure of the blood in the atria which forces open the bicuspid and tricuspid valves. Blood passes through these valves into the ventricles
3)When the ventricles are full they contract. This increases the pressure of the blood in the ventricles which closes the valves. Blood cannot return to atria
4)Ventricles continue to contract and the pressure continues to increase. Forces open the semi-lunar valves at the base of the aorta and the pulmonary artery.
1. Blood is ejected into these two arteries. The pulmonary artery carries blood to the lungs. The aorta has branches that carry blood to all parts of the body
5)As the ventricles empty,higher pressure in the aorta and pulmonary artery closes the valves in these blood vessels. The cycle then begins again as the atria start to fill with blood

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